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Line 227: |
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| (382) H. Ludwig. " Ueber die Ordnung Gastrotricha Mctschn" Zeit. f. wiss. | | (382) H. Ludwig. " Ueber die Ordnung Gastrotricha Mctschn" Zeit. f. wiss. |
| Zool., Vol. xxvi. 1876. | | Zool., Vol. xxvi. 1876. |
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| | |
| ==CHAPTER XVI. NEMATELMINTHES AND ACANTHOCEPHALA==
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| | |
| NEM ATELMINTHES '.
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| | |
| Nematoidea. Although the ova of various Nematodes have
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| formed some of the earliest, as well as the most frequent objects
| |
| of embryological observation, their development is still but very
| |
| imperfectly known. Both viviparous and oviparous forms are
| |
| common, and in the case of the oviparous forms the eggs are
| |
| usually enveloped in a hard shell. The segmentation is total
| |
| and nearly regular, though the two first segments are often
| |
| unequal. The relation of the segmentation spheres to the
| |
| germinal layers is however only satisfactorily established (through
| |
| the researches of Butschli (No. 383)) in the case of Cucullanus
| |
| elegans, a form parasitic in the Perch 2 .
| |
| | |
| The early development of this embryo takes place within
| |
| the body of the parent, and the egg is enveloped in a delicate
| |
| membrane. After the completion of the early stages of segmentation the embryo acquires the form of a thin flat plate
| |
| composed of two layers of cells (fig. 166 A and B). The two
| |
| layers of this plate give rise respectively to the epiblast and
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| hypoblast, and at a certain stage the hypoblastic layer ceases to
| |
| | |
| 1 The following classification of the Nematoda is employed in this chapter :
| |
| | |
| r Ascaridae.
| |
| Strongylidae.
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| | |
| Trichinidse. II. Gordioidea.
| |
| | |
| I. Nematoidea. , Filarid8B . m . Chaetosomoidea.
| |
| | |
| Mermithidae.
| |
| [_ Anguillulidse.
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| | |
| 2 The ova of Anguillula aceti are stated by Hallez to undergo a similar development to those of Cucullanus.
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| | |
| 242
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| | |
| | |
| 372
| |
| | |
| | |
| | |
| CUCULLANUS.
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| | |
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| | |
| grow, while the growth of the epiblastic layer continues. As a
| |
| consequence of this the sides of the plate begin to fold over
| |
| towards the side of the hypoblast (fig. 166 D.) This folding
| |
| results in the formation of a remarkably constituted gastrula,
| |
| which has the form of a hollow two-layered cylinder with an
| |
| incompletely closed slit on one side (fig. 166 E, bl.p}. This slit
| |
| has the value of a blastopore. It becomes closed by the coalescence of the two edges, a process which commences posteriorly,
| |
| | |
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| | |
| | |
| FIG. 166.
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| A.
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| B.
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| C.
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| | |
| VARIOUS STAGES IN THE DEVELOPMENT OF CUCULLANUS ELEGANS.
| |
| | |
| (From Biitschli.)
| |
| | |
| Surface view of flattened embryo at an early stage in the segmentation.
| |
| Side view of an embryo at a somewhat later stage, in optical section.
| |
| Flattened embryo at the completion of segmentation.
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| | |
| D. Embryo at the commencement of the gastrula stage.
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| | |
| E. Embryo when the blastopore is reduced to a mere slit.
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| | |
| F. Vermiform embryo after the division of the alimentary tract into oesophageal
| |
| and glandular divisions.
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| | |
| m. mouth; ep. epiblast; hy. hypoblast; me. mesoblast; a?, oesophagus; bl.p. blastopore.
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| | |
| and then gradually extends forwards. In front the blastopore
| |
| never becomes completely closed, but remains as the permanent
| |
| mouth. The embryo after these changes has a worm-like form,
| |
| which becomes the more obvious as it grows in length and
| |
| becomes curved (fig. 166 F).
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| | |
| The hypoblast of the embryo gives rise to the alimentary
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| | |
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| | |
| NEMATELM1NTHES. 373
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| | |
| | |
| | |
| canal, and soon becomes divided into an cesophageal section
| |
| (fig. 1 66 F, ce) formed of granular cells, and a posterior division
| |
| formed of clear cells. The mesoblast (fig. 166, me) takes its
| |
| origin from certain special hypoblast cells around the mouth,
| |
| and thence grows backwards towards the posterior end of the
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| body.
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| | |
| The young Cucullanus becomes hatched while still in the
| |
| generative ducts of its parent, and is distinguished by the
| |
| presence of a remarkable thread-like tail. On the dorsal surface
| |
| is a provisional boring apparatus in the form of a conical papilla.
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| A firm cuticle enveloping the body is already present. In this
| |
| condition it leaves its parent and host, and leads for a time a
| |
| free existence in the water. Its metamorphosis is dealt with in
| |
| another section.
| |
| | |
| The ova of the Oxyuridae parasitic in Insects are stated by Galeb (No.
| |
| 386) to take the form of a blastosphere at the close of segmentation. An
| |
| inner layer is then formed by delamination. What the inner layer gives rise
| |
| to is not clear, since the whole alimentary canal is stated to be derived from
| |
| two buds, which arise at opposite ends of the body, and grow inwards till
| |
| they meet.
| |
| | |
| The generative organs. The study of the development of
| |
| the generative organs of Nematodes has led to some interesting
| |
| results. In the case of both sexes the generative organs originate (Schneider, No. 390) from a single cell. This cell elongates
| |
| and its nuclei multiply. After assuming a somewhat columnar
| |
| form, it divides into (i) a superficial investing layer, and (2) an
| |
| axial portion.
| |
| | |
| In the female the superficial layer is only developed distinctly in the median part of the column. In the course of the
| |
| further development the two ends of the column become the
| |
| blind ends of the ovary, and the axial tissue they contain forms
| |
| the germinal tissue of nucleated protoplasm. The superficial
| |
| layer gives rise to the epithelium of the uterus and oviduct.
| |
| The germinal tissue, which is originally continuous, is interrupted
| |
| in the middle part (where the superficial layer gives rise to the
| |
| uterus and oviduct), and is confined to the two blind extremities
| |
| of the tube.
| |
| | |
| In the male the superficial layer, which gives rise to the epithelium of the vas deferens, is only formed at the hinder end of
| |
| | |
| | |
| | |
| 374 METAMORPHOSIS.
| |
| | |
| | |
| | |
| the original column. In other respects the development takes
| |
| place as in the female.
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| | |
| Gordioidea. The ovum of Gordius undergoes a regular segmentation.
| |
| According to Villot (No. 391) it forms at the close of segmentation a morula,
| |
| which becomes two-layered by delamination. The embryo is at first
| |
| spherical, but soon becomes elongated.
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| | |
| By an invagination at the anterior extremity the head is formed. It
| |
| consists of a basal portion, armed with three rings of stylets, and a conical
| |
| proboscis, armed with three large stylets. When the larva becomes free
| |
| the head becomes everted, though it remains retractile. By the time the
| |
| embryo is hatched a complete alimentary tract is formed with an oral opening
| |
| at the end of the proboscis, and a subterminal ventral anal opening. It is
| |
| divided into an oesophagus and stomach, and a large gland opens into it at
| |
| the base of the proboscis.
| |
| | |
| The body has a number of transverse folds, which give it a ringed
| |
| appearance.
| |
| | |
| Metamorphosis and life history.
| |
| | |
| Nematoidea. Although a large number of Nematodes have
| |
| a free existence and simple life history, yet the greater number
| |
| of known genera are parasitic, and undergo a more or less complicated metamorphosis 1 . According to this metamorphosis
| |
| they may be divided into two groups (which by no means
| |
| closely correspond with the natural divisions), viz. those which
| |
| have a single host, and those with two hosts. Each of these
| |
| main divisions may be subdivided again into two.
| |
| | |
| In the first group with one host the simplest cases are those
| |
| in which the adult sexual form of parasite lays its eggs in the
| |
| alimentary tract of its host, and the eggs are thence transported
| |
| to the exterior. The embryo still in the egg, if favoured by
| |
| sufficient warmth and moisture, completes its development up
| |
| to a certain point, and, if then swallowed by an individual of the
| |
| species in which it is parasitic in the adult condition, it is
| |
| denuded of its shell by the action of the gastric juice, and
| |
| develops directly into the sexual form.
| |
| | |
| Leuckart has experimentally established this metamorphosis in the case
| |
| of Trichocephalus affinis, Oxyurus ambigua, and Heterakis vermicularis.
| |
| The Oxyuridae of Blatta and Hydrophilus have a similar life history
| |
| | |
| 1 The following facts are mainly derived from Leuckart's exhaustive treatise
| |
| (No. 388).
| |
| | |
| | |
| | |
| NEMATELMINTHES. 375
| |
| | |
| | |
| | |
| (Caleb, No. 386), and it is almost certain that the metamorphosis of the
| |
| human parasites, Ascaris lumbricoides and Oxyurus vermicularis, is of this
| |
| nature.
| |
| | |
| A slightly more complicated metamorphosis is common in
| |
| the genera Ascaris and Strongylus. In these cases the egg-shell
| |
| is thin, and the embryo becomes free externally, and enjoys for
| |
| a shorter or longer period a free existence in water or moist
| |
| earth. During this period it grows in size, and though not
| |
| sexual usually closely resembles the adult form of the permanently free genus Rhabditis. In some cases the free larva
| |
| becomes parasitic in a freshwater Mollusc, but without thereby
| |
| undergoing any change. It eventually enters the alimentary
| |
| tract of its proper host and there become sexual.
| |
| | |
| As examples of this form of development worked out by Leuckart may
| |
| be mentioned Uochmius trigonocephalus, parasitic in the dog, and Ascaris
| |
| acuminata, in the frog. The human parasite Dochmius duodenale undergoes the same metamorphosis as Dochmius trigonocephalus.
| |
| | |
| A remarkable modification of this type of metamorphosis is found in
| |
| Ascaris (Rhabdonema) nigrovenosa, which in its most developed condition
| |
| is parasitic in the lungs of the frog (Metschnikoff, Leuckart, No. 388). The
| |
| embryos pass through their first developmental phases in the body of the
| |
| parent. They have the typical Rhabditis form, and make their way after
| |
| birth into the frog's rectum. From this they pass to the exterior, and then
| |
| living either in moist earth, or the faeces of the frog, develop into a sexual
| |
| form, but are very much smaller than in the adult condition. The sexes are
| |
| distinct, and the males are distinguished from the females by their smaller
| |
| size, shorter and rounded tails, and thinner bodies. The females have
| |
| paired ovaries with a very small number of eggs, but the testis of the males
| |
| is unpaired. Impregnation takes place in the usual way, and in summer
| |
| time about four embryos are developed in each female, which soon burst
| |
| their egg-capsules, and then move freely in the uterus. Their active movements soon burst the uterine walls, and they then come to lie freely in the
| |
| body cavity. The remaining viscera of the mother are next reduced to a
| |
| finely granular material, which serves for the nutrition of the young forms
| |
| which continue to live in the maternal skin. The larvae eventually become
| |
| free, and though in many respects different from the parent form which gave
| |
| rise to them, have nevertheless the Rhabditis form. They live in water or
| |
| slime, and sometimes become parasitic in water-snails ; in neither case however do they undergo important changes unless eventually swallowed by a
| |
| frog. They then pass down the trachea into the lungs and there rapidly
| |
| develop into the adult form. No separate males have been found in the
| |
| lungs of the frog, but it has been shewn by Schneider (No. 390) that the
| |
| so-called females are really hermaphrodites ; the same gland giving origin
| |
| | |
| | |
| | |
| 376 METAMORPHOSIS.
| |
| | |
| | |
| | |
| to both spermatozoa and ova, the former being developed before the latter 1 .
| |
| The remarkable feature of the above life history is the fact that in the stage
| |
| corresponding with the free larval stage of the previous forms the larvae of
| |
| this species become sexual, and give rise to a second free larval generation,
| |
| which develops into the adult form on again becoming parasitic in the
| |
| original host. It constitutes a somewhat exceptional case of heterogamy as
| |
| defined in the introduction.
| |
| | |
| Amongst the Nematodes with but a single host a remarkable parasite in
| |
| wheat has its place. This form, known as Anguillula scandens, inhabits in
| |
| the adult condition the ears of wheat, in which it lays its eggs. After
| |
| hatching, the larvae become encysted, but become free on the death of the
| |
| plant. They now inhabit moist earth, but eventually make their way into
| |
| the ears of the young wheat and become sexually mature.
| |
| | |
| The second group of parasitic Nematodes with two hosts
| |
| may be divided into two groups, according to whether the larva
| |
| has a free existence before passing into its first or intermediate
| |
| host, or is taken into it while still in the egg. In the majority
| |
| of cases the larval forms live in special connective tissue capsules, or sometimes free in the tissues of their intermediate
| |
| hosts ; but the adults, as in the cases of other parasitic Nematodes, inhabit the alimentary tract.
| |
| | |
| The life history of Spiroptera obtusa may be cited as an example of a
| |
| Nematode with two hosts in which the embryo is transported into its
| |
| intermediate host while still within the egg. The adult of this form is
| |
| parasitic in the mouse, and the ova pass out of the alimentary tract with the
| |
| excreta, and may commonly be found in barns, etc. If one of the ova is
| |
| now eaten by the meal-worm (larva of Tenebrio), it passes into the body
| |
| cavity of this worm and undergoes further development. After about five
| |
| weeks it becomes encapsuled between the ' fat bodies ' of the meal-worm.
| |
| It then undergoes an ecdysis, and, if the meal-worm with its parasites is
| |
| now eaten by the mouse, the parasites leave their capsule and develop into
| |
| the sexual form.
| |
| | |
| As examples of life histories in which a free state intervenes before the
| |
| intermediate host, Cucullanus elegans and Dracunculus may be selected.
| |
| The adult Cucullanus elegans is parasitic in the alimentary tract of the Perch
| |
| and other freshwater fishes. It is a viviparous form, and the young after
| |
| birth pass out into the water. They next become parasitic in Cyclops,
| |
| passing in through the mouth, so into the alimentary tract, and thence into
| |
| the body cavity. They soon undergo an ecdysis, in the course of which the
| |
| oesophagus becomes divided into a muscular pharynx and true glandular
| |
| | |
| 1 Leuckart does not appear to be satisfied as to the hermaphroditism of these
| |
| forms ; and holds that it is quite possible that the ova may develop parthenogenetically.
| |
| | |
| | |
| | |
| NEMATELMINTHES. 377
| |
| | |
| | |
| | |
| oesophagus. They then grow rapidly in length, and at a second ecdysis
| |
| acquire a peculiar beaker-like mouth cavity approaching that of the adult.
| |
| They do not become encapsuled. No further development of the worm
| |
| takes place so long as it remains in the Cyclops, but, if the Cyclops is now
| |
| swallowed by a Perch, the worm undergoes a further ecdysis, and rapidly
| |
| attains to sexual maturity.
| |
| | |
| The observations of Fedschenko on Dracunculus medinensis 1 , which is
| |
| parasitic in the subcutaneous connective tissue in Man, would seem to shew
| |
| that it undergoes a metamorphosis very similar to that of Cucullanus. There
| |
| is moreover a striking resemblance between the larvae of the two forms.
| |
| The larvae of Dracunculus become transported into water, and then make
| |
| their way into the body cavity of a Cyclops by boring through the soft skin
| |
| between the segments on the ventral surface of the body. In the body cavity
| |
| the larvae undergo an ecdysis and further development. But on reaching
| |
| a certain stage of development, though they remain a long time in the
| |
| Cyclops, they grow no further. The remaining history is unknown, but
| |
| probably the next host is man, in which the larva comes to maturity. In the
| |
| adult condition only females of Dracunculus are known, and it has been
| |
| suggested by various writers that the apparent females are in reality hermaphrodites, like Ascaris nigrovenosa, in which the male organs come to
| |
| maturity before the female.
| |
| | |
| Another very remarkable human parasite belonging to the same group
| |
| as Dracunculus is the form known as Filaria sanguinis hominis, or Filaria
| |
| Bancrofti 2 .
| |
| | |
| The sexual form is parasitic in warm climates in the human tissues, and
| |
| produces multitudes of larvae which pass into the blood, and are sometimes
| |
| voided with the urine. The larvae in the blood do not undergo a further development, and unless transported to an intermediate host die before very long.
| |
| Some, though as yet hardly sufficient, evidence has been brought forward to
| |
| shew that if the blood of an infected patient is sucked by a mosquito the
| |
| larvae develop further in the alimentary tract of the mosquito, pass through a
| |
| more or less quiescent stage, and eventually grow considerably in size, and
| |
| on the death of the mosquito pass into the water. From the water they are
| |
| probably transported directly or indirectly into the human intestines, and
| |
| then bore their way into the tissues in which they are parasitic, and become
| |
| sexually mature.
| |
| | |
| The well-known Trichina spiralis has a life history unlike that of other
| |
| known Nematodes, though there can be little doubt that this form should
| |
| be classified in respect to its life history with the last- described forms.
| |
| The peculiarity of the life history of Trichina is that the embryos set free
| |
| in the alimentary canal pass through the walls into the muscular tissues and
| |
| there encyst ; but do not in a general way pass out from the alimentary
| |
| | |
| 1 Vide Leuckart, D. men. Par., Vol. II. p. 704.
| |
| | |
| 2 Vide D. P. Manson, " On the development of Filaria sanguinis hominis."
| |
| Journal of the Linnean Society, Vol. xiv. No. 75.
| |
| | |
| | |
| | |
| 378 MKTAMORPHOSIS.
| |
| | |
| | |
| | |
| canal of one host and thence into a fresh host to encyst. It occasionally
| |
| however happens that this migration does take place, and the life history
| |
| of Trichina spiralis then becomes almost identical with that of some of the
| |
| forms of the third type. Trichina is parasitic in man, and in swine, and
| |
| also in the rat, mouse, cat, fox and other forms which feed upon them.
| |
| Artificially it can be introduced into various herbivorous forms (rabbit,
| |
| guinea-pig, horse) and even birds.
| |
| | |
| The sexual form inhabits the alimentary canal. The female is viviparous, and produces myriads of embryos, which pass into the alimentary
| |
| canal of their host, through the walls of which they make their way, and
| |
| travelling along lines of connective tissue pass into the muscles. Here the
| |
| embryos, which are born in a very imperfect condition, rapidly develop,
| |
| and eventually assume a quiescent condition in a space inclosed by sarcolemma. Within the sarcolemma a firm capsule is developed for each larva,
| |
| which after some months becomes calcified ; and after the atrophy of the
| |
| sarcolemma a connective tissue layer is formed around it. Within its
| |
| capsule the larva can live for many years, even ten or more, without
| |
| undergoing further development, but if at last the infected flesh is eaten by
| |
| a suitable form, e.g. the infected flesh of the pig by man, the quiescent
| |
| state of the larva is brought to a close, and sexual maturity is attained in
| |
| the alimentary tract of the new host.
| |
| | |
| Gordioidea. The free larva of Gordius already described usually penetrates into the larva of Chironomus where it becomes encysted. On the
| |
| Chironomus being eaten by some fish (Villot, No. 39) (Phoxinus laevis or
| |
| Cobitis barbatula), it penetrates into the wall of the intestine of its second
| |
| host, becomes again encysted and remains quiescent for some time. Eventually in the spring it leaves its capsule, and enters the intestine, and
| |
| passes to the exterior with the faeces. It then undergoes a gradual metamorphosis, in the course of which it loses its ringed structure and cephalic
| |
| armature, grows in length, acquires its ventral cord, and on the development of the generative organs loses the greater part of its alimentary tract.
| |
| | |
| Young examples of Gordius have often been found in various terrestrial
| |
| carnivorous Insecta, but the meaning of this fact is not yet clear.
| |
| | |
| | |
| | |
| BIBLIOGRAPHY.
| |
| | |
| (383) O. Biitschli. "Entwicklungsgeschichte d. Cucullanus elegans." Zdt.j.
| |
| wiss. Zool., B. xxvi. 1876.
| |
| | |
| (384) T. S. Cobbold. Entozoa. Groombridge and Son, 1864.
| |
| | |
| (385) T. S. Cobbold. Parasites; A Treatise on the Entozoa of Man mn/
| |
| Animals. Churchill, 1879.
| |
| | |
| (386) O. Galeb. "Organisation et developpement des Oxyurides," &c. Archives de Zool. expcr. et getter. , Vol. vn. 1878.
| |
| | |
| (387) R. Leu ck art. Untcrsufkutigcn itb. Trichina spiralis. 2nd ed. Leip/ig,
| |
| 1866.
| |
| | |
| (388) R. Leuckart. Die tnenschlichcn Parasitcn, Bd. II. 1876.
| |
| | |
| | |
| | |
| NEMATELMINTHES. 379
| |
| | |
| | |
| | |
| (389) H. A. Pagenstecher. Die Trichinen nach Versitchen dargestellt. Leipzig, 1865.
| |
| | |
| (390) A.Schneider. Monographic d. Nemaioden. Berlin, 1866.
| |
| | |
| (391) A. Villot. "Monographic des Dragoneaux" (Gordioidea). Archives de
| |
| Zool. exper. et gener., Vol. ill. 1874.
| |
| | |
| ACANTHOCEPHALA.
| |
| | |
| The Acanthocephala appear to be always viviparous. At the time of
| |
| impregnation the ovum is a naked cell, and undergoes in this condition the
| |
| earlier phases of segmentation.
| |
| | |
| The segmentation is unequal (Leuckart, No. 393), but whether there is an
| |
| epibolic gastrula has not clearly been made out.
| |
| | |
| Before segmentation is completed there are formed round the ovum
| |
| thick protecting membranes, which are usually three in number, the middle
| |
| one being the strongest. After segmentation the central cells of the ovum
| |
| fuse together to give rise to a granular mass, while the peripheral cells at a
| |
| slightly later period form a more transparent syncytium. At the anterior
| |
| end of the embryo there appears a superficial cuticle bearing in front a ring
| |
| of hooks.
| |
| | |
| The embryo is now carried out with the excreta from the intestine of
| |
| the vertebrate host in which its parent lives. It is then swallowed by some
| |
| invertebrate host 1 .
| |
| | |
| In the intestine of the invertebrate host the larva is freed from its
| |
| membranes, and is found to have a somewhat elongated conical form, terminating anteriorly in an obliquely placed disc, turned slightly towards the
| |
| ventral surface and armed with hooks. Between this disc and the granular
| |
| mass, already described as formed from the central cells of the embryo, is a
| |
| rather conspicuous solid body. Leuckart supposes that this body may represent a rudimentary functionless pharynx, while the granular mass in
| |
| his opinion is an equally rudimentary and functionless intestine. The body
| |
| wall is formed of a semifluid internal layer surrounding the rudimentary
| |
| intestine, if such it be, and of a firmer outer wall immediately within the cuticle.
| |
| The adult Echinorhyncus is formed by a remarkable process of development within the body of the larva, and the skin is the only part of the
| |
| larva which is carried over to the adult.
| |
| | |
| In Echinorhyncus proteus the larva remains mobile during the formation of the adult, but in other forms the metamorphosis takes place during
| |
| a quiescent condition of the larva.
| |
| | |
| The organs of the adult are differentiated from a mass of cells which
| |
| appears to be a product of the central embryonic granular mass, and is
| |
| | |
| 1 Echin. proteus, which is parasitic in the adult state in many freshwater fish,
| |
| passes through its larval condition in the body cavity of Gammarus pulex. Ech.
| |
| angustatus, parasitic in the Perch, is found in the larval condition in the body cavity
| |
| of Asellus aquaticus. Ech. gigas, parasitic in swine, is stated by Schneider (No. 394)
| |
| to pass through its larval stages in maggots.
| |
| | |
| | |
| | |
| 380 ACANTHOCEPHALA.
| |
| | |
| | |
| | |
| called by Leuckart the embryonic nucleus. The embryonic nucleus becomes
| |
| divided into four linearly arranged groups of cells, of which the hindermost
| |
| but one is the largest, and very early differentiates itself into (i) a peripheral
| |
| layer, and (2) a central mass formed of two distinct bodies. The peripheral
| |
| layer of this segment grows forwards and backwards, and embraces the
| |
| other segments, with the exception of the front end of the first one which
| |
| is left uncovered. The envelope so formed gives rise to the splanchnic and
| |
| somatic mesoblast of the adult worm. Of the four groups of cells within it
| |
| the anterior gives rise to the proboscis, the next to the nerve ganglion, the
| |
| third, formed of two bodies, to the paired generatives, and the fourth to the
| |
| generative ducts. The whole of the above complex rapidly elongates, and
| |
| as it does so the enveloping membrane becomes split into two layers ; of
| |
| which the outer forms the muscular wall of the body (somatic mesoblast),
| |
| and the inner the muscular sheath of the proboscis and the so-called generative ligament enveloping the generative organs. The inner layer may be
| |
| called the splanchnic mesoblast in spite of the absence of an intestine.
| |
| The cavity between the two mesoblastic layers forms the body cavity.
| |
| | |
| The various parts of the adult continue to differentiate themselves as
| |
| the whole increases in size. The generative masses very early shew traces
| |
| of becoming differentiated into testes or ovaries. In the male the two
| |
| generative masses remain spherical, but in the female become elongated :
| |
| the rudiment of the generative ducts becomes divided into three sections
| |
| in both sexes. The most remarkable changes are, however, those undergone
| |
| by the rudiment of the proboscis.
| |
| | |
| In its interior there is formed a cavity, but the wall bounding the front
| |
| end of the cavity soon disappears. By the time that this has taken place
| |
| the body of the adult completely fills up the larval skin, to which it very
| |
| soon attaches itself. The hollow rudiment of the proboscis then becomes
| |
| everted, and forms a papilla at the end of the body, immediately adjoining the larval skin. This papilla, with the larval skin covering it,
| |
| constitutes the permanent proboscis. The original larval cuticle is either
| |
| now or at an earlier period thrown off and a fresh cuticle developed. The
| |
| hooks of the proboscis are formed from cells of the above papilla, which
| |
| grow through the larval skin as conical prominences, on the apex of which
| |
| a chitinous hook is modelled. The remainder of the larval skin forms the
| |
| skin of the adult, and at a later period develops in its deeper layer the
| |
| peculiar plexus of vessels so characteristic of the Acanthocephala. The
| |
| anterior oval appendages of the adult cutis, known as the lemnisci, are
| |
| outgrowths from the larval skin.
| |
| | |
| The Echinorhyncus has with the completion of these changes practically
| |
| acquired its adult structure ; but in the female the ovaries undergo at this
| |
| period remarkable changes, in that they break up into a number of spherical
| |
| masses, which lie in the lumen of the generative ligaments, and also make
| |
| their way into the body cavity.
| |
| | |
| The young Echinorhyncus requires to be transported to its permanent
| |
| host, which feeds on its larval host, before attaining to sexual maturity.
| |
| | |
| | |
| | |
| ACANTHOCEPHALA. 381
| |
| | |
| | |
| | |
| BIBLIOGRAPHY.
| |
| | |
| (392) R. Greeff. " Untersuchungen ii. d. Bau u. Entwicklung des Echin. miliarius." Archiv f. Naturgesch. 1864.
| |
| | |
| (393) R. Leuckart. Die menschlichen Parasiten. Vol. n. p. 80 1 et seq.
| |
| 1876.
| |
| | |
| (394) An. Schneider. " Ueb. d. Bau d. Acanthocephalen." Archiv f. Anat.
| |
| u, Phys. 1868.
| |
| | |
| (395) G. R. Wagener. Beitrdge z. Entwicklungsgeschichte d. Eingeweidewiirmer. Haarlem, 1865.
| |
| | |
| | |
| | |
| CHAPTER XVII.
| |
| | |
| TRACHEATA.
| |
| | |
| PROTOTRACH EAT A.
| |
| THE remarkable researches of Moseley (No. 396) on Peripatus
| |
| | |
| | |
| | |
| | |
| FIG. 167. ADULT EXAMPLE OF PERIPATUS CAPENSIS, natural size.
| |
| (From Moseley.)
| |
| | |
| capensis have brought clearly to light the affinities of this form
| |
| with the tracheate Arthropoda ; and its numerous primitive
| |
| | |
| | |
| | |
| | |
| FIG. 168. Two STAGES IN THE DEVELOPMENT OF PERIPATUS CAPENSIS.
| |
| (After Moseley.)
| |
| | |
| A. Youngest stage hitherto observed before the appearance of the legs.
| |
| | |
| B. Later stage after the legs and antennae have become developed.
| |
| Both figures represent the larva as it appears within the egg.
| |
| | |
| i and i. First and second post-oral appendages.
| |
| | |
| characters, such as the generally distributed tracheal apertures,
| |
| the imperfectly segmented limbs, the diverging ventral nerve
| |
| | |
| | |
| | |
| TRACHEATA.
| |
| | |
| | |
| | |
| 383
| |
| | |
| | |
| | |
| cords with imperfectly marked ganglia, and the nephridia (segmental organs 1 ), would render its embryology of peculiar interest. Unfortunately Moseley was unable, from want of
| |
| material, to make so complete a study of its development as of
| |
| its anatomy. The youngest embryo observed was in part
| |
| distinctly segmented, and coiled up within the egg (fig. 168 A).
| |
| The procephalic lobes resemble those of the Arthropoda generally, and are unlike the prae-oral lobe of
| |
| Chaetopods or Discophora. They are not
| |
| marked off by a transverse constriction
| |
| from the succeeding segments. The three
| |
| embryonic layers are differentiated, and
| |
| the interior is filled with a brownish mass
| |
| the remnant of the yolk which is probably enclosed in a distinct intestinal wall,
| |
| and is lobed in correspondence with the
| |
| segmentation of the body. The mouth
| |
| invagination is not present, and but two
| |
| pairs of slight prominences mark the rudiments of the two anterior post-oral appendages.
| |
| | |
| The single pair of antennae is formed
| |
| in the next stage, and is followed by the
| |
| remaining post-oral appendages, which
| |
| arise in succession from before backwards
| |
| somewhat later than the segments to which
| |
| they appertain.
| |
| | |
| The posterior part of the embryo becomes uncoiled, and the whole embryo
| |
| bent double in the egg (fig. 168 B).
| |
| | |
| The mouth appears as a slit-like opening between and below the procephalic
| |
| lobes. On each side and somewhat behind it there grows out
| |
| an appendage the first post-oral pair (fig. 169, i) while in
| |
| front and behind it are formed the upper and lower lips. These
| |
| two appendages next turn inwards towards the mouth, and their
| |
| | |
| | |
| | |
| | |
| FIG. 169. EMBRYO
| |
| OF PERIPATUS CAPENSIS.
| |
| Slightly older than A in
| |
| fig. 168; unrolled. (After
| |
| Moseley.)
| |
| | |
| a. antennae ; o. mouth ;
| |
| i. intestine ; c. procephalic
| |
| lobe, i, 2, 3, etc., postoral appendages.
| |
| | |
| | |
| | |
| 1 F. M. Balfour, "On certain points in the Anatomy of Peripatus capensis."
| |
| Quart. Journ. of Micros. Science, Vol. xix. 1879.
| |
| | |
| | |
| | |
| PROTOTRACHEATA.
| |
| | |
| | |
| | |
| | |
| bases become gradually closed over by two processes of the
| |
| procephalic region (fig. 170, m)
| |
| The whole of these structures
| |
| assist in forming a kind of
| |
| secondary mouth cavity, which
| |
| is at a later period further
| |
| completed by the processes of
| |
| the procephalic region meeting
| |
| above the mouth, covering over
| |
| the labrum, and growing backwards to near the origin of the
| |
| second pair of post-oral appendages.
| |
| | |
| The antennae early become
| |
| jointed, and fresh joints continue to be added throughout
| |
| embryonic life ; in the adult
| |
| there are present fully thirty
| |
| joints. It appears to me probable (though Mr Moseley takes
| |
| the contrary view) from the late development of the paired
| |
| processes of the procephalic lobes, which give rise to the circular
| |
| lip of the adult, that they
| |
| are not true appendages.
| |
| The next pair therefore
| |
| to the antennae is the first
| |
| post-oral pair. It is the
| |
| only pair connected with
| |
| the mouth. At their extremities there is formed a
| |
| pair of claws similar to
| |
| those of the ambulatory
| |
| | |
| legs (fig. 171). The next FIG. 171. HEAD OF AN EMBRYO PERIPA, . . r TUS. (From Moseley.)
| |
| | |
| and largest pair of appen- The figure shews the jaws (mamlil)lcs)> and
| |
| | |
| dagCS in the embryo are close to them epiblastic involutions, which
| |
| | |
| | |
| | |
| FIG. 170. VENTRAL VIEW OF THE
| |
| HEAD OF AN EMBRYO OF PERIPATUS CAPENSIS AT A LATE STAGE OF DEVELOPMENT.
| |
| | |
| /. thickening of epiblast of procephalic lobe to form supra-oesophageal ganglion ; ///. process from procephalic lobe
| |
| growing over the first post-oral appendage ; o. mouth; e. eye; i and 2, first
| |
| and second pair of post-oral appendages.
| |
| | |
| | |
| | |
| | |
| the oral papillae. They
| |
| | |
| | |
| | |
| grow into the supra-oesophageal ganglia. The
| |
| antennae, oral cavity, and oral papilhe are also
| |
| | |
| are chiefly remarkable for shewn.
| |
| | |
| containing the ducts of the slime glands which open at their
| |
| bases. They are without claws. The succeeding appendages
| |
| become eventually imperfectly five-jointed ; two claws are
| |
| | |
| | |
| | |
| TRACHEATA.
| |
| | |
| | |
| | |
| 385
| |
| | |
| | |
| | |
| formed as cuticular investments of papillae in pockets of the
| |
| skin at the ends of their terminal joints.
| |
| | |
| I have been able to make a few observations on the internal structure of
| |
| the embryos from specimens supplied to me by Moseley. These are so far
| |
| confined to a few stages, one slightly earlier, the others slightly later, than
| |
| the embryo represented in fig. 168 B. The epiblast is formed of a layer of
| |
| columnar cells, two deep on the ventral surface, except along the median line
| |
| where there is a well-marked groove and the epiblast is much thinner (fig. 172).
| |
| | |
| The ventral cords of the trunk are formed as two independent epiblastic
| |
| thickenings. In my earlier stage these are barely separated from the
| |
| epiblast, but in the later ones are quite independent (fig. 172, v.n), and
| |
| partly surrounded by mesoblast.
| |
| | |
| The supra-cesophageal ganglia are formed as thickenings of the epiblast
| |
| of the ventral side of the procephalic lobes in front of the stomodaeum.
| |
| They are shewn at / in fig. 170. The thickenings of the two sides are at
| |
| first independent. At a somewhat later period an invagination of the
| |
| epiblast grows into each of these lobes. The openings of these invaginations
| |
| extend from the oral cavity forwards; and they are shewn in fig. 171 l .
| |
| Their openings become closed, and the walls of the invaginations constitute
| |
| a large part of the embryonic supra-cesophageal ganglia.
| |
| | |
| Similar epiblastic invaginations assist in forming the supra-cesophageal
| |
| ganglia of other Tracheata.
| |
| They are described in the sequel
| |
| for Insects, Spiders and Scorpions. The position of the supracesophageal ganglia on the ventral side of the procephalic lobes
| |
| is the same as that in other
| |
| Tracheata.
| |
| | |
| The mesoblast is formed, in
| |
| the earliest of my embryos, of
| |
| scattered cells in the fairly wide
| |
| space between the mesenteron
| |
| and the epiblast. There are two
| |
| distinct bands of mesoblast on
| |
| the outer sides of the nervous
| |
| cords. In the later stage the
| |
| mesoblast is divided into distinct somatic and splanchnic layers, both very thin ; but the two
| |
| layers are connected by transverse strands (fig. 172). There
| |
| | |
| | |
| | |
| sp.w
| |
| | |
| | |
| | |
| $.m
| |
| | |
| | |
| | |
| | |
| FIG. 172. SECTION THROUGH THE TRUNK
| |
| OF AN EMBRYO OF PERIPATUS. The embryo
| |
| from which the section is taken was somewhat
| |
| younger than fig. 171.
| |
| | |
| sp.m. splanchnic mesoblast.
| |
| | |
| s.m. somatic mesoblast.
| |
| | |
| me. median section of body cavity.
| |
| | |
| k. lateral section of body cavity.
| |
| | |
| v.n. ventral nerve cord.
| |
| | |
| me. mesenteron.
| |
| | |
| | |
| | |
| 1 This figure is taken from Moseley. The epiblastic invaginations are represented
| |
| in it very accurately, and though not mentioned in the text of the paper, Moseley
| |
| informs me that he has long been aware of the homology of these folds with those in
| |
| various other Tracheata.
| |
| | |
| | |
| | |
| B. II.
| |
| | |
| | |
| | |
| 2 5
| |
| | |
| | |
| | |
| 386 PROTOTRACHEATA.
| |
| | |
| | |
| | |
| are two special longitudinal septa dividing the body cavity into three
| |
| compartments, a median (me), containing the mesenteron, and two lateral
| |
| (Ic) containing the nerve cords. This division of the body cavity persists,
| |
| as I have elsewhere shewn, in the adult. A similar division is found in
| |
| some Chaetopoda, e.g. Polygordius.
| |
| | |
| I failed to make out that the mesoblast was divided into somites, and
| |
| feel fairly confident that it is not so in the stages I have investigated.
| |
| | |
| There is a section of the body cavity in the limbs as in embryo Myriapods, Spiders, etc.
| |
| | |
| In the procephalic lobe there is a well-developed section of the body
| |
| cavity, which lies dorsal to and in front of the rudiment of the supracesophageal ganglia.
| |
| | |
| The alimentary tract is formed of a mesenteron (fig. 172), a stomodaeum, and proctodaeum. The wall of the mesenteron is formed, in the
| |
| stages investigated by me, of a single layer of cells with yolk particles,
| |
| and encloses a lumen free from yolk. The forward extension of the
| |
| mesenteron is remarkable.
| |
| | |
| The stomodaeum in the earlier stage is a simple pit, which meets but does
| |
| not open into the mesenteron. In the later stage the external opening of
| |
| the pit is complicated by the structures already described. The proctodaeum is a moderately deep pit near the hinder end of the body.
| |
| | |
| The existence of a tracheal system 1 is in itself almost sufficient to
| |
| demonstrate the affinities of Peripatus with the Tracheata, in spite of the
| |
| presence of nephridia. The embryological characters of the procephalic
| |
| lobes, of the limbs and claws, place however this conclusion beyond
| |
| the reach of scepticism. If the reader will compare the figure of Peripatus
| |
| with that of an embryo Scorpion (fig. 196 A) or Spider (fig. 200 C) or better
| |
| still with Metschnikoffs figure of Geophilus (No. 399) PI. xxi. fig. u,he
| |
| will be satisfied on this point.
| |
| | |
| The homologies of the anterior appendages are not very easy
| |
| to determine ; but since there does not appear to me to be sufficient evidence to shew that any of the anterior appendages have
| |
| become aborted, the first post-oral appendages embedded in the
| |
| lips may provisionally be regarded as equivalent to the mandibles,
| |
| and the oral papillae to the first pair of maxillae, etc. Moseley is
| |
| somewhat doubtful about the homologies of the appendages,
| |
| and hesitates between considering the oral papillae as equivalent
| |
| to the second pair of maxillae (on account of their containing the
| |
| openings of the mucous glands, which he compares with the
| |
| spinning glands of caterpillars), or to the poison claws (fourth
| |
| | |
| 1 The specimens shewing tracheae which Moseley has placed in my hands are
| |
| quite sufficient to leave no doubt whatever in my mind as to the general accuracy of
| |
| his description of the tracheal system.
| |
| | |
| | |
| | |
| TRACHEATA. 387
| |
| | |
| | |
| | |
| post-oral appendages) of the Chilopoda (on account of the
| |
| poison-glands which he thinks may be homologous with the
| |
| mucous glands).
| |
| | |
| The arguments for either of these views do not appear to me conclusive. There are glands opening into various anterior appendages in
| |
| the Tracheata, such as the poison glands in the Chelicerae (mandibles) of
| |
| Spiders, and there is some evidence in Insects for the existence of a gland
| |
| belonging to the first pair of maxillae, which might be compared with the
| |
| mucous gland of Peripatus. For reasons already stated I do not regard
| |
| the processes of the cephalic lobes, which form the lips, as a pair of true
| |
| appendages.
| |
| | |
| BIBLIOGRAPHY.
| |
| | |
| (396) H. N. Moseley. "On the Structure and Development of Peripatus
| |
| capensis." Phil. Trans. Vol. 164, 1874.
| |
| | |
| MYRIAPODA 1 .
| |
| | |
| Chilognatha. The first stages in the development of the
| |
| Chilognatha have been investigated by Metschnikoffand Stecker,
| |
| but their accounts are so contradictory as hardly to admit of
| |
| reconciliation.
| |
| | |
| According to Metschnikoff, by whom the following four
| |
| species have been investigated, viz., Strongylosoma Guerinii,
| |
| Polydesmus complanatus, Polyxenus lagurus, and Julus Moneletei, the segmentation is at first regular and complete, but,
| |
| when the segments are still fairly large, the regular segmentation
| |
| is supplemented by the appearance of a number of small cells at
| |
| various points on the surface, which in time give rise to a
| |
| continuous blastoderm.
| |
| | |
| The blastoderm becomes thickened on the ventral surface,
| |
| and so forms a ventral plate 2 .
| |
| | |
| 1 The classification of the Myriapoda employed in the present section is
| |
| | |
| I. Chilognatha. (Millipedes.)
| |
| II. Chilopoda. (Centipedes.)
| |
| | |
| 2 Stecker's (No. 400) observations were made on the eggs of Julus fasciatus, Julus
| |
| fcetidus, Craspedosoma marmoratum, Polydesmus complanatus, and Strongylosoma
| |
| pallipes, and though carried on by means of sections, still leave some points very
| |
| obscure, and do not appear to me deserving of much confidence. The two species of
| |
| Julus and Craspedosoma undergo, according to Stecker, a nearly identical development. The egg before segmentation is constituted of two substances, a central protoplasmic, and a peripheral deutoplastic. It first divides into two equal segments, and
| |
| coincidentally with their formation part of the central protoplasm travels to the
| |
| | |
| 252
| |
| | |
| | |
| | |
| 388 CHILOGNATHA.
| |
| | |
| | |
| | |
| | |
| | |
| FIG. 173. THREE STAGES IN THE DEVELOPMENT OF STRONGYLOSOMA GUERINII.
| |
| (After Metschnikoff.)
| |
| | |
| A. Embryo on eleventh day with commencing ventral flexure (*).
| |
| | |
| B. Embryo with three pairs of post-oral appendages.
| |
| | |
| C. Embryo with five pairs of post-oral appendages.
| |
| | |
| gs. ventral plate; at. antenme; 15 post-oral appendages; x. point of flexure of
| |
| the ventral plate.
| |
| | |
| surface as two clear fluid segments. The ovum is thus composed of two yolk segments
| |
| to two protoplasmic segments. The two former next divide into four, with the production of two fresh protoplasmic segments. The four protoplasmic segments now
| |
| constitute the upper or animal pole of the egg, and occupy the position of the future
| |
| ventral plate. The yolk segments form the lower pole, which is however dorsal in
| |
| relation to the future animal. The protoplasmic segments increase in number by a
| |
| regular division, and arrange themselves in three rows, of which the two outermost
| |
| rapidly grow over the yolk segments. A large segmentation cavity is stated to be
| |
| present in the interior of the ovum.
| |
| | |
| It would appear from Stecker's description that the yolk segments (hypoblast)
| |
| next become regularly invaginated, so as to enclose a gastric cavity, opening externally
| |
| by a blastopore; but it is difficult to believe that a typical gastrula, such as that
| |
| represented by Sleeker, really comes into the cycle of development of the Chilognatha.
| |
| | |
| The mesoblast is stated to be derived mainly from the epiblast. This layer in the
| |
| region of the future ventral plate becomes reduced to two rows of cells, and the inner
| |
| of these by the division of its constituent elements gives rise to the mesoblast. The
| |
| development of Polydesmus and Strongylosoma is not very different from that of Julus.
| |
| The protoplasm at the upper pole occupies from the first a superficial position.
| |
| Segmentation commences at the lower pole, where the food yolk is mainly present !
| |
| The gastrula is stated to be similar to that of Julus, The mesoblast is formed in
| |
| Polydesmus as a layer of cells split off from the epiblast, but in Strongylosoma as an
| |
| outgrowth from the lips of the blastopore. Stecker, in spite of the statements in his
| |
| paper as to the origin of the mesoblast from the epiblast, sums up at the end to the
| |
| effect that both the primary layers have a share in the formation of the mesoblast,
| |
| which originates by a process of endogenous cell-division !
| |
| | |
| It may be noted that the closure of the blastopore takes place, according to
| |
| Stecker, on the dorsal side of the embryo.
| |
| | |
| | |
| | |
| TRACHEATA. 389
| |
| | |
| | |
| | |
| The most important sources of information for the general
| |
| embryology of the Chilognatha are the papers of Newport (No.
| |
| 397) and Metschnikoff (No. 398). The development of Strongylosoma may be taken as fairly typical for the group ; and the
| |
| subsequent statements, unless the reverse is stated, apply to the
| |
| species of Strongylosoma investigated by Metschnikoff.
| |
| | |
| After the segmentation and formation of the layers the first
| |
| observable structure is a transverse furrow in the thickening of
| |
| the epiblast on the ventral surface of the embryo. This furrow
| |
| rapidly deepens, and gives rise to a ventral flexure of the embryo
| |
| (fig. 173 A, x\ which is much later in making its appearance in
| |
| Julus than in Strongylosoma and Polyxenus. A pair of appendages, which become the antennae, makes its appearance
| |
| shortly after the formation of the transverse furrow, and there
| |
| soon follow in order the next three pairs of appendages. All
| |
| these parts are formed in the infolded portion of the ventral
| |
| thickening of the blastoderm (fig. 173 B). The ventral thickening has in the meantime become marked by a longitudinal
| |
| furrow, but whether this is connected with the formation of
| |
| the nervous system, or is equivalent to the mesoblastic furrow in
| |
| Insects, and connected with the formation of the mesoblast, has
| |
| not been made out. Shortly after the appearance of the three
| |
| pairs of appendages behind the antennae two further pairs become
| |
| added, and at the same time oral and anal invaginations become
| |
| formed '(fig- 173 Q. In front of the oral opening an unpaired
| |
| upper lip is developed. The prse-oral part of the ventral plate
| |
| develops into the bilobed procephalic lobes, the epiblast of
| |
| which is mainly employed in the formation of the supra-cesophageal ganglia. The next important change which takes place is
| |
| the segmentation of the body of the embryo (fig. 174 A), the
| |
| most essential feature in which is the division of the mesoblast
| |
| into somites. Segments are formed in order from before backwards, and soon extend to the region behind the appendages.
| |
| On the appearance of segmentation the appendages commence
| |
| to assume their permanent form. The two anterior pairs of
| |
| post-oral appendages become jaws ; and the part of the embryo
| |
| which carries them and the antennae is marked off from the
| |
| trunk as the head. The three following pairs of appendages
| |
| grow in length and assume a form suited for locomotion. Behind
| |
| | |
| | |
| | |
| 390 CHILOGNATHA.
| |
| | |
| | |
| | |
| the three existing pairs of limbs there are developed three fresh
| |
| pairs, of whicJi tJie two anterior belong to a single primitive segment. While the above changes take place in the appendages
| |
| the embryo undergoes an ecdysis, which gives rise to a cuticular
| |
| membrane within the single egg-membrane (chorion, Metschnikoff\ On this cuticle a tooth-like process is developed, the
| |
| function of which is to assist in the hatching of the embryo
| |
| (fig. 174 A).
| |
| | |
| In Polyxenus a cuticular membrane is present as in Strongylosoma,
| |
| but it is not provided with a tooth-like process. In the same form amoeboid
| |
| cells separate themselves from the blastoderm at an early period. These
| |
| cells have been compared to the embryonic envelopes of Insects described
| |
| below.
| |
| | |
| In Julus two cuticular membranes are present at the time of hatching :
| |
| the inner one is very strongly developed and encloses the embryo after
| |
| hatching. After leaving the chorion the embryo Julus remains connected
| |
| with it by a structureless membrane which is probably the outer of the two
| |
| cuticular membranes.
| |
| | |
| At the time when the embryo of Strongylosoma is hatched
| |
| (fig. 174 B) nine post-cephalic segments appear to be present.
| |
| | |
| | |
| | |
| | |
| | |
| FlG. 174. TWO STAGES IN THE DEVELOPMENT OF STRONGYLOSOMA GUEKINll.
| |
| | |
| (After Metschnikoff.)
| |
| | |
| A. A seventeen days' embryo, already segmented.
| |
| | |
| B. A just-hatched larva.
| |
| | |
| Of these segments the second is apparently (from MetschnikofT's
| |
| figure, 174 B) without a pair of appendages; the third and
| |
| | |
| | |
| | |
| TRACHEATA. 391
| |
| | |
| | |
| | |
| fourth are each provided with a single functional pair of limbs ;
| |
| the fifth segment is provided with two pairs of rudimentary
| |
| limbs, which are involuted in a single sack and not visible without preparation, and therefore not shewn in the figure. The
| |
| sixth segment is provided with but a single pair of" appendages,
| |
| though a second pair is subsequently developed on it 1 .
| |
| | |
| Julus, at the time it leaves the chorion, is imperfectly segmented, but is
| |
| provided with antennas, mandibles, and maxillae, and seven pairs of limbs,
| |
| of which the first three are much more developed than the remainder.
| |
| Segmentation soon makes its appearance, and the head becomes distinct
| |
| from the trunk, and on each of the three anterior trunk segments a single
| |
| pair of limbs is very conspicuous (Metschnikoff) 2 . Each of the succeeding
| |
| segments bears eventually two pairs of appendages. At the time when
| |
| the inner embryonic cuticle is cast off, the larva appears to be hexapodous,
| |
| like the young Strongylosoma, but there are in reality four pairs of rudimentary appendages behind the three functional pairs. The latter only
| |
| appear on the surface after the first post-embryonic ecdysis. Pauropus
| |
| (Lubbock) is hexapodous in a young stage. At the next moult two pairs
| |
| of appendages are added, and subsequently one pair at each moult.
| |
| | |
| There appear to be eight post-oral segments in Julus at the
| |
| time of hatching. According to Newport fresh segments are
| |
| added in post-embryonic life by successive budding from a
| |
| blastema between the penultimate segment and that in front of
| |
| it. They arise in batches of six at the successive ecdyses, till
| |
| the full number is completed. A functional, though not a real
| |
| hexapodous condition, appears to be characteristic of Chilognatha
| |
| generally at the time of hatching.
| |
| | |
| The most interesting anatomical feature of the Chilognatha
| |
| is the double character of their segments, the feet (except the
| |
| first three or four, or more), the circulatory, the respiratory, and
| |
| the nervous systems shewing this peculiarity. Newport's and
| |
| | |
| 1 Though the superficially hexapodous larva of Strongylosoma and other Chilognatha has a striking resemblance to some larval Insects, no real comparison is possible between them, even on the assumption that the three functional appendages of
| |
| both are homologous, because Embryology clearly proves that the hexapodous Insect
| |
| type has originated from an ancestor with numerous appendages by the atrophy of
| |
| those appendages, and not from an hexapodous larval form prior to the development
| |
| of the full number of adult appendages.
| |
| | |
| 2 Newport states however that a pair of limbs is present on the first, second, and
| |
| fourth post-oral segments, but that the third segment is apodous ; and this is undoubtedly the case in the adult.
| |
| | |
| | |
| | |
| 392
| |
| | |
| | |
| | |
| CHILOPODA.
| |
| | |
| | |
| | |
| Metschnikoff's observations have not thrown as much light on
| |
| the nature of the double segments as might have been hoped,
| |
| but it appears probable that they have not originated from a
| |
| fusion of two primitively distinct segments, but from a later
| |
| imperfect division of each of the primitive segments into two,
| |
| and the supply to each of the divisions of a primitive segment of
| |
| a complete set of organs.
| |
| | |
| Chilopoda. Up to the present time the development of only
| |
| one type of Chilopoda, viz. that of Geophilus, has been worked
| |
| out. Most forms lay their eggs, but Scolopendra is viviparous.
| |
| | |
| | |
| | |
| | |
| a u . i
| |
| | |
| | |
| | |
| FlG. 175. TWO STAGES IN THE DEVELOPMENT OF GEOPHILUS.
| |
| | |
| (After Metschnikoff.)
| |
| | |
| A. Side-view of embryo at the stage when the segments are beginning to be formed.
| |
| | |
| B. Later stage after the appendages have become established.
| |
| | |
| at. antenna.-; an.t. proctodseum.
| |
| | |
| The segmentation appears to resemble that in the Chilognatha,
| |
| and at its close there is present a blastoderm surrounding a
| |
| central mass of yolk cells. A ventral thickening of the blastoderm is soon formed. It becomes divided into numerous segments, which continue to be formed successively from the
| |
| posterior unsegmented part. The antennae are the first appendages to appear, and are well developed when eighteen segments
| |
| have become visible (fig. 175 A). The post-oral appendages
| |
| are formed slightly later, and in order from before backwards.
| |
| As the embryo grows in length, and fresh segments continue to
| |
| be formed, the posterior part of it becomes bent over so as to
| |
| face the ventral surface of the anterior, and it acquires an
| |
| | |
| | |
| | |
| TRACHEATA. 393
| |
| | |
| | |
| | |
| appearance something like that of many embryo Crustaceans
| |
| (fig. 175 B). Between forty and fifty segments are formed while
| |
| the embryo is still in the egg. The appendages long remain
| |
| unjointed. The fourth post-oral appendage, which becomes the
| |
| poison-claw, is early marked out by its greater size : on the
| |
| third post-oral there is formed a temporary spine to open the
| |
| egg membrane.
| |
| | |
| It does not appear, from Metschnikoff's figures of Geophilus, that any
| |
| of the anterior segments are without appendages, and it is very probable
| |
| that Newport is mistaken in supposing that the embryo has a segment without appendages behind that with the poison claws, which coalesces with the
| |
| segment of the latter. It also appears to me rather doubtful whether the
| |
| third pair of post-oral appendages, i.e. those in front of the poison-claws, can
| |
| fairly be considered as forming part of the basilar plate. The basilar plate
| |
| is really the segment of the poison-claws, and may fuse more or less completely with the segment in front and behind it, and the latter is sometimes
| |
| without a pair of appendages (Lithobius, Scutigera).
| |
| | |
| Geophilus, at the time of birth, has a rounded form like that
| |
| of the Chilognatha.
| |
| | |
| The young of Lithobius is born with only six pairs of limbs.
| |
| | |
| General observation on the homologies of the appendages of
| |
| Myriapoda.
| |
| | |
| The chief difficulty in this connection is the homology of the third pair of
| |
| post-oral appendages.
| |
| | |
| In adult Chilognatha there is present behind the mandibles a four-lobed
| |
| plate, which is usually regarded as representing two pairs of appendages,
| |
| viz. the first and second pairs of maxillae of Insects. Metschnikoff's observations seem however to shew that this plate represents but a single
| |
| pair of appendages, which clearly corresponds with the first pair of maxillae
| |
| in Insects. The pair of appendages behind this plate is ambulatory, but
| |
| turned towards the head ; it is in the embryo the foremost of the three
| |
| functional pairs of legs with which the larva is born. Is it equivalent to
| |
| the second pair of maxillae of Insects or to the first pair of limbs of Insects?
| |
| In favour of the former view is the fact (i) that in embryo Insects the
| |
| second pair of maxillae sometimes resembles the limbs rather than the
| |
| jaws, so that it might be supposed that in Chilognatha a primitive
| |
| ambulatory condition of the third pair of appendages has been retained ;
| |
| (2) that the disappearance of a pair of appendages would have to be
| |
| postulated if the second alternative is adopted, and that if Insects are
| |
| descended from forms related to the Myriapods it is surprising to find a
| |
| pair of appendages always present in the former, absent in the latter.
| |
| | |
| | |
| | |
| 394
| |
| | |
| | |
| | |
| MYRIAPODA.
| |
| | |
| | |
| | |
| The arguments which can be urged for the opposite view do not appear
| |
| to me to have much weight, so that the homology of the appendages in
| |
| question with the second pair of maxillae may be provisionally assumed.
| |
| | |
| The third pair of post-oral appendages of the Chilopoda may probably
| |
| also be assumed to be equivalent to the second pair of maxillae, though they
| |
| are limb-like and not connected with the head. The subjoined table shews
| |
| the probable homologies of the appendages.
| |
| | |
| | |
| | |
| | |
| | |
| CHILOGNATHA(Strongylo
| |
| so ma at time of birth).
| |
| | |
| | |
| CHILOPODA (Scolopendra
| |
| adult).
| |
| | |
| | |
| Pre-oral region.
| |
| | |
| | |
| Antennae.
| |
| | |
| | |
| Antennas.
| |
| | |
| | |
| ist Post-oral segment.
| |
| | |
| | |
| Mandibles.
| |
| | |
| | |
| Mandibles.
| |
| | |
| | |
| 2nd ,, ,,
| |
| | |
| | |
| Maxillae i. (Four-lobed
| |
| plate in adult, but a simple pair of appendages
| |
| in embryo).
| |
| | |
| | |
| Maxillie i.
| |
| (Palp and bilobed median
| |
| process).
| |
| | |
| | |
| 3rd
| |
| (probably equivalent to
| |
| segment bearing 2nd pair
| |
| of maxillae in Insects).
| |
| | |
| | |
| ist pair of ambulatory
| |
| limbs.
| |
| | |
| | |
| Limb-like appendages with
| |
| basal parts in contact.
| |
| | |
| | |
| 4th ,, ,,
| |
| | |
| | |
| (?) Apodous.
| |
| | |
| | |
| Poison claws.
| |
| | |
| | |
| 5th
| |
| | |
| | |
| 2nd pair of ambulatory
| |
| limbs.
| |
| | |
| | |
| ist pair of ambulatory
| |
| limbs.
| |
| | |
| | |
| 6th
| |
| | |
| | |
| 3rd ,,
| |
| | |
| | |
| 2nd ,,
| |
| | |
| | |
| 7th
| |
| | |
| | |
| 4th and sth
| |
| (rudimentary. )
| |
| | |
| | |
| 3rd
| |
| | |
| 8th ,, ,,
| |
| | |
| | |
| 6th
| |
| (the 7th pair is developed
| |
| in this segment later).
| |
| | |
| | |
| 4th
| |
| | |
| | |
| 9 th
| |
| | |
| | |
| Apodous.
| |
| | |
| | |
| 5th
| |
| | |
| | |
| loth ,,
| |
| | |
| | |
| ,, (last segment in
| |
| embryo).
| |
| | |
| | |
| 6th
| |
| | |
| | |
| | |
| The germinal layers and formation of organs.
| |
| | |
| The development of the organs of the Myriapoda, and the origin of the
| |
| germinal layers, are very imperfectly known : Myriapoda appear however
| |
| to be closely similar to Insects in this part of their development, and the
| |
| general question of the layers will be treated more fully in connection with
| |
| that group.
| |
| | |
| The greater part of the blastoderm gives rise to the epiblast, which
| |
| furnishes the skin, nervous system, tracheal system, and the stomodacum
| |
| and proctodaeum.
| |
| | |
| | |
| | |
| TRACHEATA. 395
| |
| | |
| | |
| | |
| The mesoblast arises in connection with the ventral thickening of the
| |
| blastoderm, but the details of its formation are not known. Metschnikoff
| |
| describes a longitudinal furrow which appears very early in Strongylosoma,
| |
| which is perhaps equivalent to the mesoblastic furrows of Insects, and so
| |
| connected with the formation of the mesoblast.
| |
| | |
| The mesoblast is divided up into a series of protovertebra-like bodies
| |
| the mesoblastic somites the cavities of which become the body cavity and
| |
| the walls the muscles and probably the heart. They are (Metschnikoff)
| |
| prolonged into the legs, though the prolongations become subsequently
| |
| segmented off from the main masses. The splanchnic mesoblast is,
| |
| according to Metschnikoff, formed independently of the somites, but this
| |
| point requires further observation.
| |
| | |
| The origin of the hypoblast remains uncertain, but it appears probable
| |
| that it originates, in a large measure at least, from the yolk segments. In
| |
| the Chilognatha the mesenteron is formed in the interior of the yolk segments, so that those yolk segments which are not employed in the formation
| |
| of the alimentary canal lie freely in the body cavity. In the relation of
| |
| the yolk segments to the alimentary canal the Chilopoda present a strong
| |
| contrast to the Chilognatha, in that the greater part of the yolk lies
| |
| within their mesenteron. The mesenteron is at first a closed sack, but is
| |
| eventually placed in communication with the stomodaeum and the proctodasum. The Malpighian bodies arise as outgrowths from the blind extremity of the latter.
| |
| | |
| BIBLIOGRAPHY.
| |
| | |
| (397) G. N e wp or t. " On the Organs of Reproduction and Development of the
| |
| Myriapoda." Philosophical Transactions, 1841.
| |
| | |
| (398) E. Metschnikoff. ' ' Embryologie der doppeltflissigen Myriapoden (Chilognatha)." Zeit.f. wiss. Zool., Vol. xxiv. 1874.
| |
| | |
| (399) ' ' Embryologisches iiber Geophilus." Zeit. f. wiss. ZooL y Vol. xxv.
| |
| | |
| 1875
| |
| (400) Anton Stecker. "Die Anlage d. Keimblatter bei den Diplopoden."
| |
| Archivf. mik. Anatomie, Bd. xiv. 1877.
| |
| | |
| INSECTA 1 .
| |
| | |
| The formation of the embryonic layers in Insects has not
| |
| been followed out in detail in a large number of types ; but, as
| |
| | |
| 1 The following classification of the Insecta is employed in this chapter,
| |
| ((i) Collembola.
| |
| | |
| I. Aptera. | (a) Thysanura .
| |
| | |
| !(i) Orthoptera genuina (Blatta, Locusta, etc.).
| |
| (2) pseudoneuroptera (Termes, Ephemera,
| |
| | |
| Libellula).
| |
| | |
| !(i) Hemiptera heteroptera (Cimex, Notonecta, etc.).
| |
| (2) ,, homoptera (Aphis, Cicada, etc.).
| |
| | |
| (3) ,, parasita (Pediculus, etc.).
| |
| | |
| | |
| | |
| 396
| |
| | |
| | |
| | |
| INSECTA.
| |
| | |
| | |
| | |
| in so many other instances, some of the most complete histories
| |
| we have are due to Kowalevsky (No. 416). The development
| |
| | |
| | |
| | |
| | |
| FiG. 176. FOUR EMBRYOS OF llYDROPHlLUS P1CEUS VIEWED FROM THE
| |
| | |
| VENTRAL SURFACE. (After Kowalevsky.)
| |
| The upper end is the anterior, gg. germinal groove; am. amnion.
| |
| | |
| of Hydrophilus has been worked out by him more fully than
| |
| that of any other form, and will serve as a type for comparison
| |
| with other forms.
| |
| | |
| The segmentation has not been studied, but no doubt belongs
| |
| to the centrolecithal type (vide pp. no 120). At its close
| |
| there is an uniform layer of cells enclosing a central mass of
| |
| yolk. These cells, in the earliest observed stage, were flat on
| |
| the dorsal, but columnar on part of the ventral surface of the
| |
| egg, where they form a thickening which will be called the ventral plate. At the posterior part of the ventral plate two folds,
| |
| with a furrow between them, make their appearance. They form
| |
| a structure which may be spoken of as the germinal groove (fig.
| |
| | |
| !(i) Diptera genuina (Musca, Tipula, etc.).
| |
| (2) aphaniptera (Pulex, etc.).
| |
| (3) ,, pupipara (Braula, etc.).
| |
| | |
| v .. ( (i) Neuroptera planipennia (Myrniclcon, etc.)
| |
| TOptera. j (a) ^ trichoptera (Phryganea, etc.).
| |
| | |
| VI. Coleoptera.
| |
| VII. Lepidoptera.
| |
| | |
| (i) Hymenoptera aculeata (Apis, Formica, etc.).
| |
| (a) ,, entomophaga (Ichneumon, Platy
| |
| gaster, etc).
| |
| (3) ,, phytophaga ( Tenthredo, Sirex, etc.).
| |
| | |
| | |
| | |
| VIII. Hymenoptera.
| |
| | |
| | |
| | |
| TRACHEATA.
| |
| | |
| | |
| | |
| 397
| |
| | |
| | |
| | |
| | |
| y*
| |
| | |
| | |
| | |
| FlG. 177. TWO TRANSVERSE SECTIONS THROUGH
| |
| | |
| EMBRYOS OF HvDROPHiLUS piCEUS. (After Kowalevsky.)
| |
| | |
| A. Section through an embryo of the stage represented in fig. 176 B, at the point where the two
| |
| germinal folds most approximate.
| |
| | |
| B. Section through an embryo somewhat later
| |
| than the stage fig. 176 D, through the anterior region
| |
| where the amnion has not completely closed over the
| |
| embryo.
| |
| | |
| | |
| | |
| ). The cells
| |
| which form the floor
| |
| of the groove are far
| |
| more columnar than
| |
| those of other parts
| |
| of the blastoderm (fig.
| |
| 177 A). The two
| |
| folds on each side of
| |
| it gradually approach
| |
| each other. They do
| |
| so at first behind, and
| |
| then in the middle;
| |
| from the latter point
| |
| the approximation
| |
| gradually extends
| |
| backwards and forwards (fig. 176 B and
| |
| C). In the middle
| |
| and hinder parts of
| |
| the ventral plate the
| |
| groove becomes, by
| |
| the coalescence of the folds, converted into a canal (fig. 178 A,
| |
| gg), the central cavity of which soon disappears, while at the
| |
| same time the cells of the wall undergo division, become more
| |
| rounded, and form a definite layer (me} the mesoblast beneath
| |
| the columnar cells of the surface. Anteriorly the process is
| |
| slightly different, though it leads to the similar formation of
| |
| mesoblast (fig. 177 B). The flat floor of the groove becomes in
| |
| front bodily converted into the mesoblast, but the groove itself
| |
| is never converted into a canal. The two folds simply meet
| |
| above, and form a continuous superficial layer.
| |
| | |
| During the later stages of the process last described remarkable structures, eminently characteristic of the Insecta, have
| |
| made their first appearance. These structures are certain
| |
| embryonic membranes or coverings, which present in their mode
| |
| of formation and arrangement a startling similarity to the true
| |
| and false amnion of the Vertebrata. They appear as a double
| |
| fold of the blastoderm round the edge of the germinal area,
| |
| which spreads over the ventral plate, from behind forwards, in a
| |
| | |
| | |
| | |
| gg. germinal groove ;
| |
| nion ; yk. yolk.
| |
| | |
| | |
| | |
| me. mesoblast ; am. am
| |
| | |
| | |
| INSECT A.
| |
| | |
| | |
| | |
| | |
| general way in the same
| |
| manner as the amnion in,
| |
| for instance, the chick.
| |
| The folds at their origin
| |
| are shewn in surface view
| |
| in fig. 176 D, am, and in
| |
| section in fig. 177 B, am.
| |
| The folds eventually
| |
| meet, coalesce (fig. 178,
| |
| am) and give rise to two
| |
| membranes covering the
| |
| ventral plate, viz. an
| |
| inner one, which is continuous with the edge of
| |
| the ventral plate ; and
| |
| an outer, continuous with
| |
| the remainder of the
| |
| blastoderm. The vertebrate nomenclature may
| |
| be conveniently employed for these membranes.
| |
| The inner limb of the
| |
| fold will therefore be spoken of as the amnion, and the outer
| |
| one, including the dorsal part of the blastoderm, as the
| |
| serous envelope 1 . A slight consideration of the mode of
| |
| formation of the membranes, or an inspection of the figures
| |
| illustrating their formation, makes it at once clear that the yolk
| |
| can pass in freely between the amnion and serous envelope (vide
| |
| fig. 181). At the hind end of the embryo this actually takes
| |
| place, so that the ventral plate covered by the amnion appears to
| |
| become completely imbedded in the yolk: elsewhere the two
| |
| membranes are in contact. At first (fig. 176) the ventral plate
| |
| occupies but a small portion of the ventral surface of the egg, but
| |
| during the changes above described it extends over the whole
| |
| ventral surface, and even slightly on the dorsal surface both in
| |
| front and behind. It becomes at the same time (fig. 179) divided
| |
| | |
| | |
| | |
| FIG. 178. SECTIONS THROUGH TWO EMBRYOS
| |
| OF HYDROPHILUS PICEUS. (After Kowalevsky.)
| |
| | |
| A. Section through the posterior part of the
| |
| embryo fig. 1 76 D, shewing the completely closed
| |
| amnion and the germinal groove.
| |
| | |
| B. Section through an older embryo in which
| |
| the mesoblast has grown out into a continuous
| |
| plate beneath the epiblast.
| |
| | |
| gg. germinal groove ; am. amnion ; yk. yolk ;
| |
| cp. epiblast.
| |
| | |
| | |
| | |
| 1 The reverse nomenclature to this is rather inconveniently employed by Metschnikoff.
| |
| | |
| | |
| | |
| TRACHEATA.
| |
| | |
| | |
| | |
| 399
| |
| | |
| | |
| | |
| | |
| FIG. 179. EMBRYO OF
| |
| HYDROPHILUS PICEUS
| |
| | |
| VIEWED FROM THE VEN
| |
| TRAL SURFACE. (After
| |
| Kowalevsky.)
| |
| | |
| pc.L procephalic lobe.
| |
| | |
| | |
| | |
| by a series of transverse lines into segments, which increase in
| |
| number and finally amount in all to seventeen, not including the most anterior section,
| |
| which gives off as lateral outgrowths the
| |
| two procephalic lobes (pc.l). The changes
| |
| so far described are included within what
| |
| Kowalevsky calls his first embryonic period;
| |
| at its close the parts contained within the
| |
| chorion have the arrangement shewn in fig.
| |
| 178 B. The whole of the body of the
| |
| embryo is formed from the ventral plate,
| |
| and no part from the amnion or serous
| |
| envelope.
| |
| | |
| The general history of the succeeding
| |
| stages may be briefly told.
| |
| | |
| The appendages appear as very small
| |
| rudiments at the close of the last stage, but
| |
| soon become much more prominent (fig.
| |
| 1 80 A). They are formed as outgrowths of both layers, and
| |
| arise nearly simultaneously. There
| |
| are in all eight pairs of appendages.
| |
| The anterior or antennae (at) spring
| |
| from the procephalic lobes, and
| |
| the succeeding appendages from
| |
| the segments following. The last
| |
| pair of embryonic appendages,
| |
| which disappears very early, is
| |
| formed behind the third pair of
| |
| the future thoracic limbs. Paired
| |
| epiblastic involutions, shewn as pits
| |
| in the posterior segments in fig.
| |
| 1 80 A, give rise to the tracheae;
| |
| and the nervous system is formed
| |
| as two lateral epiblastic thickenings, one on each side of the midventral line. These eventually become split off from the skin ; while
| |
| between them there passes in a
| |
| median invagination of the skin
| |
| | |
| | |
| | |
| | |
| FlG. 1 80. TWO STAGES IN THE
| |
| DEVELOPMENT OF HYDROPHILUS
| |
| | |
| PICEUS. (From Gegenbaur, after
| |
| Kowalevsky.)
| |
| | |
| Is. labrum ; at. antenna ; md.
| |
| | |
| | |
| | |
| 400 INSECTA.
| |
| | |
| | |
| | |
| (fig. 189 C). The two nervous strands are continuous in front
| |
| with the supra-oesophageal ganglia, which are formed of the
| |
| epiblast of the procephalic lobes. These plates gradually grow
| |
| round the dorsal side of the embryo, and there is formed
| |
| immediately behind them an oral invagination, in front of which
| |
| there appears an upper lip (fig. 180, Is). A proctodaeum is formed
| |
| at the hind end of the body slightly later than the stomodaeum.
| |
| The mesoblast cells become divided into two bands, one on
| |
| each side of the middle line (fig. 189 A), and split into
| |
| splanchnic and somatic layers. The central yolk mass at about
| |
| the stage represented in fig. 179 begins to break up into
| |
| yolk spheres. The hypoblast is formed first on the ventral
| |
| side at the junction of the mesoblast and the yolk, and
| |
| gradually extends and forms a complete sack-like mesenteron,
| |
| enveloping the yolk (fig. 185 al). The amnion and serous
| |
| membrane retain their primitive constitution for some time, but
| |
| gradually become thinner on the ventral surface, where a rupture
| |
| appears eventually to take place. The greater part of them
| |
| disappears, but in the closure of the dorsal parietes the serous
| |
| envelope plays a peculiar part, which is not yet understood. It
| |
| is described on p. 404. The heart is formed from the mesoblastic layers, where they meet in the middle dorsal line (fig. 185 C,
| |
| hi]. The somatic mesoblast gives rise to the muscles and
| |
| connective tissue, and the splanchnic mesoblast to the muscular
| |
| part of the wall of the alimentary tract, which accompanies the
| |
| hypoblast in its growth round the yolk. The proctodaeum
| |
| forms the rectum and Malpighian bodies 1 , and the stomodseum
| |
| the oesophagus and proventriculus. The two epiblastic sections
| |
| of the alimentary tract are eventually placed in communication
| |
| with the mesenteron.
| |
| | |
| The development of Hydrophilus is a fair type of that of
| |
| Insects generally, but it is necessary to follow with somewhat
| |
| greater detail the comparative history of the various parts which
| |
| have been briefly described for this type.
| |
| | |
| TJte embryonic membranes and the formation of the layers.
| |
| | |
| All Insects have at the close of segmentation a blastoderm
| |
| formed of a single row of cells enclosing a central yolk mass,
| |
| | |
| 1 This has not been shewn in the case of Hydrophilus,
| |
| | |
| | |
| | |
| TRACHEATA.
| |
| | |
| | |
| | |
| 401
| |
| | |
| | |
| | |
| which usually contains nuclei, and in the Poduridae is divided up
| |
| in the ordinary segmentation into distinct yolk cells. The first
| |
| definite structure formed is a thickening of the blastoderm,
| |
| which forms a ventral plate.
| |
| | |
| The ventral plate is very differently situated in relation to the yolk in
| |
| different types. In most Diptera, Hymenoptera and (?) Neuroptera (Phryganea) it forms from the first a thickening extending over nearly the
| |
| whole ventral surface of the ovum, and in many cases extends in its subsequent growth not only over the whole ventral surface, but over a considerable part of the apparent dorsal surface as well (Chironomus, Simulia,
| |
| Gryllotalpa, etc.). In Coleoptera, so far as is known, it commences as a less
| |
| extended thickening either of the central part (Donacia) or posterior part
| |
| (Hydrophilus) of the ventral surface, and gradually grows in both directions,
| |
| passing over to the dorsal surface behind.
| |
| | |
| Embryonic membranes. In the majority of Insects there
| |
| are developed enveloping membranes like those of Hydrophilus.
| |
| | |
| The typical mode of formation of these membranes is represented diagrammatically in fig. 181 A and B. A fold of the
| |
| blastoderm arises round the edge of the ventral plate. This
| |
| fold, like the amniotic fold of the
| |
| higher Vertebrata,
| |
| is formed of two
| |
| limbs, an outer,
| |
| the serous membrane (se), and an
| |
| inner, the true amnion (am). Both
| |
| limbs extend so
| |
| as to cover over
| |
| the ventral plate,
| |
| and finally meet
| |
| and coalesce, so
| |
| thatadouble membrane is present
| |
| over the ventral
| |
| plate. At the same
| |
| time (fig. 181 B)
| |
| the point where the fold originates is carried dorsalwards by the
| |
| B. II. 26
| |
| | |
| | |
| | |
| Sf
| |
| | |
| | |
| | |
| | |
| FIG. 181. DIAGRAMMATIC LONGITUDINAL SECTIONS
| |
| OF AN INSECT EMBRYO AT TWO STAGES TO SHEW THE
| |
| | |
| DEVELOPMENT OF THE EMBRYONIC ENVELOPES.
| |
| | |
| In A the amniotic folds have not quite met so as to
| |
| cover the ventral plate. The yolk is represented as divided
| |
| into yolk cells. In B the sides of the ventral plate have
| |
| extended so as nearly to complete the dorsal integument.
| |
| The mesenteron is represented as a closed sack filled with
| |
| yolk cells, am. amnion; se. serous envelope; v.p. ventral plate ; d. i. dorsal integument ; me. mesenteron ; st.
| |
| stomodaeum ; an i. proctodaeum.
| |
| | |
| | |
| | |
| 4O2 INSECTA.
| |
| | |
| | |
| | |
| dorsal extension of the edges of the ventral plate, which give
| |
| rise to the dorsal integument (d.i). This process continues
| |
| till the whole dorsal surface is covered by the integument.
| |
| The amnion then separates from the dorsal integument, and the
| |
| embryo becomes enveloped in two membranes an inner, the
| |
| amnion, and an outer, the serous membrane. In fig. 181 B the
| |
| embryo is represented at the stage immediately preceding the
| |
| closure of the dorsal surface.
| |
| | |
| By the time that these changes are effected, the serous
| |
| membrane and amnion are both very thin and not easily
| |
| separable. The amnion appears to be usually absorbed before
| |
| hatching; but in hatching both membranes, if present, are either
| |
| absorbed, or else ruptured and thrown off.
| |
| | |
| The above mode of development of the embryonic membranes has been
| |
| especially established by the researches of Kowalevsky (No. 416) and Graber
| |
| (No. 412) for various Hymenoptera (Apis), Diptera (Chironomus\ Lepidoptera and Coleoptera (Melolontha, Lino).
| |
| | |
| Considerable variations in the development of the enveloping membranes
| |
| are known.
| |
| | |
| When the fold which gives rise to the membranes is first formed, there
| |
| is, as is obvious in fig. 181 A, a perfectly free passage by which the yolk can
| |
| pass in between the amnion and serous membrane. Such a passage of the
| |
| yolk between the two membranes takes place posteriorly in Hydrophilus and
| |
| Donacia: in Lepidoptera the yolk passes in everywhere, so that in this form
| |
| the ventral plate becomes first of all imbedded in the yolk, and finally, on the
| |
| completion of the dorsal integument, the embryo is enclosed in a complete
| |
| envelope of yolk contained between the amnion and the serous membrane.
| |
| During the formation of the dorsal integument the external yolk sack communicates by a dorsally situated umbilical canal with the yolk cavity within
| |
| the body. On the rupture of the amnion the embryo is nourished at the
| |
| expense of the yolk contained in the external yolk sack.
| |
| | |
| In the Hemiptera and the Libellulidae the ventral plate also becomes
| |
| imbedded in the yolk, but in a somewhat different fashion to the Lepidoptera, which more resembles on an exaggerated scale what takes place in
| |
| Hydrophilus.
| |
| | |
| In the Libellulidas (Calopteryx) there is first of all formed (Brandt, No.
| |
| 403) a small ventral and posterior thickening of the blastoderm (fig. 182 A).
| |
| The hinder part of this becomes infolded into the yolk as a projection (fig.
| |
| 182 B), which consists of two laminae, an anterior and a posterior, continuous
| |
| at the apex of the invagination. The whole structure, which is completely
| |
| imbedded within the yolk, rapidly grows in length, and turns towards
| |
| the front end of the egg (fig. 182 C). Its anterior lamina remains thick and
| |
| gives rise to the ventral plate (ps), the posterior (am) on the other hand
| |
| | |
| | |
| | |
| TRACHEATA.
| |
| | |
| | |
| | |
| 403
| |
| | |
| | |
| | |
| | |
| becomes very thin, and
| |
| forms a covering corresponding with the amnion
| |
| of the more ordinary types.
| |
| The remainder of the blastoderm covering the yolk
| |
| (se) forms the homologue
| |
| of the serous membrane
| |
| of other types. The ventral surface of the ventral
| |
| plate is turned towards
| |
| the dorsal side (retaining
| |
| the same nomenclature as
| |
| in ordinary cases) of the
| |
| egg, and the cephalic
| |
| extremity is situated at
| |
| the point of origin of the
| |
| infolding.
| |
| | |
| The further history is
| |
| however somewhat peculiar. The amnion is at first
| |
| (fig. 182 C) continuous with
| |
| the serous envelope on the
| |
| posterior side only, so that
| |
| the serous envelope does
| |
| not form a continuous sack,
| |
| but has an opening close
| |
| to the head of the embryo.
| |
| In the Hemiptera parasita this opening (Melnikow, No. 422) remains permanent, and the embryo, after it has reached a certain stage of development,
| |
| becomes everted through it, while the yolk, enclosed in the continuous membrane formed by the amnion and serous envelope, forms a yolk sack on the
| |
| dorsal surface. In the Libellulidae however and most Hemiptera, a fusion of
| |
| the two limbs of the serous membrane takes place in the usual way, so as to
| |
| convert it into a completely closed sack (fig. 183 A). After the formation of
| |
| the appendages a fusion takes place between the amnion and serous envelope over a small area close to the head of the embryo. In the middle of
| |
| this area a rupture is then effected, and the head of the embryo followed by
| |
| the body is gradually pushed through the opening (fig. 183 B and C). The
| |
| embryo becomes in the process completely rotated, and carried into a
| |
| position in the egg-shell identical with that of the embryos of other orders of
| |
| Insects (fig. 183 C).
| |
| | |
| Owing to the rupture of the embryonic envelopes taking place at the
| |
| point where they are fused into one, the yolk does not escape in the above
| |
| process, but is carried into a kind of yolk sack, on the dorsal surface of the
| |
| embryo, formed of the remains of the amnion and serous envelope. The
| |
| | |
| 262
| |
| | |
| | |
| | |
| FIG. 182. THREE STAGES IN THE DEVELOPMENT
| |
| | |
| OF THE EMBRYO OF CALOPTERYX. (After Brandt.)
| |
| | |
| The embryo is represented in the egg-shell.
| |
| | |
| A. Embryo with ventral plate.
| |
| | |
| B. Commencing involution of ventral plate.
| |
| | |
| C. Involution of ventral plate completed.
| |
| | |
| ps. vefitral plate; g. edge of ventral plate; am.
| |
| amnion ; se- serous envelope.
| |
| | |
| | |
| | |
| 404
| |
| | |
| | |
| | |
| INSECTA.
| |
| | |
| | |
| | |
| walls of the yolk sack either
| |
| assist in forming the dorsal
| |
| parietes of the body, or are
| |
| more probably enclosed
| |
| within the body by the
| |
| growth of the dorsal parietes from the edge of the
| |
| ventral plate.
| |
| | |
| In Hydrophilus and
| |
| apparently in the Phryganidae also, there are certain remarkable peculiarities in the closure of the
| |
| dorsal surface. The fullest
| |
| observations on the subject
| |
| have been made by Kowalevsky (No. 416), but Dohrn
| |
| (No. 408) has with some
| |
| probability thrown doubts
| |
| on Kowalevsky's interpretations. According to Dohrn
| |
| the part of the serous envelope which covers the dorsal surface becomes thickened, and gives rise to a
| |
| peculiar dorsal plate which
| |
| is shewn in surface view in
| |
| ventral parts of the amnion
| |
| and serous membrane have
| |
| either been ruptured or
| |
| have disappeared. While
| |
| the dorsal plate is being
| |
| formed, the mesoblast, and
| |
| somewhat later the lateral
| |
| parts of the epiblast of the
| |
| ventral plate gradually
| |
| grow towards the dorsal
| |
| side and enclose the dorsal
| |
| plate, the wall of which in
| |
| the process appears to be
| |
| folded over so as first of
| |
| all to form a groove and
| |
| finally a canal. The stages
| |
| in this growth are shewn
| |
| from the surface in fig. 184
| |
| B and C and in section in
| |
| | |
| | |
| | |
| | |
| FlG. 183. THREE STAGES IN THE DEVELOPMENT
| |
| | |
| OF CALOPTERYX. (After Brandt.)
| |
| | |
| The embryo is represented in the egg-shell; B.
| |
| and C. shew the inversion of the embryo.
| |
| | |
| sf. serous envelope ; am. amnion ; ab. abdomen ;
| |
| v. anterior end of head ; at. antennae ; md. mandible ;
| |
| mx l . maxilla i ; mx*. maxilla 2 ; p 1 ^. three pairs
| |
| of legs; oe. oesophagus.
| |
| | |
| fig. 184 A, doi and in section in fig. 185 A, do. The
| |
| | |
| | |
| | |
| | |
| FIG. 184. THREE LARVAL STAGES OF HYDROPHILUS FROM THE DORSAL SIDE, SHEWING THE
| |
| GRADUAL CLOSING IN OF THE DORSAL REGION WITH
| |
| THE FORMATION < >! THK I'l.CULIAR DORSAL ORGAN
| |
| | |
| do. (After Kowalevsky.)
| |
| | |
| do. dorsal organ ; at. antennae.
| |
| | |
| | |
| | |
| TRACHEATA.
| |
| | |
| | |
| | |
| 405
| |
| | |
| | |
| | |
| fig. 185 B, do. The canal is buried on the dorsal part of the yolk, but for
| |
| some time remains open by a round aperture in front (fig. 184 C). The
| |
| whole structure is known as the dorsal canal. It appears to atrophy without
| |
| leaving a trace. The heart when formed lies immediately dorsal to it 1 .
| |
| | |
| | |
| | |
| | |
| A.
| |
| B.
| |
| C.
| |
| | |
| | |
| | |
| vn
| |
| | |
| | |
| | |
| FIG. 185. THREE TRANSVERSE SECTIONS THROUGH ADVANCED
| |
| | |
| EMBRYOS OF HYDROPHILUS.
| |
| | |
| Section through the posterior part of the body of the same age as fig. 184 A.
| |
| Section through the embryo of the same age as fig. 184 C.
| |
| Section through a still older embryo.
| |
| do. dorsal plate ; vn. ventral nerve cord ; al. mesenteron ; ht. heart.
| |
| The large spaces at the sides are parts of the body cavity.
| |
| | |
| In the Poduridas the embryonic membranes appear to be at any rate
| |
| imperfect. Metschnikoff states in his paper on Geophilus that in some ants
| |
| no true embryonic membranes are found, but merely scattered cells which
| |
| take their place. In the Ichneumonidas the existence of two embryonic
| |
| membranes is very doubtful.
| |
| | |
| Formation of the embryonic layers. The formation of the
| |
| layers has been studied in sections by Kowalevsky (No. 416),
| |
| | |
| 1 According to Kowalevsky the history of the dorsal plate is somewhat different.
| |
| He believes that on the absorption of the amnion the ventral plate unites with the
| |
| serous membrane, and that the latter directly gives rise to the dorsal integument,
| |
| while the thickened part of it becomes involuted to form the dorsal tube already
| |
| described.
| |
| | |
| | |
| | |
| 406 INSECTA.
| |
| | |
| | |
| | |
| Hatschek (No. 414), and Graber (No. 412), etc. From their
| |
| researches it would appear that the formation of the mesoblast
| |
| always takes place in a manner closely resembling that in
| |
| Hydrophilus. The essential features of the process (figs. 177
| |
| and 178) appear to be that a groove is formed along the median
| |
| line of the ventral plate, and that the sides of this groove either
| |
| (i) simply close over like the walls of the medullary groove in
| |
| Vertebrates, and so convert the groove into a tube, which soon
| |
| becomes solid and forms a mass or plate of cells internal to the
| |
| epiblast ; or (2) that the cells on each side of the groove grow
| |
| over it and meet in the middle line, forming a layer external
| |
| to the cells which lined the groove. The former of these
| |
| processes is the most usual ; and in the Muscidae the dimensions
| |
| of the groove are very considerable (Graber, No. 411). In both
| |
| cases the process is fundamentally the same, and causes the
| |
| ventral plate to become divided into two layers 1 . The external
| |
| layer or epiblast is an uniform sheet forming the main part of
| |
| the ventral plate (fig. 178 B, ep). It is continuous at its edge
| |
| with the amnion. The inner layer or mesoblast constitutes an
| |
| independent plate of cells internal to the epiblast (fig. 178 B, me).
| |
| The mesoblast soon becomes divided into two lateral bands.
| |
| | |
| The origin of the hypoblast is still in dispute. It will be
| |
| remembered (vide pp. 1 14 and 1 16) that after the segmentation a
| |
| number of nuclei remain in the yolk ; and that eventually a
| |
| secondary segmentation of the yolk takes place around these
| |
| nuclei, and gives rise to a mass of yolk cells, which fill up the
| |
| interior of the embryo. These cells are diagrammatically shewn
| |
| in figs. 181 and 189, and it is probable that they constitute the
| |
| true hypoblast. Their further history is given below.
| |
| | |
| Formation of the organs and their relation to the germinal
| |
| | |
| layers.
| |
| | |
| The segments and appendages. One of the earliest
| |
| phenomena in the development is the appearance of transverse
| |
| lines indicating segmentation (fig. 186). The transverse lines
| |
| are apparently caused by shallow superficial grooves, and also in
| |
| | |
| 1 Tichomiroff (No. 420) denies the existence of a true invagination to form the
| |
| mesoblast, and also asserts that a separation of mesoblast cells from the epiblast can
| |
| take place at other parts besides the median ventral line.
| |
| | |
| | |
| | |
| TRACHEATA.
| |
| | |
| | |
| | |
| 407
| |
| | |
| | |
| | |
| many cases by the division of the mesoblastic bands into
| |
| separate somites. The most anterior line marks off a prae-oral
| |
| segment, which soon sends out two lateral wings the procephalic
| |
| lobes. The remaining segments are at first fairly uniform.
| |
| Their number does not, however, appear to be very constant.
| |
| So far as is known they never exceed seventeen, and this
| |
| number is probably the typical one (figs. 186 and 187).
| |
| | |
| In Diptera the number appears to be usually fifteen though it may be
| |
| only fourteen. In Lepidoptera and in Apis there appear to be sixteen
| |
| segments. These and other variations affect only the number of the segments
| |
| which form the abdomen of the adult.
| |
| | |
| The appendages arise as paired pouchlike outgrowths of the epiblast and mesoblast ; and their number and the order of
| |
| their appearance are subject to considerable
| |
| variation, the meaning of which is not yet
| |
| clear. As a rule they arise subsequently to
| |
| the segmentation of the parts of the body
| |
| to which they belong. There is always
| |
| formed one pair of appendages which spring
| |
| from the lateral lobes of the procephalic
| |
| region, or from the boundary line between
| |
| these and the median ventral part of this
| |
| region. These appendages are the antennae.
| |
| They have in the embryo a distinctly ventral position as compared to that which
| |
| they have in the adult.
| |
| | |
| In the median ventral part of the procephalic region there arises the labrum (fig. 187, Is}. It is formed
| |
| by the coalescence of a pair of prominences very similar to true
| |
| appendages, though it is probable that they have not this
| |
| value 1 .
| |
| | |
| 1 If these structures are equivalent to appendages, they may correspond to one of
| |
| the pairs of antennae of Crustacea. From a figure by Fritz Miiller of the larva of
| |
| Calotermes (Jenaische Zeit. Vol. XI. pi. n, fig. 12) it would appear that they lie in
| |
| front of the true antennae, and would therefore on the above hypothesis correspond to
| |
| the first pair of antennae of Crustacea. Biitschli (No. 405) describes in the Bee a pair
| |
| of prominences immediately in front of the mandibles which eventually unite to form
| |
| a kind of underlip ; they in some ways resemble true appendages.
| |
| | |
| | |
| | |
| | |
| FIG. 1 86. EMBRYO
| |
| OF HYDROPHILUS PI
| |
| CEUS VIEWED FROM THE
| |
| VENTRAL SURFACE.
| |
| | |
| (After Kowalevsky.)
| |
| pc. I. procephalic lobe.
| |
| | |
| | |
| | |
| 408
| |
| | |
| | |
| | |
| INSECTA.
| |
| | |
| | |
| | |
| The antennae themselves can hardly be considered to have
| |
| the same morphological value as the succeeding appendages.
| |
| They are rather equivalent to paired processes of the prae-oral
| |
| lobes of the Chaetopoda.
| |
| | |
| From the first three post-oral segments there grow out the
| |
| mandibles and two pairs of maxillae, and from the three following
| |
| segments the three pairs of thoracic appendages. In many
| |
| Insects (cf. Hydrophilus) a certain .number of appendages of the
| |
| same nature as the anterior ones are visible in the embryo on
| |
| the abdominal segments, a fact which shews that Insects are
| |
| descended from ancestors with more than three pairs of ambulatory appendages.
| |
| | |
| In Apis according to Biitschli (No. 405) all the abdominal segments are
| |
| provided with appendages, which always
| |
| remain in a very rudimentary condition.
| |
| All trace of them as well as of the thoracic
| |
| appendages is lost by the time the embryo
| |
| is hatched. In the phytophagous Hymenoptera the larva is provided with
| |
| 9 ii pairs of legs.
| |
| | |
| In the embryo of Lepidoptera there
| |
| would appear from Kowalevsky's figures
| |
| to be rudiments of ten pairs of post-thoracic appendages. In the caterpillar of
| |
| this group there are at the maximum five
| |
| pairs of such rudimentary feet, viz. a pair
| |
| on the 3rd, 4th, 5th, and 6th, and on the
| |
| last abdominal segment. The embryos
| |
| of Hydrophilus (fig. 187), Mantis, etc. are
| |
| also provided with additional appendages.
| |
| In various Thysanura small prominences
| |
| are present on more or fewer of the abdominal segments (fig. 192), which may
| |
| probably be regarded as rudimentary
| |
| feet.
| |
| | |
| Whether all or any of the appendages
| |
| of various kinds connected with the
| |
| hindermost segments belong to the same
| |
| category as the legs is very doubtful. Their usual absence in the embryo or
| |
| in any case their late appearance appears to me against so regarding them ;
| |
| but Biitschli is of opinion that in the Bee the parts of the sting are related
| |
| genetically to the appendages of the penultimate and antepenultimate abdominal segments, and this view is to some extent supported by more recent
| |
| | |
| | |
| | |
| | |
| FlG. 187. TWO STAGES IN THE
| |
| DEVELOPMENT OF HYDROPHILUS
| |
| | |
| PICEUS. (From Gegenbaur, after
| |
| Kowalevsky. )
| |
| | |
| Is. labrum; at. antenna; tnd.
| |
| mandible; nix. maxilla I.; li. maxilla II.; //>"/" feet; a. anus.
| |
| | |
| | |
| | |
| TRACHEATA.
| |
| | |
| | |
| | |
| 409
| |
| | |
| | |
| | |
| observations (Kraepelin, etc.), and if it holds true for the Bee must be regarded
| |
| as correct for other cases also.
| |
| | |
| As to the order of the appearance of the appendages observations are as
| |
| yet too scanty to form any complete scheme. In many cases all the appendages appear approximately at the same moment, e.g. Hydrophilus, but
| |
| whether this holds good for all Coleoptera is by no means certain. In Apis
| |
| the appendages are stated by Biitschli to arise simultaneously, but according
| |
| to Kowalevsky the two mouth appendages first appear, then the antennae,
| |
| and still later the thoracic appendages. In the Diptera the mouth appendages are first formed, and either simultaneously with these, or slightly later,
| |
| the antennae. In the Hemiptera and Libellulidae the thoracic appendages
| |
| are the first to be formed, and the second pair of maxillae makes its appearance before the other cephalic appendages.
| |
| | |
| The history of the changes in the embryonic appendages during the
| |
| attainment of the 'adult con- .
| |
| | |
| dition is beyond the scope
| |
| of this treatise, but it may
| |
| be noted that the second
| |
| pair of maxillae are relatively very large in the
| |
| embryo, and not infrequently (Libellula, etc.)
| |
| have more resemblance to
| |
| the ambulatory than to the
| |
| masticatory appendages.
| |
| | |
| The exact nature of the
| |
| wings and their relation to
| |
| the other segments is still
| |
| very obscure. They appear as dorsal leaf-like appendages on the 2nd and
| |
| 3rd thoracic segments, and
| |
| are in many respects similar to the tracheal gills
| |
| of the larvae of Ephemeridae and Phryganidae (fig.
| |
| 1 88 A), of which they are
| |
| supposed by Gegenbaur
| |
| and Lubbock to be modifications. The undoubtedly
| |
| secondary character of the
| |
| closed tracheal system of
| |
| larvae with tracheal gills
| |
| tells against this view.
| |
| Fritz Miiller finds in the
| |
| larvae of Calotermes ru
| |
| | |
| | |
| | |
| FIG. 188. FIGURES ILLUSTRATING AQUATIC RESPIRATION IN INSECTS. (After Gegenbaur.)
| |
| | |
| A. Hinder portion of the body of Ephemera
| |
| vulgata. a. longitudinal tracheal trunks; b. alimentary canal ; c. tracheal gills.
| |
| | |
| B. Larva of ^Eschna grandis. a. superior longitudinal tracheal trunks ; b. their anterior end ; c. portion branching on proctodaeum ; o. eyes.
| |
| | |
| C. Alimentary canal of the same larva from the
| |
| side, a, b, and c. as in B ; d. inferior tracheal trunk ;
| |
| e. transverse branches between upper and lower
| |
| tracheal trunks.
| |
| | |
| | |
| | |
| 410 INSECTA.
| |
| | |
| | |
| | |
| gosus (one of the Termites) that peculiar and similar dorsal appendages are
| |
| present on the two anterior of the thoracic segments. They are without
| |
| tracheae. The anterior atrophies, and the posterior acquires tracheas and gives
| |
| rise to the first pair of wings. The second pair of wings is formed from
| |
| small processes on the third thoracic segment like those on the other two.
| |
| Fritz Miiller concludes from these facts that the wings of Insects are
| |
| developed from dorsal processes of the body, not equivalent to the ventral
| |
| appendages. What the primitive function of these appendages was is not
| |
| clear. Fritz Miiller suggests that they may have been employed as respiratory organs in the passage from an aqueous to a terrestrial existence, when
| |
| the Termite ancestors lived in moist habitations a function for which processes supplied with blood-channels would be well adapted. The undoubted
| |
| affinity of Insects to Myriapods, coupled with the discovery by Moseley of a
| |
| tracheal system in Peripatus, is however nearly fatal to the view that Insects
| |
| can have sprung directly from aquatic ancestors not provided with tracheae.
| |
| But although this suggestion of Fritz Miiller cannot be accepted, it is still
| |
| possible that the processes discovered by him may have been the earliest
| |
| rudiments of wings, which were employed first as organs of propulsion by a
| |
| water-inhabiting Insect ancestor which had not yet acquired the power of
| |
| flying.
| |
| | |
| The nervous system. The nervous system arises entirely
| |
| from the epiblast; but the development of the prae-oral and
| |
| post-oral sections may be best considered separately.
| |
| | |
| The post-oral section, or ventral cord of the adult, arises as
| |
| two longitudinal thickenings of the epiblast, one on each side of
| |
| the median line (fig. 189 B, vn), which are subsequently split ofif
| |
| from the superficial skin and give rise to the two lateral strands
| |
| of the ventral cord. At a later period they undergo a differentiation into ganglia and connecting cords.
| |
| | |
| Between these two embryonic nerve cords there is at first a shallow
| |
| furrow, which soon becomes a deep groove (fig. 189 C). At this stage the
| |
| differentiation of the lateral elements into ganglia and commissures takes
| |
| place, and, according to Hatschek (No. 414), the median groove becomes in
| |
| the region of the ganglia converted into a canal, the walls of which soon fuse
| |
| with those of the ganglionic enlargements of the lateral cords, and connect
| |
| them across the middle line. Between the ganglia on the other hand the
| |
| median groove undergoes atrophy, becoming first a solid cord interposed
| |
| between the lateral strands of the nervous system, and finally disappearing
| |
| without giving rise to any part of the nervous system. It is probable that
| |
| Hatschek is entirely mistaken about the entrance of a median element into
| |
| the ventral cord, and that the appearances he has described are due to
| |
| shrinkage. In Spiders the absence of a median element can be shewn with
| |
| great certainty, and, as already stated, this element is not present in
| |
| | |
| | |
| | |
| TRACHEATA.
| |
| | |
| | |
| | |
| 411
| |
| | |
| | |
| | |
| Peripatus. Hatschek states that in the mandibular segment the median
| |
| element is absorbed, and that the two lateral cords of that part give rise to
| |
| the oesophageal commissures, while the sub-cesophageal ganglion is formed
| |
| from the fusion of the ganglia of the two maxillary segments.
| |
| | |
| The prae-oral portion of the nervous system consists entirely
| |
| of the supra-cesophageal ganglion. It is formed, according to
| |
| Hatschek, of three parts. Firstly and mainly, of a layer sepa
| |
| | |
| | |
| | |
| | |
| FIG. 189. THREE TRANSVERSE SECTIONS THROUGH THE EMBRYO OF
| |
| HYDROPHILUS. (After Kowalevsky.)
| |
| | |
| A. Transverse section through the larva represented in fig. 187 A.
| |
| | |
| B. Transverse section through a somewhat older embryo in the region of one of
| |
| the stigmata.
| |
| | |
| C. Transverse section through the larva represented in fig. 187 B.
| |
| | |
| vn. ventral nerve cord; am. amnion and serous membrane ; me. mesoblast ; me.s.
| |
| somatic mesoblast ; hy. hypoblast (?) ; yk. yolk cells (true hypoblast) ; st. stigma of
| |
| trachea.
| |
| | |
| rated from the thickened inner part of the cephalic lobe on each
| |
| side ; secondly, of an anterior continuation of the lateral cords ;
| |
| and thirdly, of a pit of skin invaginated on each side close to the
| |
| | |
| | |
| | |
| 412 IN SECT A.
| |
| | |
| | |
| | |
| dorsal border of the antennae. This pit is at first provided with
| |
| a lumen, which is subsequently obliterated; while the walls of
| |
| the pit become converted into true ganglion cells. The two
| |
| supra-cesophageal ganglia remain disconnected on the dorsal
| |
| side till quite the close of embryonic life.
| |
| | |
| The tracheae and salivary glands. The tracheae, as was
| |
| first shewn by Butschli (No. 405), arise as independent segmentally arranged paired invaginations of the epiblast (fig. 189 B and
| |
| C, st). Their openings are always placed on the outer sides of
| |
| the appendages of their segments, where such are present.
| |
| | |
| Although in the adult stigmata are never found in the space
| |
| between the prothorax and head 1 , in the embryo and the larva
| |
| tracheal invaginations may be developed in all the thoracic (and
| |
| possibly in the three jaw-bearing segments) and in all the
| |
| abdominal segments except the two posterior.
| |
| | |
| In the embryo of the Lepidoptera, according to Hatschek (No. 414),
| |
| there are 14 pairs of stigmata, belonging to the 14 segments of the body
| |
| behind the mouth ; but Tichomiroff states that Hatschek is in error in
| |
| making this statement for the foremost post-oral segments. The last two
| |
| segments are without stigmata. In the larvae of Lepidoptera as well as those
| |
| of many Hymenoptera, Coleoptera and Diptera, stigmata are present on all
| |
| the postcephalic segments except the 2nd and 3rd thoracic and the two last
| |
| abdominal. In Apis there are eleven pairs of tracheal invaginations according to Kowalevsky (No. 416), but according to Butschli (No. 405) only ten,
| |
| the prothorax being without one. In the Bee they appear simultaneously,
| |
| and before the appendages.
| |
| | |
| The blind ends of the tracheal invaginations frequently (e.g.
| |
| Apis) unite together into a common longitudinal canal, which
| |
| forms a longitudinal tracheal stem. In other cases (eg. Gryllotalpa, Dohrn, No. 408) they remain distinct, and each tracheal
| |
| stem has a system of branches of its own.
| |
| | |
| The development of the tracheae strongly supports the view,
| |
| arrived at by Moseley from his investigations on Peripatus, that
| |
| they are modifications of cutaneous glands.
| |
| | |
| The salivary and spinning glands are epiblastic structures,
| |
| which in their mode of development are very similar to the
| |
| tracheae, and perhaps have a similar origin. The salivary glands
| |
| | |
| 1 In Smynthurus, one of the Collembola, there are, according to Lubbock, only
| |
| two stigmata, which are placed on the head.
| |
| | |
| | |
| | |
| TRACHEATA. 413
| |
| | |
| | |
| | |
| arise as paired epiblastic imaginations, not, as might be
| |
| expected, of the Stomodaeum, but of the ventral plate behind
| |
| the mouth on the inner side of the mandibles. At first independent, they eventually unite in a common duct, which falls into
| |
| the mouth. The spinning glands arise on the inner side of the
| |
| second pair of maxillae in Apis and Lepidoptera, and form
| |
| elongated glands extending through nearly the whole length
| |
| of the body. They are very similar in their structure and development to salivary glands, and are only employed during larval
| |
| life. They no doubt resemble the mucous glands of the oral
| |
| papillae of Peripatus, with which they have been compared by
| |
| Moseley. The mucous glands of Peripatus may perhaps be the
| |
| homologous organs of the first pair of maxillae, for the existence
| |
| of which there appears to be some evidence amongst Insects.
| |
| | |
| Mesoblast. It has been stated that the mesoblast becomes
| |
| divided in the region of the body into two lateral bands (fig. 189
| |
| A). These bands in many, if not all forms, become divided
| |
| into a series of somites corresponding with the segments of the
| |
| body. In each of them a cavity appears the commencing
| |
| perivisceral cavity which divides them into a somatic plate in
| |
| contact with the epiblast, and a splanchnic plate in contact with
| |
| the hypoblast (fig. 189). In the interspaces between the
| |
| segments the mesoblast is continuous across the median ventral
| |
| line. The mesoblast is prolonged into each of the appendages
| |
| as these are formed, and in the appendages there is present a
| |
| central cavity. By Metschnikoff these cavities are stated to be
| |
| continuous, as in Myriapods and Arachnida, with those of the
| |
| somites ; but by Hatschek (No. 414) they are stated to be
| |
| independent of those in the somites and to be open to the yolk.
| |
| | |
| The further details of the history of the mesoblast are very imperfectly
| |
| known, and the fullest account' we have is that by Dohrn (No. 408) for
| |
| Gryllotalpa. It would appear that the mesoblast grows round and encloses
| |
| the dorsal side of the yolk earlier than the epiblast. In Gryllotalpa it forms
| |
| a pulsating membrane. As the epiblast extends dorsalwards the median
| |
| dorsal part of the membrane is constricted off as a tube which forms the
| |
| heart. At the same time the free space between the pulsating membrane
| |
| and the yolk is obliterated, but transverse passages are left at the lines
| |
| between the somites, through which the blood passes from the ventral part of
| |
| the body to corresponding openings in the wall of the heart. The greater
| |
| part of the membrane gives rise to the muscles of the trunk.
| |
| | |
| | |
| | |
| 414 INSECTA.
| |
| | |
| | |
| | |
| Ventrally the mesoblastic bands soon meet across the median line. The
| |
| cavities in the appendages become obliterated and their mesoblastic walls
| |
| form the muscles, etc. The cavities in the separate mesoblastic somites also
| |
| cease to be distinctly circumscribed.
| |
| | |
| The splanchnic mesoblast follows the hypoblast in its growth, and gives
| |
| rise to the connective tissue and muscular parts of the walls of the alimentary tract. The mesoblastic wall of the proctodaeum is probably formed
| |
| independently of the mesoblastic somites. In the head the mesoblast is
| |
| stated to form at first a median ventral mass, which does not pass into the
| |
| procephalic lobe ; though it assists in forming both the antennae and upper
| |
| lip.
| |
| | |
| The alimentary canal. The alimentary tract of Insects is
| |
| formed of three distinct sections (fig. 181) a mesenteron or
| |
| middle section (me), a stomodaeum (st) and a proctodaeum (an).
| |
| The stomodaeum and proctodaeum are invaginations of the
| |
| epiblast, while the mesenteron is lined by the hypoblast. The
| |
| distinction between the three is usually well marked in the adult
| |
| by the epiblastic derivatives being lined by chitin. The stomodaeum consists of mouth, oesophagus, crop, and proventriculus or
| |
| gizzard, when such are present. The mesenteron includes the
| |
| stomach, and is sometimes (Orthoptera, etc.) provided at its
| |
| front end with pyloric diverticula posteriorly it terminates just
| |
| in front of the Malpighian bodies. These latter fall into the
| |
| proctodaeum, which includes the whole of the region from their
| |
| insertion to the anus.
| |
| | |
| The oral invagination appears nearly coincidently. with the
| |
| first formation of segments at the front end of the groove
| |
| between the lateral nerve cords, and the anal invagination
| |
| appears slightly later at the hindermost end of the ventral plate.
| |
| | |
| The Malpighian bodies arise as two pairs of outgrowths of the
| |
| epiblast of t/te proctodceum, whether solid at first is not certain.
| |
| The subsequent increase which usually takes place in their
| |
| number is due to sproutings (at first solid) of the two original
| |
| vessels.
| |
| | |
| The glandular walls of the mesenteron are formed from the hypoblast ;
| |
| but the exact origin of the layer has not been thoroughly worked out in all
| |
| cases. In Hydrophilus it is stated by Kowalevsky (No. 416) to appear as
| |
| two sheets split off from the lateral masses of mesoblast, which gradually
| |
| grow round the yolk, and a similar mode of formation would seem to hold
| |
| good for Apis. Tichomiroff (No. 420) confirms Kowalevsky on this point,
| |
| | |
| | |
| | |
| TR ACHE AT A. 415
| |
| | |
| | |
| | |
| and further states that these two masses meet first ventrally and much later
| |
| on the dorsal side. In Lepidoptera, on the other hand, Hatschek finds that
| |
| the hypoblast arises as a median mass of polygonal cells in the anterior part
| |
| of the ventral plate. These cells increase by absorbing material from the
| |
| yolk, and then gradually extend themselves and grow round the yolk.
| |
| | |
| Dohrn (No. 408) believes that the yolk cells, the origin of which has
| |
| already been spoken of, give rise to the hypoblastic walls of the mesenteron,
| |
| and this view appears to be shared by Graber (No. 412), though the latter
| |
| author holds that some of the yolk cells are derived by budding from the
| |
| blastoderm 1 .
| |
| | |
| From the analogy of Spiders I am inclined to accept Dohrn's and
| |
| Graber's view. It appears to me probable that Kowalevsky's observations
| |
| are to be explained by supposing that the hypoblast plates which he believes
| |
| to be split off from the mesoblast are really separated from the yolk.
| |
| | |
| .It will be convenient to add here a few details to what has already been
| |
| stated as to the origin of the yolk cells. As mentioned above, the central
| |
| yolk breaks up at a period, which is not constant in the different forms, into
| |
| polygonal or rounded masses, in each of which a nucleus has in many
| |
| instances been clearly demonstrated although in others such nuclei have not
| |
| been made out. It is probable however that nuclei are in all cases really
| |
| present, and that these masses must be therefore regarded as cells. They
| |
| constitute in fact the yolk cells. The periphery of the yolk breaks up into
| |
| cells while the centre is still quite homogeneous.
| |
| | |
| The hypoblastic walls of the mesenteron appear to be formed
| |
| in the first instance laterally (fig. 189 B and C, hy). They then
| |
| meet ventrally (fig. 185 A and B), and finally close in the
| |
| mesenteron on the dorsal side.
| |
| | |
| The mesenteron is at first a closed sack, independent of both
| |
| stomodaeum and proctodaeum ; and in the case of the Bee it so
| |
| remains even after the close of embryonic life. The only glandular organs of the mesenteron are the not unfrequent pyloric
| |
| tubes, which are simple outgrowths of its anterior end. It is
| |
| possible that in some instances they may be formed in situ
| |
| around the lateral parts of the yolk.
| |
| | |
| In many instances the whole of the yolk is enclosed in the walls of the
| |
| mesenteron, but in other cases, as in Chironomus and Simulia (Weismann,
| |
| No. 430 ; Metschnikoff, No. 423), part of the yolk may be left between the
| |
| ventral wall of the mesenteron and the ventral plate. In Chironomus the
| |
| | |
| 1 Graber's view on this point may probably be explained by supposing that he has
| |
| mistaken a passage of yolk cells into the blastoderm for a passage of blastoderm cells
| |
| into the yolk. The former occurrence takes place, as I have found, largely in Spiders,
| |
| and probably therefore also occurs in Insects.
| |
| | |
| | |
| | |
| 41 6 INSECTA.
| |
| | |
| | |
| | |
| mass of yolk external to the mesenteron takes the form of a median and two
| |
| lateral streaks. Some of the yolk cells either prior to the establishment of
| |
| the mesenteron, or derived from the unenclosed portions of the yolk, pass
| |
| into the developing organs (Dohrn, 408) and serve as a kind of nutritive cell.
| |
| They also form blood corpuscles and connective-tissue elements. Such yolk
| |
| cells may be compared to the peculiar bodies described by Reichenbach in
| |
| Astacus, which form the secondary mesoblast. Similar cells play a very
| |
| important part in the development of Spiders.
| |
| | |
| Generative organs. The observations on the development of the
| |
| generative organs are somewhat scanty. In Diptera certain cells known
| |
| as the pole cells are stated by both Metschnikoff (No. 423) and Leuckart to
| |
| give rise to the generative organs. The cells in question (in Chironomus
| |
| and Musca vomitoria, Weismann, No. 430) appear at the hinder end of the
| |
| ovum before any other cells of the blastoderm. They soon separate from
| |
| the blastoderm and increase by division. In the embryo, produced by the
| |
| viviparous larva of Cecidomyia, there is at first a single pole cell, which
| |
| eventually divides into four, and the resulting cells become enclosed within
| |
| the blastoderm. They next divide into two masses, which are stated by
| |
| Metschnikoff (No. 423) to become surrounded by indifferent embryonic cells 1 .
| |
| Their protoplasm then fuses, and their nuclei divide, and they give rise to
| |
| the larval ovaries, for which the enclosing cells form the tunics.
| |
| | |
| In Aphis Metschnikoff (No. 423) detected at a very early stage a mass
| |
| of cells which give rise to the generative organs. These cells are situated
| |
| at the hind end of the ventral plate ; and, except in the case of one of the
| |
| cells which gives rise by division to a green mass adjoining the fat body,
| |
| the protoplasm of the separate cells fuses into a syncytium. Towards the
| |
| close of embryonic life the syncytium assumes a horse-shoe form. The mass
| |
| is next divided into two, and the peripheral layer of each part gives rise
| |
| to the tunic, while from the hinder extremity of each part an at first solid
| |
| duct the egg- tube grows out. The masses themselves form the germogens. The oviduct is formed by a coalescence of the ducts from each
| |
| germogen.
| |
| | |
| Ganin derives the generative organs in Platygaster (vide p. 347) from
| |
| the hind end of the ventral plate close to the proctodaeum ; while Suckow
| |
| states that the generative organs are outgrowths of the proctodicum.
| |
| According to these two sets of observations the generative organs would
| |
| appear to have an epiblastic origin an origin which is not incompatible
| |
| with that from the pole cells.
| |
| | |
| In Lepidoptera the genital organs are present in the later periods of
| |
| embryonic life as distinct paired organs, one on each side of the heart, in
| |
| the eighth postcephalic segment. They are elliptical bodies with a duct
| |
| passing off from the posterior end in the female or from the middle in the
| |
| male. The egg-tubes or seminal tubes are outgrowths of the elliptical
| |
| bodies.
| |
| | |
| 1 This point requires further observation.
| |
| | |
| | |
| | |
| TRACHEATA.
| |
| | |
| | |
| | |
| 417
| |
| | |
| | |
| | |
| In other Insects the later stages in the development of the generative
| |
| organs closely resemble those in the Lepidoptera, and the organs are usually
| |
| distinctly visible in the later stages of embryonic life.
| |
| | |
| It may probably be laid down, in spite of some of Metschnikoff's
| |
| observations above quoted, that the original generative mass gives rise to
| |
| both the true genital glands and their ducts. It appears also to be fairly
| |
| clear that the genital glands of both sexes have an identical origin.
| |
| | |
| Special types of larva.
| |
| | |
| Certain of the Hymenopterous forms, which deposit their eggs in the
| |
| eggs or larvae of other Insects, present very peculiar modifications in their
| |
| development. Platygaster, which lays its egg in the larvae of Cecidomyia,
| |
| undergoes perhaps the most remarkable development amongst these forms.
| |
| It has been studied especially by Ganin (No. 410), from whom the following
| |
| account is taken.
| |
| | |
| The very first stages are unfortunately but imperfectly known, and the
| |
| interpretations offered by Ganin do not in all cases appear quite satisfactory. In the earliest stage after being laid the egg is enclosed in a
| |
| capsule produced into a stalk (fig. 190 A). In the interior of the egg
| |
| there soon appears a single spherical body, regarded by Ganin as a cell
| |
| (fig. 190 B). In the next stage three similar bodies appear in the vitellus,
| |
| no doubt derived from the first one (fig. 190 C). The central one presents
| |
| somewhat different characters to the two others, and, according to Ganin,
| |
| gives rise to the whole embryo. The two peripheral bodies increase by
| |
| division, and soon appear as nuclei imbedded in a layer of protoplasm (fig. 190 D,
| |
| E, F). The layer so
| |
| formed serves as a
| |
| covering for the embryo, regarded by
| |
| Ganin as equivalent
| |
| to the amnion (? serous membrane) of
| |
| other Insect embryos. In the embryo cell new cells
| |
| are stated to be
| |
| formed by a process
| |
| of endogenous cell formation (fig. 190 D, E). It appears probable that
| |
| Ganin has mistaken nuclei for cells in the earlier stages, and that a blastoderm is formed as in other Insects, and that this becomes divided in a way
| |
| not explained into a superficial layer which gives rise to the serous
| |
| envelope, and a deeper layer which forms the embryo. However this
| |
| | |
| B. II. 27
| |
| | |
| | |
| | |
| | |
| FlG. 190. A SERIES OF STAGES IN THE DEVELOPMENT
| |
| | |
| OF PLATYGASTER. (From Lubbock ; after Ganin.)
| |
| | |
| | |
| | |
| 41 8 INSECTA.
| |
| | |
| | |
| | |
| may be, a differentiation into an epiblastic layer of columnar cells and
| |
| a hypoblastic layer of more rounded cells soon becomes apparent in the body
| |
| of the embryo. Subsequently to this the embryo grows rapidly, till by a
| |
| deep transverse constriction on the ventral surface it becomes divided into an
| |
| anterior cephalothoracic portion and a posterior caudal portion (fig. 190 F).
| |
| The cephalothorax grows in breadth, and near its anterior end an invagination appears, which gives rise to the mouth and cesophagus. On
| |
| the ventral side of the cephalothorax there is first formed a pair of
| |
| claw-like appendages on each side of the mouth, then a posterior pair of
| |
| appendages near the junction of the cephalothorax and abdomen, and
| |
| lastly a pair of short conical antennae in front.
| |
| | |
| At the same time the hind end of the abdomen becomes bifid, and gives
| |
| rise to a fork-like caudal appendage ; and at a slightly later period four
| |
| grooves make their appearance in the caudal region, and divide this part of
| |
| the embryo into successive segments. While these changes have been
| |
| taking place in the general form of the embryo, the epiblast has given rise
| |
| to a cuticle, and the hypoblastic cells have become differentiated into a
| |
| central hypoblastic axis the mesenteron and a surrounding layer of
| |
| mesoblast, some of the cells of which form longitudinal muscles.
| |
| | |
| With this stage closes what may be regarded as the embryonic development of Platygaster. The embryo becomes free from the amnion, and presents itself as a larva, which from its very remarkable characters has been
| |
| spoken of as the Cyclops larva by Ganin.
| |
| | |
| The larvae of three species have been described by Ganin, which are represented in fig. 1 9 1 A, B, C. These larvae are strangely dissimilar to the ordinary
| |
| Hexapod type, whether larval or adult. They are formed of a cephalothoracic
| |
| shield with the three pairs of appendages (a, kf, lfg\ the development of
| |
| which has already been described, and of an abdomen formed of five segments, the last of which bears the somewhat varying caudal appendages.
| |
| The nervous system is as yet undeveloped.
| |
| | |
| The larvae move about in the tissues of their hosts by means of their
| |
| claws.
| |
| | |
| The first larval condition is succeeded by a second with very different
| |
| characters, and the passage from the first to the second is accompanied by
| |
| an ecdysis.
| |
| | |
| The ecdysis commences at the caudal extremity, and the whole of the
| |
| last segment is completely thrown off. As the ecdysis extends forwards
| |
| the tail loses its segmentation and becomes strongly compressed, the
| |
| appendages of the cephalothorax are thrown off, and the whole embryo
| |
| assumes an oval form without any sharp distinction into different regions
| |
| and without the slightest indication of segmentation (fig. 191 D). Of the
| |
| internal changes which take place during the shedding of the cuticle, the
| |
| first is the formation of a proctodaeum (gfi) by an invagination, which ends
| |
| blindly in contact with the mesenteron. Shortly after this a thickening of
| |
| the epiblast (bsm} appears along the ventral surface, which gives rise mainly
| |
| to the ventral nerve cord ; this thickening is continuous behind with the
| |
| | |
| | |
| | |
| TRACHEATA.
| |
| | |
| | |
| | |
| 419
| |
| | |
| | |
| | |
| epiblast which is invaginated to form the proctodaeum, and in front is prolonged on each side into two procephalic lobes, in which there are also
| |
| thickenings of the epiblast (gsae), which become converted into supraoesophageal ganglia, and possibly other parts.
| |
| | |
| Towards the close of the second larval period the muscles (/;) become
| |
| segmentally arranged, and give indications of the segmentation which
| |
| | |
| | |
| | |
| | |
| FlG. 191. A SERIES OF STAGES IN THE DEVELOPMENT OF PLATYGASTER.
| |
| | |
| (From Lubbock ; after Ganin.)
| |
| | |
| A. B. C. Cyclops larvae of three species of Platygaster.
| |
| D. Second larval stage. E. Third larval stage.
| |
| | |
| mo. mouth ; a. antenna ; kf. hooked feet ; Ifg. lateral feet ; /. branches of tail ;
| |
| ul. lower lip ; slkf. oesophagus ; gsae. supra- oesophageal ganglion ; bsm. ventral epiblastic plate ; Im. lateral muscles (the letters also point in D to the salivary glands) ;
| |
| gh. proctodseum ; ga. generative organs ; md. mandibles ; ag. ducts of salivary glands ;
| |
| sp. (in E) salivary glands ; mis. stomach ; ed. intestine ; ew. rectum ; ao. anus ;
| |
| tr. tracheae ; fk. fat body.
| |
| | |
| becomes apparent in the third larval period. The third and last larval
| |
| stage (fig. 191 E) of Platygaster, during which it still remains in the tissues
| |
| of its host, presents no very peculiar features. The passage from the second
| |
| to the third form is accompanied by an ecdysis.
| |
| | |
| Remarkable as are the larvae just described, there can I think be
| |
| no reason, considering their parasitic habits, for regarding them as ancestral.
| |
| | |
| 272
| |
| | |
| | |
| | |
| 420 INSECTA.
| |
| | |
| | |
| | |
| Metamorphosis and heterogamy.
| |
| | |
| Metamorphosis. The majority of Insects are born in a
| |
| condition in which they obviously differ from their parents. The
| |
| extent of this difference is subject to very great variations, but
| |
| as a rule the larvae pass through a very marked metamorphosis
| |
| before reaching the adult state. The complete history of this
| |
| metamorphosis in the different orders of Insects involves a far
| |
| too considerable amount of zoological detail to be dealt with in
| |
| this work ; and I shall confine myself to a few observations on
| |
| the general characters and origin of the metamorphosis, and of
| |
| the histological processes which take place during its occurrence 1 .
| |
| | |
| In the Aptera the larva differs from the adult only in the
| |
| number of facets in the cornea and joints in the antennae.
| |
| | |
| In most Orthoptera and Hemiptera the larvae differ from the
| |
| adult in the absence of wings and in other points. The wings,
| |
| etc., are gradually acquired in the course of a series of successive
| |
| moultings. In the Ephemeridae and Libellulidae, however, the
| |
| metamorphosis is more complicated, in that the larvae have
| |
| provisional tracheal gills which are exuviated before the final
| |
| moult. In the Ephemeridae there are usually a great number of
| |
| moultings ; the tracheal gills appear after the second moult, and
| |
| the rudiments of the wings when the larva is about half grown.
| |
| Larval life may last for a very long period.
| |
| | |
| In all the other groups of Insects, viz. the Diptera, Neuroptera, Coleoptera, Lepidoptera, and Hymenoptera, the larva
| |
| passes with a few exceptions through a quiescent stage, in
| |
| which it is known as a pupa, before it attains the adult stage.
| |
| These forms are known as the Holometabola.
| |
| | |
| In the Diptera the larvae are apodous. In the true flies (Muscidae) they
| |
| are without a distinct head and have the jaws replaced by hooks. In the
| |
| Tipulidae there is on the other hand a well-developed head with the normal
| |
| appendages. The pupae of the Muscidae are quiescent, and are enclosed in
| |
| the skin of the larva which shrinks and forms a firm oval case. In the
| |
| | |
| 1 For a systematic account of this subject the reader is referred to Lubbock (No.
| |
| 420) and to Graber (No. 411). He will find in Weismann (Nos. 430 and 431) a detailed
| |
| account of the internal changes which take place.
| |
| | |
| | |
| | |
| TRACHEATA. 42 1
| |
| | |
| | |
| | |
| Tipulidae the larval skin is thrown off at the pupa stage, and in some cases
| |
| the pupae continue to move about.
| |
| | |
| The larvae of the Neuroptera are hexapodous voracious forms. When the
| |
| larva becomes a pupa all the external organs of the imago are already
| |
| established. The pupa is often invested in a cocoon. It is usually quiescent,
| |
| though sometimes it begins to move about shortly before the imago emerges.
| |
| | |
| In the Coleoptera there is considerable variety in the larval forms. As a
| |
| rule the larvae are hexapodous and resemble wingless Insects. But some
| |
| herbivorous larvae (e.g. the larva of Melolontha) closely resemble true
| |
| caterpillars, and there are also grub-like larvae without feet (Curculio) which
| |
| resemble the larvae of Hymenoptera. The pupa is quiescent, but has all
| |
| the parts of the future beetle plainly visible. The most interesting larvae
| |
| among the Coleoptera are those of Sitaris, one of the Meloidae (Fabre, No.
| |
| 409). They leave the egg as active hexapodous larvae which attach themselves to the bodies of Hymenoptera, and are thence transported to a cell
| |
| filled with honey. Here they eat the ovum of the Hymenopterous form.
| |
| They then undergo an ecdysis, in which they functionally lose their appendages, retaining however small rudiments of them, and become grubs. They
| |
| feed on the honey and after a further ecdysis become pupae.
| |
| | |
| In the Lepidoptera the larva has the well-known form of a caterpillar.
| |
| The caterpillars have strong jaws, adapted for biting vegetable tissues,
| |
| which are quite unlike the oral appendages of the adult. They have three
| |
| pairs of jointed thoracic legs, and a variable number (usually five) of pairs
| |
| of rudimentary abdominal legs the so-called pro-legs. The larva undergoes
| |
| numerous ecdyses, and the external parts of the adult such as the wings, etc.,
| |
| are formed underneath the chitinous exoskeleton before the pupa stage.
| |
| The pupa is known as a chrysalis and in some Lepidoptera is enveloped in
| |
| a cocoon.
| |
| | |
| The Hymenoptera present considerable variations in the character of the
| |
| larvae. In the Aculeata, many Entomophaga, the Cynipidae, etc., the larvae
| |
| are apodous grubs, incapable of going in search of their food ; but in the
| |
| Siricidse they are hexapodous forms like caterpillars, which are sometimes
| |
| even provided with pro-legs. In some of the Entomophaga the larvae have
| |
| very remarkable characters which have already been described in a special
| |
| section, 'vide pp. 418, 419.
| |
| | |
| Before proceeding to the consideration of the value of the
| |
| various larval forms thus shortly enumerated, it is necessary to
| |
| say a few words as to the internal changes which take place
| |
| during the occurrence of the above metamorphosis. In the
| |
| simplest cases, such as those of the Orthoptera and Hemiptera,
| |
| where the metamorphosis is confined to the gradual formation
| |
| of the wings, etc. in a series of moults, the wings first appear as
| |
| two folds of the epidermis beneath the cuticle on the two
| |
| posterior thoracic segments. At the next moult these processes
| |
| | |
| | |
| | |
| 422 INSECTA.
| |
| | |
| | |
| | |
| become covered by the freshly formed cuticle, and appear as
| |
| small projections. At every successive moult these projections
| |
| become more prominent owing to a growth in the epidermis
| |
| which has taken place in the preceding interval. Accompanying
| |
| the formation of such organs as the wings, internal changes
| |
| necessarily take place in the arrangement of the muscles, etc. of
| |
| the thorax, which proceed pari passu with the formation of the
| |
| organs to which they belong. The characters of the metamorphosis in such forms as the Ephemeridae only differ from the
| |
| above in the fact that provisional organs are thrown off at the
| |
| same time that the new ones are formed.
| |
| | |
| In the case of the Holometabola the internal phenomena of
| |
| the metamorphosis are of a very much more remarkable character. The details of our knowledge are largely due to Weismann (Nos. 430 and 431). The larvae of the Holometabola have
| |
| for the most part a very different mode of life to the adults.
| |
| A simple series of transitions between the two is impossible,
| |
| because intermediate forms would be for the most part incapable
| |
| of existing. The transition from the larval to the adult state is
| |
| therefore necessarily a more or less sudden one, and takes place
| |
| during the quiescent pupa condition. Many of the external
| |
| adult organs are however formed prior to the pupa stage, but do
| |
| not become visible on the surface. The simplest mode of Holometabolic metamorphosis may be illustrated by the development
| |
| of Corethra plumicornis, one of the Tipulidae. This larva, like
| |
| that of other Tipulidae, is without thoracic appendages, but
| |
| before the last larval moult, and therefore shortly before the
| |
| pupa stage, certain structures are formed, which Weismann has
| |
| called imaginal discs. These imaginal discs are in Corethra
| |
| simply invaginations of the epidermis. There are in the thorax
| |
| six pairs of such structures, three dorsal and three ventral. The
| |
| three ventral are attached to the terminations of the sensory
| |
| nerves, and the limbs of the imago are formed as simple
| |
| outgrowths of them, which as they grow in length take a spiral
| |
| form. In the interior of these outgrowths are formed the
| |
| muscles, tracheae, etc., of the limbs; which are believed by
| |
| Weismann (it appears to me without sufficient ground) to be
| |
| derived from a proliferation of the cells of the neurilemma.
| |
| The wings are formed from the two posterior dorsal imaginal
| |
| | |
| | |
| | |
| TRACHEATA. 423
| |
| | |
| | |
| | |
| discs. The hypodermis of the larva passes directly into that of
| |
| the imago.
| |
| | |
| The pupa stage of Corethra is relatively very short, and the
| |
| changes in the internal parts which take place during it are not
| |
| considerable. The larval abdominal muscles pass for the most
| |
| part unchanged into those of the imago, while the special
| |
| thoracic muscles connected with the wings, etc., develop directly
| |
| during the latest larval period from cords of cells already formed
| |
| in the embryo.
| |
| | |
| In the Lepidoptera the changes in the passage from the
| |
| larval to the adult state are not very much more considerable
| |
| than those in Corethra. Similar imaginal discs give rise during
| |
| the later larval periods to the wings, etc. The internal changes
| |
| during the longer pupa period are somewhat more considerable.
| |
| Important modifications and new formations arise in connection with the alimentary tract, the nervous and muscular
| |
| systems.
| |
| | |
| The changes which take place in the true flies (Muscidse) are
| |
| far more complicated than either those in Corethra or in the
| |
| Lepidoptera. The abdomen of the larva of Musca becomes
| |
| bodily converted into the abdomen of the imago as in the above
| |
| types, but the whole epidermis and appendages of the head and
| |
| thorax are derived from imaginal discs which are formed within
| |
| and (so far as is known) independently of the epidermis of the
| |
| larva or embryo. These imaginal discs are simple masses of
| |
| apparently indifferent cells, which for the most part appear at
| |
| the close of embryonic life, and are attached to nerves or
| |
| tracheae. They grow in size during larval life, but during the
| |
| relatively long pupa stage they unite together to give rise to a
| |
| continuous epidermis, from which the appendages grow out as
| |
| processes. The epidermis of the anterior part of the larva is
| |
| simply thrown off, and has no share in forming the epidermis of
| |
| the adult.
| |
| | |
| There are a pair of cephalic imaginal discs and six pairs of
| |
| thoracic discs. Two pairs, a dorsal and a ventral, give rise to
| |
| each thoracic ring, and the appendages attached to it.
| |
| | |
| Though, as mentioned above, no evidence has yet been
| |
| produced to shew that the imaginal discs of Musca are derived
| |
| from the embryonic epiblast, yet their mode of growth and
| |
| | |
| | |
| | |
| 424 1NSECTA.
| |
| | |
| | |
| | |
| eventual fate proves beyond the shadow of a doubt that they are
| |
| homologous with the imaginal discs of Corethra. Their earliest
| |
| origin is well worth further investigation.
| |
| | |
| The metamorphosis of the internal organs is still more
| |
| striking than that of the external. There is a disruption, total
| |
| or partial, of all the internal organs except the generative
| |
| organs. In the case of the alimentary tract, the Malpighian
| |
| vessels, the heart and the central nervous system, the disruption
| |
| is of a partial kind, which has been called by Weismann
| |
| histolysis. The cells of these organs undergo a fatty degeneration, the nuclei alone in some cases remaining. The kind of
| |
| plasma resulting from this degeneration retains the shape of the
| |
| organs, and finally becomes built up again into the corresponding organs of the imago. The tracheae, muscles and peripheral
| |
| nerves, and an anterior part of the alimentary tract, are entirely
| |
| disrupted. They seem to be formed again from granular cells
| |
| derived from the enormous fat body.
| |
| | |
| The phenomena of the development of the Muscidse are undoubtedly of
| |
| rather a surprising character. Leaving for the moment the question of the
| |
| origin of the pupa stage to which I return below, it will be admitted on all
| |
| hands that during the pupa stage the larva undergoes a series of changes
| |
| which, had they taken place by slow degrees, would have involved, in such a
| |
| case as Musca, a complete though gradual renewal of the tissues. Such
| |
| being the case, the cells of the organs common to the larva and the imago
| |
| would, in the natural course of things, not be the same cells as those of the
| |
| larva but descendants of them. We might therefore expect to find in the
| |
| rapid conversion of the larval organs into those of the adult some condensation, so to speak, of the process of ordinary cell division. Such condensations
| |
| are probably represented in the histolysis in the case of the internal organs,
| |
| and in the formation of imaginal discs in the case of the external ones, and
| |
| I think it probable that further investigation will shew that the imaginal
| |
| discs of the Muscidae are derivatives of the embryonic epiblast. The above
| |
| considerations by no means explain the whole of Weismann's interesting
| |
| observations, but an explanation is I believe to be found by following up
| |
| these lines.
| |
| | |
| More or less parallel phenomena to those in Insects are found in the
| |
| development of the Platyelminthes and Echinoderms. The four disc-like
| |
| invaginations of the skin in many larval Nemertines (vide p. 198), which
| |
| give rise to the permanent body wall of the Nemertine, may be compared to
| |
| the imaginal discs. The subsequent throwing off of the skin of Pilidium or
| |
| larva of Desor is a phenomenon like the absorption of part of the larval
| |
| skin of Musca. The formation of an independent skin within the first larval
| |
| | |
| | |
| | |
| TR ACHE AT A.
| |
| | |
| | |
| | |
| 425
| |
| | |
| | |
| | |
| form in the Distomeaeand in the Cestoda may be compared to the apparently
| |
| independent formation of the imaginal discs in Musca.
| |
| | |
| The fact that in a majority of instances it is possible to trace
| |
| an intimate connection between the surroundings of a larva and
| |
| its organization proves in the clearest way that the characters of
| |
| the majority of existing larval forms of Insects have owed their
| |
| origin to secondary adaptations. A few instances will illustrate
| |
| this point.
| |
| | |
| In the simplest types of metamorphosis, e.g. those of the
| |
| Orthoptera genuina, the larva has precisely the same habits as
| |
| the adult. We find that a caterpillar
| |
| form is assumed by phytophagous larvae
| |
| amongst the Lepidoptera, Hymenoptera
| |
| and Coleoptera. Where the larva has
| |
| not to go in search of its nutriment the
| |
| grub-like apodous form is assumed. The
| |
| existence of such an apodous larva is
| |
| especially striking in the Hymenoptera,
| |
| in that rudiments of thoracic and abdominal appendages are present in the
| |
| embryo and disappear again in the larva.
| |
| The case of the larva of Sitaris, already
| |
| described (p. 421), affords another very
| |
| striking proof that the organization of
| |
| the larva is adapted to its habits.
| |
| | |
| It follows from the above that the
| |
| development of such forms as the Orthoptera genuina is more primitive than
| |
| that of the holometabolous forms; a
| |
| conclusion which tallies with the fact
| |
| | |
| | |
| | |
| | |
| FIG. 102.
| |
| | |
| | |
| | |
| ANTERIOR
| |
| | |
| | |
| | |
| HALF OF CAMPODEA FRAGILIS. (From Gegenbaur; after Palmen.)
| |
| | |
| a. antennae ; p. feet ; j> ',
| |
| post-tho
| |
| feet; s.
| |
| | |
| | |
| | |
| stigma.
| |
| | |
| that both palaeontological and anatomical evidence shew the
| |
| Orthoptera to be a very primitive group of Insects.
| |
| | |
| The above argument probably applies with still greater force
| |
| to the case of the Thysanura ; and it seems to be probable that
| |
| this group is more nearly related than any other to the primitive
| |
| wingless ancestors of Insects 1 . The characters of the oral
| |
| | |
| 1 Brauer and Lubbock (No. 421) have pointed out the primitive characters of these
| |
| forms, especially of Campodea.
| |
| | |
| | |
| | |
| 426 INSECTA.
| |
| | |
| | |
| | |
| appendages in this group, the simplicity of their metamorphosis,
| |
| and the presence of abdominal appendages (fig. 192), all tell in
| |
| favour of this view, while the resemblance of the adult to the
| |
| larvae of the Pseudoneuroptera, etc., points in the same direction.
| |
| The Thysanura and Collembola are not however to be regarded
| |
| as belonging to the true stock of the ancestors of Insects, but as
| |
| degenerated relations of this stock ; much as Amphioxus and
| |
| the Ascidians are degenerate relations of the ancestral stock of
| |
| Vertebrates, and Peripatus of that of the Tracheata. It is
| |
| probable that all these forms have succeeded in retaining their
| |
| primitive characters from their degenerate habits, which prevented them from entering into competition in the struggle for
| |
| existence with their more highly endowed relatives. While in a
| |
| general way it is clear that the larval forms of Insects cannot be
| |
| expected to throw much light on the nature of Insect ancestors,
| |
| it does nevertheless appear to me probable that such forms as
| |
| the caterpillars of the Lepidoptera are not without a meaning in
| |
| this respect. It is easy to conceive that even a secondary larval
| |
| form may have been produced by the prolongation of one of the
| |
| embryonic stages ; and the general similarity of a caterpillar to
| |
| Peripatus, and the retention by it of post-thoracic appendages, are
| |
| facts which appear to favour this view of the origin of the caterpillar form.
| |
| | |
| The two most obscure points which still remain to be dealt
| |
| with in the metamorphosis of Insects are (i) the origin of the
| |
| quiescent pupa stage ; (2) the frequent dissimilarity between the
| |
| masticatory apparatus of the larva and adult.
| |
| | |
| These two points may be conveniently dealt with together,
| |
| and some valuable remarks about them will be found in Lubbock
| |
| (No. 420).
| |
| | |
| On grounds already indicated it may be considered certain
| |
| that the groups of Insects without a pupa stage, and with a larva
| |
| very similarly organised to the adult, preceded the existing
| |
| holometabolic groups. The starting-point in the metamorphosis
| |
| of the latter groups was therefore something like that of the
| |
| Orthoptera. Suppose it became an advantage to a species that
| |
| the larva and adult should feed in a somewhat different way, a
| |
| difference in the character of their mouth parts would soon make
| |
| itself manifest ; and, since an intermediate type of mouth parts
| |
| | |
| | |
| | |
| TRACHEATA. 427
| |
| | |
| | |
| | |
| would probably be disadvantageous, there would be a tendency
| |
| to concentrate into a single moult the transition from the larval
| |
| to the adult form of mouth parts. At each ordinary moult there
| |
| is a short period of quiescence, and this period of quiescence
| |
| would naturally become longer in the important moult at which
| |
| the change in the mouth parts was effected. In this way a
| |
| rudimentary pupa stage might be started. The pupa stage,
| |
| once started, might easily become a more important factor in
| |
| the metamorphosis. If the larva and imago diverged still more
| |
| from each other, a continually increasing amount of change
| |
| would have to be effected at the pupa stage. It would probably
| |
| be advantageous to the species that the larva should not have
| |
| rudimentary functionless wings ; and the establishment of the
| |
| wings as external organs would therefore become deferred to
| |
| the pupa stage. The same would probably apply to other
| |
| organs.
| |
| | |
| Insects usually pass through the pupa stage in winter in cold
| |
| climates and during the dry season in the tropics, this stage
| |
| serving therefore apparently for the protection of the species
| |
| during the inclement season of the year. These facts are easily
| |
| explained on the supposition that the pupa stage has become
| |
| secondarily adapted to play a part in the economy of the
| |
| species quite different from that to which it owes its origin.
| |
| | |
| Heterogamy. The cases of alternations of generations
| |
| amongst Insects all fall under the heading already defined in
| |
| the introduction as Heterogamy. Heterogamy amongst Insects
| |
| has been rendered possible by the existence of parthenogenesis,
| |
| which, as stated in the introduction, has been taken hold of by
| |
| natural selection, and has led to the production of generations of
| |
| parthenogenetic forms, by which a clear economy in reproduction
| |
| is effected. Parthenogenesis without heterogamy occurs in a
| |
| large number of forms. In Bees, Wasps, and a Sawfly (Nematus
| |
| ventricosus) the unfertilized ova give rise to males. In two
| |
| Lepidopterous genera (Psyche and Solenobia) the unfertilized
| |
| ova give rise mainly, if not entirely, to females. Heterogamy
| |
| occurs in none of the above types, but in Psyche and Solenobia
| |
| males are only occasionally found, so that a series of generations
| |
| producing female young from unfertilized ova are followed by a
| |
| generation producing young of both sexes from fertilized ova. It
| |
| | |
| | |
| | |
| 428 INSECTA.
| |
| | |
| | |
| | |
| would be interesting to know if the unimpregnated female would
| |
| not after a certain number of generations give rise to both males
| |
| and females ; such an occurrence might be anticipated on
| |
| grounds of analogy. In the cases of true heterogamy parthenogenesis has become confined to special generations, which differ
| |
| in their character from the generations which reproduce themselves sexually. The parthenogenetic generations generally
| |
| flourish during the season when food is abundant; while the
| |
| sexual generations occur at intervals which are often secondarily
| |
| regulated by the season, supply of food, etc.
| |
| | |
| A very simple case of this kind occurs, if we may trust the
| |
| recent researches of Lichtenstein 1 , in certain Gall Insects
| |
| (Cynipidae). He finds that the female of a form known as
| |
| Spathegaster baccarum, of which both males and females are
| |
| plentiful, pricks a characteristic gall in certain leaves, in which
| |
| she deposits the fertilized eggs. The eggs from these galls give
| |
| rise to a winged and apparently adult form, which is not, however, Spathegaster, but is a species considered to belong to a
| |
| distinct genus known as Neuroterus ventricularis. Only females
| |
| of Neuroterus are found, and they lay unfertilized ova in peculiar
| |
| galls which develop into Spathegaster baccarum. Here we have
| |
| a true case of heterogamy, the females which produce parthenogenetically having become differentiated from those which produce sexually. Another interesting type of heterogamy is that
| |
| which has been long known in the Aphides. In the autumn
| |
| impregnated eggs are deposited by females, which give rise in
| |
| the course of the spring to females which produce parthenogenetically and viviparously. The viviparous females always
| |
| differ from the females which lay the fertilized eggs. The generative organs are of course differently constituted, and the ova of
| |
| the viviparous females are much smaller than those of the oviparous females, as is generally the case in closely allied viviparous and oviparous forms; but in addition the former are
| |
| usually without wings, while the latter are winged. The reverse
| |
| is however occasionally the case. An indefinite number of generations of viviparous females may be produced if they are artificially kept warm and supplied with food ; but in the course of
| |
| | |
| 1 Petites Nouvelles Entomolog iyues, May, 1878.
| |
| | |
| | |
| | |
| TRACHEATA. 429
| |
| | |
| | |
| | |
| nature the viviparous females produce in the autumn males and
| |
| females which lay eggs with firm shells, and so preserve the
| |
| species through the winter. The heterogamy of the allied
| |
| Coccidae is practically the same as that of the Aphidae. In the
| |
| case of Chermes and Phylloxera the parthenogenetic generations
| |
| lay their eggs in the normal way.
| |
| | |
| The complete history of Phylloxera quercus has been worked
| |
| out by Balbiani (No. 401). The apterous females during the
| |
| summer lay eggs developing parthenogenetically into apterous
| |
| females, which continue the same mode of reproduction. In the
| |
| autumn, however, the eggs which are laid give rise in part to
| |
| winged forms and in part to apterous forms. Both of these
| |
| forms lay smaller and larger eggs, which develop respectively
| |
| into very minute males and females without digestive organs.
| |
| The fertilized eggs laid by these forms probably give rise to the
| |
| parthenogenetic females.
| |
| | |
| A remarkable case of heterogamy accompanied by paedogenesis was discovered by Wagner to take place in certain
| |
| species of Cecydomyia (Miastor), a genus of the Diptera. The
| |
| female lays a few eggs in the bark of trees, etc. These eggs
| |
| develop in the winter into larvae, in which ovaries are early
| |
| formed. The ova become detached into the body cavity,
| |
| surrounded by their follicles, and grow at the cost of the
| |
| follicles. They soon commence to undergo a true development,
| |
| and on becoming hatched they remain for some time in the
| |
| body cavity of the parent, and are nourished at the expense of
| |
| its viscera. They finally leave the empty skin of their parent,
| |
| and subsequently reproduce a fresh batch of larvae in the same
| |
| way. After several generations the larvae undergo in the
| |
| following summer a metamorphosis, and develop into the sexual
| |
| form.
| |
| | |
| Another case of paedogenesis is that of the larvae of Chironomus, which have been shewn by Grimm (No. 413) to lay eggs
| |
| which develop exactly in the same way as fertilized eggs into
| |
| larvae.
| |
| | |
| BIBLIOGRAPHY.
| |
| | |
| (401) M. Balbiani. " Observations s. la reproduction d. Phylloxera du Chene."
| |
| An. Sc. Nat. Ser. v. Vol. xix. 1874.
| |
| | |
| | |
| | |
| 430 INSECTA.
| |
| | |
| | |
| | |
| (402) E. Bess els. " Studien u. d. Entwicklung d. Sexualdriisen bei den Lepidoptera." Ztit.f. wiss. Zool. Bd. xvii. 1867.
| |
| | |
| (403) Alex. Brandt. "Beitrage zur Entwicklungsgeschichte d. Libellulida u.
| |
| Hemiptera, mil besonderer Berucksichtigung d. Embryonalhiillen derselben." Mem.
| |
| Ac. Petersbourg, Ser. vn. Vol. xm. 1869.
| |
| | |
| (404) Alex. Brandt. Ueber das Ei u. seine Bildungsstdttt. Leipzig, 1878.
| |
| | |
| (405) O. Biitschli. "Zur Entwicklungsgeschichte d. Biene." Zeit. f. wiss.
| |
| Zool. Bd. xx. 1870.
| |
| | |
| (406) H. Dewitz. "Bau u. Entwicklung d. Stachels, etc." Zeit.f. wiss. Zool.
| |
| Vols. xxv. and xxvin. 1875 and 1877.
| |
| | |
| (407) H. Dewitz. "Beitrage zur Kenntniss d. Postembryonalentwicklung d.
| |
| Gliedmassen bei den Insecten." Zeit.f. wiss. Zool. xxx. Supplement. 1878.
| |
| | |
| (408) A. Dohrn. "Notizen zur Kenntniss d. Insectenentwicklung." Zeitschrift
| |
| f. wiss. Zool. Bd. xxvi. 1876.
| |
| | |
| (409) M. Fabre. " L'hypermetamorphose et lesmoeursdes Meloides." An.Sci.
| |
| Nat. Series iv. Vol. vn. 1857.
| |
| | |
| (410) Ganin. " Beitrage zur Erkenntniss d. Entwicklungsgeschichte d. Insecten."
| |
| Zeit.f. wiss. Zool. Bd. xix. 1869.
| |
| | |
| (411) V. Graber. Die Insecten. MUnchen, 1877.
| |
| | |
| (412) V. Graber. "Vorlauf. Ergeb. lib. vergl. Embryologie d. Insecten."
| |
| Archivf. mikr. Anat. Vol. XV. 1878.
| |
| | |
| (413) O. v. Grimm. " Ungeschlechtliche Fortpflanzung einer Chironomus Art-u.
| |
| deren Entwicklung aus dem unbefruchteten Ei." Mem. Acad. Petersbourg. 1870.
| |
| | |
| (414) B. Hatschek. " Beitrage zur Entwicklung d. Lepidopteren." Jenaische
| |
| Zeitschrift, Bd. XI.
| |
| | |
| (415) A. K 6 1 1 i k e r. " Observationes de prima insectorum genese, etc. " Ann. Sc.
| |
| Nat. Vol. xx. 1843.
| |
| | |
| (416) A. Kowalevsky. " Embryologische Studien an Wurmern u. Arthropoden."
| |
| Mem. Ac. imp. Petersbourg, Ser. vn. Vol. xvi. 1871.
| |
| | |
| (417) C. Kraepelin. 4 ' Untersuchungen Ub. d. Bau, Mechanismus u. d. Entwick. des Stachels d. bienartigen Thiere." Zeit.f. wiss. Zool. Vol. xxni. 1873.
| |
| | |
| (418) C. Kupffer. "Faltenblatt an d. Embryonen d. Gattung Chironomus."
| |
| Arch.f. mikr. Anat. Vol. u. 1866.
| |
| | |
| (419) R. Leuckart. Zur Kenntniss d. Generationswechsels u. d. Parthenogenese
| |
| b. d. Insecten. Frankfurt, 1858.
| |
| | |
| (420) Lubbock. Origin and Metamorphosis of Insects. 1874.
| |
| | |
| (421) Lubbock. Monograph on Collembola and Thysanura. Ray Society, 1873.
| |
| | |
| (422) Melnikow. " Beitrage z. Embryonalentwicklung d. Insecten." Archiv
| |
| f. Naturgeschichte, Bd. xxxv. 1869.
| |
| | |
| (423) E. Metschnikoff. "Embryologische Studien an Insecten." Zeit. f.
| |
| wiss. Zool. Bd. xvi. 1866.
| |
| | |
| (424) P. Meyer. "Ontogenie und Phylogenie d. Insecten." Jenaische Zeitschrift, Vol. x. 1876.
| |
| | |
| (425) FritzMiiller. " Beitrage z. Kenntniss d. Termiten." Jenaische Zeitschrift, Vol. IX. 1875.
| |
| | |
| (426) A. S. Packard. " Embryological Studies on Diplex, Perithemis, and
| |
| the Thysanurous genus Isotoma." Mem. Peabody Acad. Science, I. i. 1871.
| |
| | |
| (427) Suckow. " Geschlechtsorgane d. Insecten." Ileusinger's Zeitschrift f.
| |
| organ. Physik, Bd. n. 1828.
| |
| | |
| | |
| | |
| TRACHEATA.
| |
| | |
| | |
| | |
| 431
| |
| | |
| | |
| | |
| (428) Tichomiroff. " Ueber die Entwicklungsgeschichte des Seidenwiirms."
| |
| Zoologischer Anzeiger, n. Jahr. No. 20 (Preliminary Notice).
| |
| | |
| (429) Aug. Weismann. "Zur Embryologie d. Insecten." Archiv f. Anat.
| |
| und Phys. 1864.
| |
| | |
| (430) Aug. Weismann. " Entwicklung d. Dipteren." Zeit. f. wiss. Zool.
| |
| Vols. xin. and xiv. Leipzig, 1863 4.
| |
| | |
| (431) Aug. Weismann. " Die Metamorphose d. Corethra plumicornis. " Zeit.
| |
| f. wiss. Zool. Vol. xvi. 1866.
| |
| | |
| (432) N. Wagner. "Beitrag z. Lehre d. Fortpflanzung d. Insectenlarven."
| |
| Zeit.f. wiss. Zool. Vol. xin. 1860.
| |
| | |
| (433) Zaddach. Untersuchungen iib. d. Bau u. d. Entwicklungd. Gliederthiere.
| |
| Berlin, 1854.
| |
| | |
| ARACHNIDA 1 .
| |
| | |
| The development of several divisions of this interesting
| |
| group has been worked out ; and it will be convenient to deal in
| |
| the first instance with the special history of each of these
| |
| divisions, and then to treat in a
| |
| separate section the development of the organs for the
| |
| whole group.
| |
| | |
| Scorpionidae. The embryonic development always takes
| |
| place within the female Scorpion. In Buthus it takes place
| |
| within follicle-like protuberances of the wall of the ovary.
| |
| In Scorpio also development
| |
| commences while the egg is
| |
| still in the follicle, but when the
| |
| trunk becomes segmented the
| |
| embryo passes into the ovarian
| |
| tube. The chief authority for
| |
| the development of the Scorpionidae is Metschnikoff (No. 434).
| |
| | |
| At the pole of the ovum facing the ovarian tube there is
| |
| | |
| | |
| | |
| | |
| | |
| FIG. 193. OVUM OF SCORPION WITH
| |
| THE ALREADY -FORMED BLASTODERM
| |
| SHEWING THE PARTIAL SEGMENTATION.
| |
| (After Metschnikoff.)
| |
| | |
| bl. blastoderm.
| |
| | |
| | |
| | |
| 1 The classification of the Arachnida adopted in the present work is shewn below.
| |
| c Scorpionidse. . . ( Tetrapneumones.
| |
| | |
| Pedipalpi. IL Aranema - JDipneumones.
| |
| | |
| I. ArthrOgastra. \ Pseudoscorpionidae.
| |
| | |
| I Soiifugse. in. Acarina,
| |
| | |
| ^ Phalangidse.
| |
| | |
| | |
| | |
| 432
| |
| | |
| | |
| | |
| SCORPIONID^E.
| |
| | |
| | |
| | |
| formed a germinal disc which undergoes a partial segmentation
| |
| (fig. 193 bl). A somewhat saucer-shaped one-layered blastoderm is then formed, which soon becomes thickened in the
| |
| centre and then divided into two layers. The outer of these
| |
| is the epiblast, the inner the mesoblast. Beneath the mesoblast
| |
| there subsequently appear granular cells, which form the
| |
| commencement of the hypoblast 1 .
| |
| | |
| During the formation of the blastoderm a cellular envelope is formed
| |
| round the embryo. Its origin is doubtful, though it is regarded by
| |
| Metschnikoff as probably derived from the blastoderm and homologous
| |
| with the amnion of Insects. It becomes double in the later stages (fig. 195).
| |
| | |
| During the differentiation of the three embryonic layers the
| |
| germinal disc becomes somewhat pyriform, the pointed end
| |
| being the posterior. At this extremity there is a special thickening which is perhaps
| |
| equivalent to the primitive cumulus of Spiders.
| |
| The germinal disc continues gradually to spread
| |
| over the yolk, but the
| |
| original pyriform area is
| |
| thicker than the remainder, and is marked off
| |
| anteriorly and posteriorly by a shallow furrow.
| |
| It constitutes a structure
| |
| corresponding with the
| |
| ventral plate of other
| |
| Tracheata. It soon becomes grooved by a FIG. 194. THREE SURFACE VIEWS OF THE
| |
| | |
| . A ,. , f VENTRAL PLATE OF A DEVELOPING SCORPION.
| |
| | |
| shallow longitudinal fur- (After Metschnikoff.)
| |
| | |
| A. Before segmentation.
| |
| | |
| B. After five segments have become formed.
| |
| | |
| C. After the appendages have begun to be
| |
| | |
| formed.
| |
| | |
| | |
| | |
| | |
| row (fig. 194 A) which
| |
| subsequently becomes
| |
| less distinct. It is then
| |
| divided by two transverse lines into three parts 2 .
| |
| | |
| | |
| | |
| 1 The origin of the hypoblast cells, if such these cells are, is obscure. Metschnikoff
| |
| doubtfully derives them from the blastoderm cells ; from my investigations on Spiders
| |
| it appears to me more probable that they originate in the yolk.
| |
| | |
| * The exact fate of the three original segments is left somewhat obscure by
| |
| | |
| | |
| | |
| TRACHEATA.
| |
| | |
| | |
| | |
| 433
| |
| | |
| | |
| | |
| In succeeding stages the anterior of the three parts is clearly
| |
| marked out as the procephalic lobe, and soon becomes somewhat
| |
| broader. Fresh segments are added from before backwards,
| |
| and the whole ventral plate increases rapidly in length (fig.
| |
| 194 B).
| |
| | |
| When ten segments have become formed, appendages appear
| |
| as paired outgrowths of the nine posterior segments (fig. 194 C).
| |
| The second segment bears the pedipalpi, the four succeeding
| |
| segments the four ambulatory appendages, and the four hindermost segments smaller provisional appendages which subsequently disappear, with the possible exception of the second.
| |
| The foremost segment, immediately behind the procephalic
| |
| lobes, is very small, and still without a rudiment of the chelicerae, which are subsequently formed on it. It would appear
| |
| from Metschnikoff's figures to
| |
| be developed later than the
| |
| other post-oral segments present at this stage. The still
| |
| unsegmented tail has become
| |
| very prominent and makes an
| |
| angle of 180 with the remainder of the body, over the
| |
| ventral surface of which it is
| |
| flexed.
| |
| | |
| By the time that twelve
| |
| segments are definitely formed, the procephalic region is
| |
| distinctly bilobed, and in the
| |
| median groove extending
| |
| along it the stomodaeum has
| |
| become formed (fig. 196 A).
| |
| The chelicerae (ck) appear as
| |
| small rudiments on the first
| |
| post-oral segment, and the
| |
| | |
| | |
| | |
| | |
| FlG. 195. A FAIRLY-ADVANCED EMBRYO OF THE SCORPION ENVELOPED IN
| |
| | |
| ITS MEMBRANES. (After Metschnikoff. )
| |
| | |
| ch. chelicerae ; pd. pedipalpi ; p^p 4 .
| |
| ambulatory appendages ; al>. post-abdomen
| |
| (tail).
| |
| | |
| | |
| | |
| Metschnikoff. He believes however that the anterior segment forms the procephalic
| |
| lobes, the posterior probably the telson and five adjoining caudal segments, and
| |
| the middle one the remainder of the body. This view does not appear to me quite
| |
| satisfactory, since on the analogy of Spiders and other Arthropoda the fresh somites
| |
| ought to be added by a continuous segmentation of the posterior lobe.
| |
| | |
| B. II. 28
| |
| | |
| | |
| | |
| | |
| | |
| | |
| 434 1 SEUDOSCORPIONID^E.
| |
| | |
| nerve cords are distinctly differentiated and ganglionated. In
| |
| the embryonic state there is one ganglion for each segment.
| |
| The ganglion in the first segment (that bearing the chelicerse) is
| |
| very small, but is undoubtedly post-oral.
| |
| | |
| At this stage, by a growth in which all the three germinal
| |
| layers have a share, the yolk is completely closed in by the
| |
| blastoderm. It is a remarkable fact with only few parallels, and
| |
| those amongst the Arthropoda, that the blastopore, or point
| |
| where the embryonic membranes meet in closing in the yolk, is
| |
| situated on the dorsal surface of the embryo.
| |
| | |
| The general relations of the embryo at about this stage are
| |
| shewn in fig. 195, where the embryo enclosed in its double
| |
| cellular membrane is seen in a side view. This embryo is about
| |
| the same age as that seen from the ventral surface in fig. 196 A.
| |
| | |
| The general nature of the further changes may easily be
| |
| gathered from an inspection of fig. 196 B and C, but a few
| |
| points may be noted.
| |
| | |
| An upper lip or labrum is formed as an unpaired organ in
| |
| the line between the procephalic lobes. The pedipalpi become
| |
| chelate before becoming jointed, and the chelicerae also early
| |
| acquire their characteristic form. Rudimentary appendages
| |
| appear on the six segments behind the ambulatory legs, five of
| |
| which are distinctly shewn in fig. 195 ; they persist only on the
| |
| second segment, where they appear to form the comb-like
| |
| organs or pectines. The last abdominal segment, Le. that next
| |
| the tail, is without provisional appendages. The embryonic tail
| |
| is divided into six segments including the telson (fig. 196 C, ab).
| |
| The lungs (st) are formed by paired invaginations, the walls of
| |
| which subsequently become plicated, on the four last segments
| |
| which bear rudimentary limbs, and simultaneously with the
| |
| disappearance of the rudimentary limbs.
| |
| | |
| PseudoscorpionidaB. The development of Qielifer has been investigated by Metschnikoff (436), and although (except that it is provided
| |
| with tracheae instead of pulmonary sacks) it might be supposed to be closely
| |
| related to Scorpio, yet in its development is strikingly different.
| |
| | |
| The eggs after being laid are carried by the female attached to the first
| |
| segment of the abdomen. The segmentation (vide p. 93) is intermediate
| |
| between the types of complete and superficial segmentation. The ovum,
| |
| mainly formed of food-yolk, divides into two, four, and eight equal segments
| |
| | |
| | |
| | |
| TRACHEATA.
| |
| | |
| | |
| | |
| 435
| |
| | |
| | |
| | |
| (fig. 197 A). There then appear one or more clear segments on the surface
| |
| of these, and finally a complete layer of cells is formed round the central
| |
| yolk spheres (fig. 197 B), which latter subsequently agglomerate into a
| |
| central mass. The superficial cells form what may be called a blastoderm,
| |
| which soon becomes divided into two layers (fig. 197 C). There now
| |
| appears a single pair of appendages (the pedipalpi) (fig. 198 A,/^/), while at
| |
| the same time the front end of the embryo grows out into a remarkable
| |
| proboscis-like prominence a temporary upper lip (concealed in the figure
| |
| | |
| | |
| | |
| | |
| flf
| |
| | |
| | |
| | |
| ab
| |
| | |
| | |
| FIG. 196. THREE STAGES IN THE DEVELOPMENT OF THE SCORPION. THE
| |
| | |
| EMBRYOS ARE REPRESENTED AS IF SEEN EXTENDED ON A PLANE.
| |
| | |
| (After Metschnikoff.)
| |
| | |
| ch. chelicerae ; pd. pedipalpi ; p l />*. ambulatory appendages ; pe. pecten ; st.
| |
| stigmata ; ab. post abdomen (tail).
| |
| | |
| behind the pedipalpus), and the abdomen (ab) becomes bent forwards towards the ventral surface. In this very rudimentary condition, after undergoing an ecdysis, the larva is hatched, although it still remains attached to
| |
| its parent. After hatching it grows rapidly, and becomes filled with a
| |
| peculiar transparent material. The first pair of ambulatory appendages is
| |
| formed behind the pedipalpi and then the three suceeding pairs, while at the
| |
| same time the chelicerae appear as small rudiments in front. External signs
| |
| of segmentation have not yet appeared, but about this period the nervous
| |
| system is formed. The supra-cesophageal ganglia are especially distinct,
| |
| and provided with a central cavity, probably formed by an invagination, as
| |
| in other Arachnida. In the succeeding stages (fig. 198 B) four provisional
| |
| | |
| 282
| |
| | |
| | |
| | |
| ARANETNA.
| |
| | |
| | |
| | |
| pairs of appendages (shewn as small knobs at ati] appear behind the ambulatory feet. The abdomen is bent forwards so as to reach almost to the
| |
| pedipalpi. In the later stages (fig. 198 C) the adult form is gradually
| |
| attained. The enormous upper lip persists for some time, but subsequently
| |
| atrophies and is replaced by a normal labrum. The appendages behind the
| |
| | |
| | |
| | |
| | |
| FIG. igj. SEGMENTATION AND FORMATION OF THE BLASTODERM IN CHELIFER.
| |
| | |
| (After Metschnikoff.)
| |
| | |
| In A the ovum is divided into a number of separate segments. In B a number of
| |
| small cells have appeared (bl) which form a blastoderm enveloping the large yolk
| |
| spheres. In C the blastoderm has become divided into two layers.
| |
| | |
| ambulatory feet atrophy, and the tail is gradually bent back into its final
| |
| position. The segmentation and the gradual growth of the limbs do not call
| |
| for special description, and the formation of the organs, so far as is known,
| |
| agrees with other types.
| |
| | |
| The segmentation of Chthonius is apparently similar to that of Chelifer
| |
| (Stecker, No. 437).
| |
| | |
| Phalangidae. Our knowledge of the development of Phalangium is
| |
| unfortunately confined to the later stages (Balbiani, No. 438). These stages
| |
| do not appear however to differ very greatly from those of true Spiders.
| |
| | |
| Araneina. The eggs of true Spiders are either deposited in
| |
| nests made specially for them, or are carried about by the
| |
| females. Species belonging to a considerable number of genera,
| |
| viz. Pholcus, Epeira, Lycosa, Clubione, Tegenaria and Agelcna
| |
| | |
| | |
| | |
| TRACHEATA.
| |
| | |
| | |
| | |
| 437
| |
| | |
| | |
| | |
| have been studied by Claparede (No. 442), Balbiani (No. 439),
| |
| Barrois (No. 441) and myself (No. 440), and the close similarity
| |
| between their embryos leaves but little doubt that there are no
| |
| great variations in development within the group.
| |
| | |
| The ovum is enclosed in a delicate vitelline membrane,
| |
| enveloped in its turn by a chorion secreted by the walls of the
| |
| oviduct. The chorion is covered by numerous rounded prominences, and occasionally exhibits a pattern corresponding with
| |
| the areas of the cells which formed it. The segmentation has
| |
| already been fully described, pp. 1 18 and 1 19. At its close there
| |
| is present an enveloping blastoderm formed of a single layer of
| |
| large flattened cells. The yolk within is formed of a number of
| |
| | |
| | |
| | |
| ' r .v-ii~-cr^ ^H 1
| |
| | |
| ooo^ o^*afe
| |
| | |
| | |
| | |
| | |
| Cll
| |
| | |
| | |
| | |
| ab
| |
| | |
| | |
| | |
| FIG. 198. THREE STAGES IN THE DEVELOPMENT OF CHELIFKR.
| |
| | |
| (After Metschnikoff.)
| |
| pd. pedipalpi ; ab. abdomen ; an.i. anal invagination ; c/i. chelicerse.
| |
| | |
| large polygonal segments ; each of which is composed of large
| |
| yolk spherules, and contains a nucleus surrounded by a layer of
| |
| protoplasm, which is prolonged into stellate processes holding
| |
| together the yolk spherules. The nucleus, surrounded by the
| |
| major part of the protoplasm of each yolk cell, appears, as a rule,
| |
| | |
| | |
| | |
| 438 ARANEINA.
| |
| | |
| | |
| | |
| to be situated not at the centre, but on one side of its yolk
| |
| segment.
| |
| | |
| The further description of the development of Spiders applies
| |
| more especially to Agelena labyrinthica, the species which
| |
| formed the subject of my own investigations.
| |
| | |
| The first differentiation of the blastoderm consists in the
| |
| cells of nearly the whole of one hemisphere becoming somewhat
| |
| more columnar than those of the other hemisphere, and in the
| |
| cells of a small area near one end of the thickened hemisphere
| |
| becoming distinctly more columnar than elsewhere, and two
| |
| layers thick. This area forms a protuberance on the surface of
| |
| the ovum, originally discovered by Claparede, and called by him
| |
| the primitive cumulus. In the next stage the cells of the
| |
| thickened hemisphere of the blastoderm become still more
| |
| columnar; and a second area, at first connected by a whitish
| |
| streak with the cumulus, makes its appearance. In the second
| |
| area the blastoderm is also more than one cell deep (fig. 199).
| |
| It will be noticed that the blastoderm, though more than one
| |
| cell thick over a large part of the ventral surface, is not divided
| |
| into distinct layers. The second area appears as a white patch
| |
| and soon becomes more distinct, while the streak continued to
| |
| it from the cumulus is no longer visible. It is shewn in surface
| |
| view in fig. 200 A. Though my observations on this stage are
| |
| not quite satisfactory, yet it appears to me probable that there
| |
| is a longitudinal thickened ridge of the blastoderm extending
| |
| from the primitive cumulus to the large white area. The section
| |
| represented in fig. 199, which I believe to be oblique, passes
| |
| through this ridge at its most projecting part.
| |
| | |
| The nuclei of the yolk cells during the above stages multiply
| |
| rapidly, and cells are formed in the yolk which join the blastoderm ; there can however be no doubt that the main increase in
| |
| the cells of the blastoderm has been due to the division of the
| |
| original blastoderm cells.
| |
| | |
| In the next stage I have been able to observe there is, in the
| |
| place of the previous thickened half of the blastoderm, a well
| |
| developed ventral plate with a procephalic lobe in front, a
| |
| caudal lobe behind, and an intermediate region marked by
| |
| about three transverse grooves, indicating a division into
| |
| segments. This plate is throughout two or more rows of
| |
| | |
| | |
| | |
| TRACHEATA.
| |
| | |
| | |
| | |
| 439
| |
| | |
| | |
| | |
| | |
| FIG. 199. SECTION THROUGH THE EMBRYO OF AGELENA LABYRINTHICA.
| |
| | |
| The section is from an embryo of the
| |
| same age as fig. 200 A, and is represented
| |
| with the ventral plate upwards. In the
| |
| ventral plate is seen a keel-like thickening,
| |
| which gives rise to the main mass of the
| |
| mesoblast.
| |
| | |
| yk. yolk divided into large polygonal
| |
| cells, in several of which nuclei are shewn.
| |
| | |
| | |
| | |
| cells thick, and the cells
| |
| which form it are divided into
| |
| two distinct layers a columnar superficial layer of epiblast
| |
| cells, and a deeper layer of
| |
| mesoblast cells (fig. 203 A).
| |
| In the latter layer there are
| |
| several very large cells which
| |
| are in the act of passing from
| |
| the yolk into the blastoderm.
| |
| The identification of the structures visible in the previous
| |
| stage with those visible in
| |
| the present stage is to a
| |
| great extent a matter of
| |
| guess-work, but it appears
| |
| to me probable that the
| |
| primitive cumulus is still present as a slight prominence visible
| |
| in surface views on the caudal lobe, and that the other thickened
| |
| patch persists as the procephalic lobe. However this may be,
| |
| the significance of the primitive cumulus appears to be that it is
| |
| the part of the blastoderm where two rows of cells become first
| |
| established \
| |
| | |
| The whole region of the blastoderm other than the ventral
| |
| plate is formed of a single row of flattened epiblast cells. The
| |
| yolk retains its original constitution.
| |
| | |
| By this stage the epiblast and mesoblast are distinctly
| |
| differentiated, and the homologue of the hypoblast is to be
| |
| sought for in the yolk-cells. The yolk-cells are not however
| |
| entirely hypoblastic, since they continue for the greater part of
| |
| the development to give rise to fresh cells which join the mesoblast.
| |
| | |
| The Spider's blastoderm now resembles that of an Insect
| |
| (except for the amnion) after the establishment of the mesoblast,
| |
| and the mode of origin of the mesoblast in both groups is very
| |
| similar, in that the longitudinal ridge-like thickening of the
| |
| | |
| 1 Various views have been put forward by Claparfede and Balbiani about the
| |
| position and significance of the primitive cumulus. For a discussion of which vide
| |
| self, No. 440.
| |
| | |
| | |
| | |
| 440 AKANEINA.
| |
| | |
| | |
| | |
| mesoblast shewn in fig. 199 is probably the homologue of the
| |
| mesoblastic groove of the Insects' blastoderm.
| |
| | |
| The ventral plate continues to grow rapidly, and at a somewhat later stage (fig. 200 B) there are six segments interposed
| |
| between the procephalic and caudal lobes. The two anterior of
| |
| these (ch and pd), especially the foremost, are less distinct than
| |
| the remainder ; and it is probable that both of them, and in any
| |
| case the anterior one, are formed later than the three segments
| |
| following. These two segments are the segments of the chelicenc
| |
| and pedipalpi. The four segments following belong to the four
| |
| pairs of ambulatory legs. The segments form raised transverse
| |
| bands separated by transverse grooves. There is at this stage a
| |
| faintly marked groove extending along the median line of the
| |
| ventral plate. This groove is mainly caused by the originally
| |
| single mesoblastic plate having become divided throughout the
| |
| whole region of the ventral plate, except possibly the procephalic
| |
| lobes, into two bands, one on each side of the middle line (fig.
| |
| 203 B).
| |
| | |
| The segments continue to increase in number by the continuous addition of fresh segments between the one last formed
| |
| and the caudal lobe. By the stage with nine segments the first
| |
| rudiments of the limbs make their appearance. The first
| |
| rudiments to appear are those of the pedipalpi and four ambulatory limbs : the chelicerae, like the segment to which they
| |
| belong, lag behind in development. The limbs appear as small
| |
| protuberances at the borders of their segments. By the stage
| |
| when they are formed the procephalic region has become
| |
| bilobed, and the two lobes of which it is composed are separated
| |
| by a shallow groove.
| |
| | |
| By a continuous elongation the ventral plate comes to form
| |
| a nearly complete equatorial ring round the ovum, the procephalic and caudal lobes being only separated by a very narrow
| |
| space, the undeveloped dorsal region of the embryo. This is
| |
| shewn in longitudinal section in fig. 204. In this condition the
| |
| embryo may be spoken of as having a dorsal flexure. By the
| |
| time that this stage is reached (fig. 200 C) the full number of
| |
| segments and appendages has become established. There are
| |
| in all sixteen segments (including the caudal lobe). The first
| |
| six of these bear the permanent appendages of the adult ; the
| |
| | |
| | |
| | |
| TRACHEATA. 44!
| |
| | |
| | |
| | |
| next four are provided with provisional appendages ; while the
| |
| last six are without appendages. The further features of this
| |
| stage which deserve notice are (i) the appearance of a shallow
| |
| depression (st) the rudiment of the stomodaeum between the
| |
| hinder part of the two procephalic lobes ; (2) the appearance of
| |
| | |
| | |
| | |
| | |
| FIG. aoo. FOUR STAGES IN THE DEVELOPMENT OF AGELENA LABYRINTHICA.
| |
| | |
| A. Stage when the ventral plate is very imperfectly differentiated, pr.c. primitive
| |
| cumulus.
| |
| | |
| B. Ovum viewed from the side when the ventral plate has become divided into
| |
| six segments, ch. segment of chelicerae imperfectly separated from procephalic lobe ;
| |
| pd. segment of pedipalpi.
| |
| | |
| C. Ventral plate ideally unrolled after the full number of segments and
| |
| appendages are established, st. stomodoeum between the two proe-oral lobes.
| |
| Behind the six pairs of permanent appendages are seen four pairs of provisional
| |
| appendages.
| |
| | |
| D and E. Two views of an embryo at the same stage. D ideally unrolled,
| |
| E seen from the side. st. stomodseum ; ch. chelicerse ; on their inner side is seen
| |
| the ganglion belonging to them. pd. pedipalpi ; pr.p. provisional appendages.
| |
| | |
| raised areas on the inner side of the six anterior appendagebearing segments. These are the rudiments of the ventral
| |
| ganglia. It deserves to be especially noted that the segment of
| |
| | |
| | |
| | |
| 44 2 AKANEINA.
| |
| | |
| | |
| | |
| the chelicera, like the succeeding segments, is provided with
| |
| ganglia ; and that the ganglia of the chelicerae are quite distinct
| |
| from the supra-cesophageal ganglia derived from the procephalic
| |
| lobes. (3) The pointed form of the caudal lobe. In Pholcus
| |
| (Claparede, No. 442) the caudal lobe forms a projecting structure
| |
| which, like the caudal lobe of the Scorpion, bends forward so as
| |
| to face the ventral surface of the part of the body immediately
| |
| in front. In most Spiders such a projecting caudal lobe is not
| |
| found. While the embryo still retains its dorsal flexure considerable changes are effected in its general constitution. The
| |
| appendages (fig. 200 D and E) become imperfectly jointed, and
| |
| grow inwards so as to approach each other in the middle line.
| |
| Even in the stage before this, the ventral integument between
| |
| the rudiments of the ganglia had become very much thinner,
| |
| and had in this way divided the ventral plate into two halves.
| |
| At the present stage the two halves of the ventral plate are still
| |
| further separated, and there is a wide space on the ventral side
| |
| only covered by a delicate layer of epiblast. This is shewn in
| |
| surface view (fig. 200 D) and in section in fig. 203 C.
| |
| | |
| The stomodaeum (j/) is much more conspicuous, and is
| |
| bounded in front by a prominent upper lip, and by a less
| |
| marked lip behind. The upper lip becomes less conspicuous in
| |
| later stages, and is perhaps to be compared with the provisional
| |
| upper lip of Chelifer. Each procephalic lobe is now marked by
| |
| a deep semicircular groove.
| |
| | |
| The next period in the development is characterised by the
| |
| gradual change in the flexure of the embryo from a dorsal to a
| |
| ventral one ; accompanied by the division of the body into an
| |
| abdomen and cephalo-thorax, and the gradual assumption of the
| |
| adult characters.
| |
| | |
| The change in the flexure of the embryo is caused by the
| |
| elongation of the dorsal region, which has hitherto been hardly
| |
| developed. Such an elongation increases the space on the
| |
| dorsal surface between the procephalic and caudal regions, and
| |
| therefore necessarily separates the caudal and procephalic lobes ;
| |
| but, since the ventral plate does not become shortened in the
| |
| process, and the embryo cannot straighten itself in the egg-shell,
| |
| it necessarily becomes ventrally flexed.
| |
| | |
| If there were but little food yolk this flexure would naturally
| |
| | |
| | |
| | |
| TRACHEATA. 443
| |
| | |
| | |
| | |
| cause the whole embryo to be bent in so as to have the ventral
| |
| surface concave. But instead of this the flexure is at first confined to the two bands which form the ventral plate. These
| |
| bands, as shewn in fig. 201 A, acquire a true ventral flexure, but
| |
| the yolk forms a projection a kind of yolk sack as Barrois
| |
| (No. 441) calls it distending the thin integument between the
| |
| two ventral bands. This yolk sack is shewn in surface view in
| |
| | |
| | |
| | |
| | |
| FlG. 201. TWO LATE STAGES IN THE DEVELOPMENT OF AGELENA LABYRINTHICA.
| |
| | |
| A. Embryo from the side at the stage when there is a large ventral protuberance
| |
| of yolk. The angle between the line of insertion of the permanent and provisional
| |
| appendages shews the extent of the ventral flexure.
| |
| | |
| B. Embryo nearly ready to be hatched. The abdomen which has not quite
| |
| acquired its permanent form is seen to be pressed against the ventral side of the
| |
| thorax.
| |
| | |
| prJ. procephalic lobe; pd. pedipalpi ; ch. chelicerae ; c,L caudal lobe; pr.p. provisional appendages.
| |
| | |
| fig. 20 1 A and in section in fig. 206. At a later period, when
| |
| the yolk has become largely absorbed, the true nature of the
| |
| ventral flexure becomes quite obvious, since the abdomen of the
| |
| young Spider, while still in the egg, is found to be bent over so
| |
| as to press against the ventral surface of the thorax (fig. 201 B).
| |
| The general character of the changes which take place
| |
| during this period in the development is shewn in fig. 201 A and
| |
| B representing two stages in it. In the first of these stages
| |
| there is no constriction between the future thorax and abdomen.
| |
| | |
| | |
| | |
| 444 ACARINA.
| |
| | |
| | |
| | |
| The four pairs of provisional appendages exhibit no signs of
| |
| atrophy ; and the extent of the ventral flexure is shewn by the
| |
| angle formed between the line of their insertion and that of the
| |
| appendages in front. The yolk has enormously distended the
| |
| integument between the two halves of the ventral plate, as is
| |
| illustrated by the fact that, at a somewhat earlier stage than
| |
| that figured, the limbs cross each other in the median ventral
| |
| line, while at this stage they do not nearly meet The limbs
| |
| have acquired their full complement of joints, and the pedipalpi
| |
| bear a cutting blade on their basal joint.
| |
| | |
| The dorsal surface between the prominent caudal lobe and
| |
| the procephalic lobes forms more than a semicircle. The terga
| |
| are fully established, and the boundaries between them, especially
| |
| in the abdomen, are indicated by transverse markings. A large
| |
| lower lip now bounds the stomodaeum, and the upper lip has
| |
| somewhat atrophied. In the later stage (fig. 201 B) the greater
| |
| part of the yolk has passed into the abdomen, which is now to
| |
| some extent constricted off from the cephalo-thorax. The
| |
| appendages of the four anterior abdominal somites have disappeared, and the caudal lobe has become very small. In front
| |
| of it are placed two pairs of spinning mammillae. A delicate
| |
| cuticle has become established, which is very soon moulted.
| |
| | |
| Acarina. The development of the Acarina, which has been mainly
| |
| investigated by Claparede (No. 446), is chiefly remarkable from the frequent
| |
| occurrence of several larval forms following each other after successive
| |
| ecdyses. The segmentation (vide p. 116) ends in the formation of a blastoderm of a single layer of cells enclosing a central yolk mass.
| |
| | |
| A ventral plate is soon formed as a thickening of the blastoderm, in which
| |
| an indistinct segmentation becomes early observable. In Myobia, which is
| |
| parasitic on the common mouse, the ventral plate becomes divided by five
| |
| constrictions into six segments (fig. 202 A), from the five anterior of which
| |
| paired appendages very soon grow out (fig. 202 B) The appendages are the
| |
| chelicerae (ch} and pedipalpi (pd] and the first three pairs of limbs (p^fi 1 }.
| |
| On the dorsal side of the chelicerae a thickened prominence of the ventral
| |
| plate appears to correspond to the procephalic lobes of other Arachnida.
| |
| The part of the body behind the five primitive appendage-bearing segments
| |
| appears to become divided into at least two segments. In other mites the
| |
| same appendages are formed as in Myobia, but the preceding segmentation
| |
| of the ventral plate is not always very obvious.
| |
| | |
| In Myobia two moultings take place while the embryo is still within the
| |
| primitive egg-shell. The first of these is accompanied by the apparently
| |
| total disappearance of the three pediform appendages, and the complete
| |
| | |
| | |
| | |
| TRACK EAT A.
| |
| | |
| | |
| | |
| 445
| |
| | |
| | |
| | |
| coalescence of the two gnathiform appendages into a proboscis (fig. 202 C).
| |
| The feet next grow out again, and a second ecdysis then takes place. The
| |
| embryo becomes thus inclosed within three successive membranes, viz. the
| |
| original egg-shell and two cuticular membranes (fig. 202 D). After the
| |
| second ecdysis the appendages assume their final form, and the embryo
| |
| leaves the egg as an hexapodous larva. The fourth pair of appendages is
| |
| | |
| | |
| | |
| | |
| FIG. 202. FOUR SUCCESSIVE STAGES IN THE DEVELOPMENT OF MYOBIA MUSCULI.
| |
| (After Claparede.)
| |
| | |
| J 1 j 4 . post-oral segments ; ch. chelicerae ; pd. pedipalpi ; pr. proboscis formed by
| |
| the coalescence of the chelicerse and pedipalpi ; p l , /*, etc. ambulatory appendages.
| |
| | |
| acquired by a post-embryonic metamorphosis. From the proboscis are
| |
| formed the rudimentary palpi of the second pair of appendages, and two
| |
| elongated needles representing the chelicerae.
| |
| | |
| In the cheese mite (Tyroglyphus) the embryo has two ecdyses which are
| |
| not accompanied by the peculiar changes observable in Myobia : the
| |
| cheliceras and pedipalpi fuse however to form the proboscis. The first
| |
| larval form is hexapodous, and the last pair of appendages is formed at a
| |
| subsequent ecdysis.
| |
| | |
| In Atax Bonzi, a form parasitic on Unio, the development and metamorphosis are even more complicated than in Myobia. The first ecdysis
| |
| occurs before the formation of the limbs, and shortly after the ventral plate
| |
| has become divided into segments. Within the cuticular membrane resulting
| |
| from the first ecdysis the anterior five pairs of limbs spring out in the usual
| |
| fashion. They undergo considerable differentiation ; the chelicerae and
| |
| pedipalpi approaching each other at the anterior extremity of the body, and
| |
| the three ambulatory legs becoming segmented and clawed. An oesophagus,
| |
| a stomach, and an oesophageal nerve-ring are also formed. When the larva
| |
| | |
| | |
| | |
| 446 ACARINA.
| |
| | |
| | |
| | |
| has attained this stage the original egg-shell is split into two valves and
| |
| eventually cast off, but the embryo remains enclosed within the cuticular
| |
| membrane shed at the first ecdysis. This cuticular membrane is spoken of
| |
| by Claparede as the deutovum. In the deutovum the embryo undergoes
| |
| further changes ; the chelicerae and pedipalpi coalesce and form the
| |
| proboscis ; a spacious body cavity with blood corpuscles appears ; and the
| |
| alimentary canal enclosing the yolk is formed.
| |
| | |
| The larva now begins to move, the cuticular membrane enclosing it is
| |
| ruptured, and the larva becomes free. It does not long remain active, but
| |
| soon bores its way into the gills of its host, undergoes a fresh moult, and
| |
| becomes quiescent. The cuticular membrane of the moult just effected
| |
| swells up by the absorption of water and becomes spherical. Peculiar
| |
| changes take place in the tissues, and the limbs become, as in Myobia,
| |
| nearly absorbed, remaining however as small knobs. The larva swims
| |
| about as a spherical body within its shell. The feet next grow out afresh,
| |
| and the posterior pair is added. From the proboscis the palpi (of the
| |
| pedipalpi) grow out below. The larva again becomes free, and amongst
| |
| other changes the chelicerae grow out from the proboscis. A further ecdysis,
| |
| with a period of quiescence, intervenes between this second larval form and
| |
| the adult state.
| |
| | |
| The changes in the appendages which appear common to the Mites
| |
| generally are (i) the late development of the fourth pair of appendages, which
| |
| results in the constant occurrence of an hexapodous larva ; and (2) the early
| |
| fusion of the chelicerae and pedipalpi to form a proboscis in which no trace
| |
| of the original appendages can be discerned. In most instances palpi and
| |
| stilets of variable form are subsequently developed in connexion with the
| |
| proboscis, and, as indicated in the above descriptions, are assumed to correspond with the two original embryonic appendages.
| |
| | |
| TJie history of tJie germinal layers.
| |
| | |
| It is a somewhat remarkable fact that each of the groups of
| |
| the Arachnida so far studied has a different form of segmentation. The types of Chelifer and the Spiders are simple modifications of the centrolecithal type, while that of Scorpio, though
| |
| apparently meroblastic, is probably to be regarded in the same
| |
| light (vide p. 120 and p. 434). The early development begins in
| |
| the Scorpion and Spiders with the formation of a ventral plate,
| |
| and there can be but little doubt that Chelifer is provided
| |
| with an homologous structure, though very probably modified,
| |
| owing to the small amount of food-yolk and early period of
| |
| hatching.
| |
| | |
| The history of the layers and their conversion into the organs
| |
| has been studied in the case of the Scorpion (Metschnikoff, No.
| |
| | |
| | |
| | |
| TRACHEATA. 447
| |
| | |
| | |
| | |
| 434), and of the Spiders ; and a close agreement has been found
| |
| to obtain between them.
| |
| | |
| It will be convenient to take the latter group as type, and
| |
| simply to call attention to any points in which the two groups
| |
| differ.
| |
| | |
| The epiblast. The epiblast, besides giving rise to the skin
| |
| (hypodermis and cuticle), also supplies the elements for the
| |
| nervous system and organs of sense, and for the respiratory
| |
| sacks, the stomodaeum and proctodaeum.
| |
| | |
| At the period when the mesoblast is definitely established,
| |
| the epiblast is formed of a single layer of columnar cells in the
| |
| region of the ventral plate, and of a layer of flat cells over other
| |
| parts of the yolk.
| |
| | |
| When about six segments are present the first changes take
| |
| place. The epiblast of the ventral plate then becomes somewhat
| |
| thinner in the median line than at the two sides (fig. 203 B). In
| |
| succeeding stages the contrast between the median and the
| |
| lateral parts becomes still more marked, so that the epiblast
| |
| becomes finally constituted of two lateral thickened bands, which
| |
| meet in front in the procephalic lobes, and behind in the caudal
| |
| lobe, and are elsewhere connected by a very thin layer (fig.
| |
| 203 C). Shortly after the appendages begin to be formed, the
| |
| first rudiments of the ventral nerve-cord become established as
| |
| epiblastic thickenings on the inner side of each of the lateral
| |
| bands. The thickenings of the epiblast of the two sides are
| |
| quite independent, as may be seen in fig. 203 C, vn, taken from a
| |
| stage somewhat subsequent to their first appearance. They are
| |
| developed from before backwards, but either from the first, or in
| |
| any case very soon afterwards, cease to form uniform thickenings,
| |
| but constitute a linear series of swellings the future ganglia
| |
| connected by very short less prominent thickenings of the epiblast (fig. 200 C). The rudiments of the ventral nerve-cord are
| |
| for a long time continuous with the epiblast, but shortly after the
| |
| establishment of the dorsal surface of the embryo they become
| |
| separated from the epiblast and constitute two independent
| |
| cords, the histological structure of which is the same as in other
| |
| Tracheata (fig. 206, vn\
| |
| | |
| The ventral cords are at first composed of as many ganglia
| |
| as there are segments. The foremost pair, belonging to the
| |
| | |
| | |
| | |
| 448
| |
| | |
| | |
| | |
| ARACHNIDA.
| |
| | |
| | |
| | |
| segment of the chelicerae, lie immediately behind the stomodaeum,
| |
| and are as independent of each other as the remaining ganglia.
| |
| Anteriorly they border on the supra-cesophageal ganglia. When
| |
| the yolk sack is formed in connection with the ventral flexure of
| |
| the embryo, the two nerve-cords become very widely separated
| |
| (fig. 206, vn) in their middle region. At a later period, at the
| |
| stage represented in fig. 201 B, they again become approximated
| |
| in the ventral line, and delicate commissures are formed uniting
| |
| | |
| | |
| | |
| | |
| | |
| FIG. 203. TRANSVERSE SECTIONS THROUGH THE VENTRAL PLATE OF AGELBNA
| |
| LABYRINTHICA AT THREE STAGES.
| |
| | |
| A. Stage when about three segments are formed. The mesoblastic plate is not
| |
| divided into two bands.
| |
| | |
| B. Stage when six segments are present (fig. ?oo B). The mesoblast is now
| |
| divided into two bands.
| |
| | |
| C. Stage represented in fig. 200 D. The ventral cords have begun to be formed
| |
| on thickenings of the epiblast, and the limbs are established.
| |
| | |
| ep. epiblast ; me. mesoblast ; me.s. mesoblastic somite ; 7>n. ventral nerve-cord ;
| |
| yk. yolk.
| |
| | |
| the ganglia of the two sides, but there is no trace at this or any
| |
| other period of a median invagination of epiblast between the
| |
| two cords, such as Hatschek and other observers have attempted
| |
| to establish for various Arthropoda and Chaetopoda. At the
| |
| stage represented in fig. 201 A the nerve ganglia are still present
| |
| in the abdomen, though only about four ganglia can be distinguished. At a later stage these ganglia fuse into two continuous
| |
| | |
| | |
| | |
| TRACHEATA. 449
| |
| | |
| | |
| | |
| cords, united however by commissures corresponding with the
| |
| previous ganglia.
| |
| | |
| The ganglia of the chelicerae have, by the stage represented
| |
| in fig. 20 1 B, completely fused with the supra-oesophageal ganglia
| |
| and form part of the oesophageal commissure. The cesophageal
| |
| commissure is however completed ventrally by the ganglia of
| |
| the pedipalpi.
| |
| | |
| The supra-cesophageal ganglia are formed independently of
| |
| the ventral cords as two thickenings of the procephalic lobes (fig.
| |
| 205). The thickenings of the two lobes are independent, and
| |
| each of them becomes early marked out by a semicircular groove
| |
| (fig. 200 D) running outwards from the upper lip. Each thickening eventually becomes detached from the superficial epiblast,
| |
| but before this takes place the two grooves become deeper,
| |
| and on the separation of the ganglia from the epiblast, the
| |
| cells lining the grooves become involuted and detached from
| |
| the skin, and form an integral part of the supra-oesophageal
| |
| ganglia.
| |
| | |
| At the stage represented in fig. 201 B the supra-oesophageal ganglia
| |
| are completely detached from the epiblast, and are constituted of the
| |
| following parts : (i) A dorsal section formed of two hemispherical lobes,
| |
| mainly formed of the invaginated lining of the semicircular grooves. The
| |
| original lumen of the groove is still present on the outer side of these
| |
| lobes. (2) Two central masses, one for each ganglion, formed of punctiform tissue, and connected by a transverse commissure. (3) A ventral
| |
| anterior lobe. (4) The original ganglia of the chelicerae, which form the
| |
| ventral parts of the ganglia 1 .
| |
| | |
| The later stages in the development of the nervous system have not
| |
| been worked out.
| |
| | |
| The development of the nervous system in the Scorpion is almost
| |
| identical with that in Spiders, but Metschnikoff believes, though without
| |
| adducing satisfactory evidence, that the median integument between the
| |
| two nerve cords assists in forming the ventral nerve cord. Grooves are
| |
| present in the supra-cesophageal ganglia similar to those in Spiders.
| |
| | |
| The mesoblast. The history of the mesoblast, up to the
| |
| formation of a ventral plate subjacent to the thickened plate of
| |
| epiblast, has been already given. The ventral plate is shewn
| |
| in fig. 203 A. It is seen to be formed mainly of small cells,
| |
| | |
| 1 For further details vide self, No. 440.
| |
| B. II. 29
| |
| | |
| | |
| | |
| 45O ARACHNIDA.
| |
| | |
| | |
| | |
| but some large cells are shewn in the act of passing into it
| |
| from the yolk. During a considerable section of the subsequent development the mesoblast is confined to the ventral
| |
| plate.
| |
| | |
| The first important change takes place when about six
| |
| somites are established ; the mesoblast then becomes divided
| |
| | |
| | |
| | |
| f/0
| |
| | |
| | |
| | |
| | |
| FIG. 204. LONGITUDINAL SECTION THROUGH AN EMBRYO OF AGELENA
| |
| | |
| LABYRINTHICA.
| |
| | |
| The section is through an embryo of the same age as that represented in fig.
| |
| 200 C, and is taken slightly to one side of the middle line so as to shew the relation
| |
| of the mesoblastic somites to the limbs. In the interior are seen the yolk segments
| |
| and their nuclei.
| |
| | |
| i 16. the segments; pr.l. procephalic lobe ; do. dorsal integument.
| |
| | |
| into two lateral bands, shewn in section in fig. 203 B, which meet
| |
| however in front in the procephalic lobes, and behind in the
| |
| caudal lobes. Very shortly afterwards these bands become
| |
| broken up into a number of parts corresponding to the segments,
| |
| each of which soon becomes divided into two layers, which
| |
| enclose a cavity between them (vide fig. 204 and fig. 207). The
| |
| outer layer (somatic) is thicker and attached to the epiblast,
| |
| and the inner layer (splanchnic) is thinner and mainly, if not
| |
| entirely, derived (in Agelena) from cells which originate in the
| |
| yolk. These structures constitute the mesoblastic somites. In
| |
| the appendage-bearing segments the somatic layer of each of
| |
| them, together with a prolongation of the cavity, is continued
| |
| | |
| | |
| | |
| TRACHEATA.
| |
| | |
| | |
| | |
| 451
| |
| | |
| | |
| | |
| into the appendage (fig. 203 C). Since the cavity of the mesoblastic somites is part of the body cavity, all the appendages
| |
| contain prolongations of the body cavity. Not only is a pair of
| |
| mesoblastic somites formed for each segment of the body, but
| |
| also for the procephalic lobes (fig. 205). The mesoblastic somites
| |
| for these lobes are established somewhat later than for the true
| |
| segments, but only differ from them in the fact that the somites
| |
| of the two sides are united by a median bridge of undivided mesoblast. The development of a somite for the procephalic lobes
| |
| is similar to what has been described by Kleinenberg for Lumbricus (p. 339),
| |
| but must not be
| |
| necessarily supposed to indicate
| |
| that the procephalic lobes form a
| |
| segment equivalent to the segments of the trunk.
| |
| They are -rather
| |
| equivalent to the
| |
| | |
| | |
| | |
| ce.s
| |
| | |
| | |
| | |
| | |
| FIG. 205. SECTION THROUGH THE PROCEPHALIC
| |
| LOBES OF AN EMBRYO OF AGELENA LABYRINTHICA.
| |
| | |
| The section is taken from an embryo of the same age
| |
| as fig. 200 D.
| |
| | |
| | |
| | |
| Drae oral lobe of g roove
| |
| | |
| | |
| | |
| stomodseum ; gr. section through semi-circular
| |
| procephalic lobe ; ce.s. cephalic section of body
| |
| cavitv.
| |
| | |
| Chaetopod larvae.
| |
| When the dorsal surface of the embryo is established a thick
| |
| layer of mesoblast becomes formed below the epiblast. This
| |
| layer is not however derived from an upgrowth of the mesoblast
| |
| of the somites, but from cells which originate in the yolk. The
| |
| first traces of the layer are seen in fig. 204, do, and it is fully
| |
| established as a layer of large round cells in the stage shewn in
| |
| fig. 206. This layer of cells is seen to be quite independent of
| |
| the mesoblastic somites (ine.s). The mesoblast of the dorsal
| |
| surface becomes at the stage represented in fig. 201 B divided
| |
| into splanchnic and somatic layers, and in the abdomen at any
| |
| rate into somites continuous with those of the ventral part of the
| |
| mesoblast. At the lines of junction of successive somites the
| |
| splanchnic layer of mesoblast dips into the yolk, and forms a
| |
| number of transverse septa, which do not reach the middle of
| |
| the yolk, but leave a central part free, in which the mesenteron
| |
| is subsequently formed. At the insertion of these septa there
| |
| | |
| 29 2
| |
| | |
| | |
| | |
| 452
| |
| | |
| | |
| | |
| ARACHNIDA.
| |
| | |
| | |
| | |
| me.s
| |
| | |
| | |
| | |
| are developed widish spaces between the layers of somatic
| |
| and splanchnic mesoblast, which form transversely directed
| |
| channels passing
| |
| from the heart outwards. They are
| |
| probably venous.
| |
| At a later stage
| |
| the septa send out
| |
| lateral offshoots,
| |
| and divide the
| |
| peripheral part of
| |
| the abdominal cavity into a number
| |
| of compartments
| |
| filled with yolk. It
| |
| is probable that
| |
| the hepatic diverticula are eventually
| |
| formed in these
| |
| compartments.
| |
| | |
| The somatic
| |
| layer of mesoblast
| |
| | |
| | |
| | |
| | |
| FIG. 206. TRANSVERSE SECTION THROUGH THE THORACIC REGION OF AN EMBRYO OF AGELENA LABYRINTHICA.
| |
| | |
| The section is taken from an embryo of the same age
| |
| as fig. 201 A, and passes through the maximum protuberance of the ventral yolk sack.
| |
| | |
| vn. ventral nerve cord ; yk. yolk ; me.s. mesoblastic
| |
| somite ; ao. aorta.
| |
| | |
| | |
| | |
| is converted into the muscles, both of the limbs and trunk, the
| |
| superficial connective tissue, nervous sheath, etc. It probably
| |
| also gives rise to the three muscles attached to the suctorial
| |
| apparatus of the oesophagus.
| |
| | |
| The heart and aorta are formed as a solid rod of cells of the
| |
| dorsal mesoblast, before it is distinctly divided into splanchnic
| |
| and somatic layers. Eventually the central cells of the heart
| |
| become blood corpuscles, while its walls are constituted of an
| |
| outer muscular and inner epithelioid layer. It becomes functional, and acquires its valves, arterial branches, etc., by the
| |
| stage represented in fig. 201 B.
| |
| | |
| The history of the mesoblast, more especially of the mesoblastic somites,
| |
| of the Scorpion is very similar to that in Spiders : their cavity is continued
| |
| in the same way into the limbs. The general character of the somites
| |
| in the tail is shewn in fig. 207. The caudal aorta is stated by MetschnikofT
| |
| to be formed from part of the mesenteron, but this is too improbable to be
| |
| accepted without further confirmation.
| |
| | |
| | |
| | |
| TRACHEATA.
| |
| | |
| | |
| | |
| 453
| |
| | |
| | |
| | |
| The hypoblast and alimentary tract. It has already
| |
| been stated that the yolk is to be regarded as corresponding to
| |
| the hypoblast of other types.
| |
| | |
| For a considerable period it is composed of the polygonal
| |
| yolk cells already described and shewn in figs. 203, 204, and 205.
| |
| The yolk cells divide and become somewhat smaller as development proceeds ; but the
| |
| main products of the division
| |
| of the yolk nuclei and the protoplasm around them are undoubtedly cells which join the
| |
| mesoblast (fig. 203 A). The
| |
| permanent alimentary tract is
| |
| formed of three sections, viz.
| |
| stomodaeum, proctodaeum, and
| |
| mesenteron. The stomodaeum
| |
| and proctodaeum are both
| |
| formed before the mesenteron.
| |
| The stomodaeum is formed as
| |
| an epiblastic pit between the
| |
| two procephalic lobes (figs. 200
| |
| and 205, st). It becomes
| |
| deeper, and by the latest stage
| |
| figured is a deep pit lined by a
| |
| cuticle and ending blindly. To
| |
| its hinder section, which forms
| |
| the suctorial apparatus of the adult, three powerful muscles (a
| |
| dorsal and two lateral) are attached.
| |
| | |
| The proctodaeum is formed considerably later than the
| |
| stomodaeum. It is a comparatively shallow involution, which
| |
| forms the rectum of the adult. It is dilated at its extremity, and
| |
| two Malpighian vessels early grow out from it.
| |
| | |
| The mesenteron is formed in the interior of the yolk. Its
| |
| walls are derived from the cellular elements of the yolk, and the
| |
| first section to be formed is the hinder extremity, which appears
| |
| as a short tube ending blindly behind in contact with the proctodaeum, and open to the yolk in front. The later history of the
| |
| mesenteron has not been followed, but it undoubtedly includes
| |
| | |
| | |
| | |
| | |
| FlG. 207. TAIL OF AN ADVANCED EMBRYO OF THE SCORPION TO ILLUSTRATE
| |
| THE STRUCTURE OF THE MESOBLASTIC
| |
| | |
| SOMITES. (After Metschnikoff.)
| |
| | |
| al. alimentary tract; an.i. anal invagination ; ep. epiblast ; me.s. mesoblastic somite.
| |
| | |
| | |
| | |
| 454 ARACHNIDA.
| |
| | |
| | |
| | |
| the whole of the abdominal section of the alimentary canal of
| |
| the adult, except the rectum, and probably also the thoracic
| |
| section. The later history of the yolk which encloses the mesenteron has not been satisfactorily studied, though it no doubt
| |
| gives rise to the hepatic tubes, and probably also to the thoracic
| |
| diverticula of the alimentary tract.
| |
| | |
| The general history of the alimentary tract in Scorpio is much the same
| |
| as in Spiders. The hypoblast, the origin of which as mentioned above is
| |
| somewhat uncertain, first appears on the ventral side and gradually spreads
| |
| so as to envelop the yolk, and form the wall of the mesenteron, from
| |
| which the liver is formed as a pair of lateral outgrowths. The proctodaeum and stomodseum are both short, especially the former (vide fig. 207).
| |
| | |
| Summary and general conclusions.
| |
| | |
| The embryonic forms of Scorpio and Spiders are very
| |
| similar, but in spite of the general similarity of Chelifer to
| |
| Scorpio, the embryo of the former differs far more from that of
| |
| Scorpio than the latter does from Spiders. This peculiarity is
| |
| probably to be explained by the early period at which Chelifer
| |
| is hatched ; and though a more thorough investigation of this
| |
| interesting form is much to be desired, it does not seem probable
| |
| that its larva is a primitive type.
| |
| | |
| The larvae of the Acarina with their peculiar ecdyses are to
| |
| be regarded as much modified larval forms. It is not however
| |
| easy to assign a meaning to the hexapodous stage through
| |
| which they generally pass.
| |
| | |
| With reference to the segments and appendages, some interesting points are brought out by the embryological study of
| |
| these forms.
| |
| | |
| The maximum number of segments is present in the
| |
| Scorpion, in which nineteen segments (not including the praeoral lobes, but including the telson) are developed. Of these the
| |
| first twelve segments have traces of appendages, but the appendages of the six last of these (unless the pecten is an appendage)
| |
| atrophy. In Spiders there are indications in the embryo of
| |
| sixteen segments ; and in all the Arachnida, except the Acarina,
| |
| at the least four segments bear appendages in the embryo
| |
| which are without them in the adult. The morphological bearings of this fact are obvious.
| |
| | |
| | |
| | |
| TRACHEATA. 455
| |
| | |
| | |
| | |
| It deserves to be noted that, in both Scorpio and the Spider,
| |
| the chelicerae are borne in the embryo by the first post-oral
| |
| segment, and provided with a distinct ganglion, so that they
| |
| cannot correspond (as they are usually supposed to do) with the
| |
| antennae of Insects, which are always developed on the prae-oral
| |
| lobes, and never supplied by an independent ganglion.
| |
| | |
| The chelicerae would seem probably to correspond with the
| |
| mandibles of Insects, and the antennae to be absent. In favour
| |
| of this view is the fact that the embryonic ganglion of the
| |
| mandibles of Insects is stated (cf. Lepidoptera, Hatschek, p. 340)
| |
| to become, like the ganglion of the chelicerae, converted into
| |
| part of the cesophageal commissure.
| |
| | |
| If the above considerations are correct, the appendages of
| |
| the Arachnida retain in many respects a very much more primitive condition than those of Insects. In the first place, both the
| |
| chelicerae and pedipalpi are much less differentiated than the
| |
| mandibles and first pair of maxillae with which they correspond.
| |
| In the second place, the first pair of ambulatory limbs must be
| |
| equivalent to the second pair of maxillae of Insects, which, for
| |
| reasons stated above, were probably originally ambulatory. It
| |
| seems in fact a necessary deduction from the arguments stated
| |
| that the ancestors of the present Insecta and Arachnida must
| |
| have diverged from a common stem of the Tracheata at a time
| |
| when the second pair of maxillae were still ambulatory in
| |
| function.
| |
| | |
| With reference to the order of the development of the appendages
| |
| and segments, very considerable differences are noticeable in the different
| |
| Arachnoid types. This fact alone appears to me to be sufficient to prove
| |
| that the order of appearance of the appendages is often a matter of
| |
| embryonic convenience, without any deep morphological significance. In
| |
| Scorpio the segments develop successively, except perhaps the first postoral, which is developed after some of the succeeded segments have
| |
| been formed. In Spiders the segment of the chelicerae, and probably also
| |
| of the pedipalpi, appears later than the next three or four. In both these
| |
| types the segments arise before the appendages, but the reverse appears to
| |
| be the case in Chelifer. The permanent appendages, except the chelicerae,
| |
| appear simultaneously in Scorpions and Spiders. The second pair appears
| |
| long before the others in Chelifer, then the third, next the first, and finally
| |
| the three hindermost.
| |
| | |
| | |
| | |
| 456 ARACHNIDA.
| |
| | |
| | |
| | |
| BIBLIOGRAPHY.
| |
| | |
| Scorpionidcz.
| |
| | |
| (434) El. Metschnikoff. " Embryologie des Scorpions." Zeit.f.wiss. Zool.
| |
| Bd. xxi. 1870.
| |
| | |
| (435) H. Rathke. Reisebemerkungen aus Taurien (Scorpio), Leipzig, 1837.
| |
| | |
| Pseudoscorpionidce.
| |
| | |
| (436) El. Metschnikoff. " Entwicklungsgeschichte d. Chelifer." Zeit.f.wiss.
| |
| Zool., Bd. xxi. 1870.
| |
| | |
| (437) A. Stecker. " Entwicklung der Chthonius-Eier im Mutterleibe und die
| |
| Bildung des Blastoderms." Sitzung. konigl. bohmisch. Gesellschaft Wissensch., 1876,
| |
| 3. Heft, and Aimed, and Mag. Nat. History, 1876, xvm. 197.
| |
| | |
| Phalangida.
| |
| | |
| (438) M. Balbiani. " Memoire sur le developpement des Phalangides." Ann.
| |
| Scien. Nat. Series v. Vol. xvi. 1872.
| |
| | |
| A raneina.
| |
| | |
| (439) M. Balbiani. "Memoire sur le developpement des Araneides." Ann.
| |
| Scien. Nat. Series v. Vol. xvn. 1873.
| |
| | |
| (440) F. M. Balfour. "Notes on the development of the Araneina." Quart.
| |
| Journ. of Micr. Science, Vol. xx. 1880.
| |
| | |
| (441) J. Barrois. " Recherches s. 1. developpement des Araigndes. " Journal
| |
| de 1'Anat. et de la Physiol. 1878.
| |
| | |
| (442) E. Claparede. Recherches s. t evolution des Araignees. Utrecht, 1862.
| |
| | |
| (443) Hero Id. De generatione Araneorum in Ovo. Marburg, 1824.
| |
| | |
| (444) H. Ludwig. "Ueber die Bildung des Blastoderms bei den Spinnen."
| |
| Zeit.f. wiss. Zool., Vol. xxvi. 1876.
| |
| | |
| Acarina.
| |
| | |
| (445) P. van Beneden. " Developpement de 1'Atax ypsilophora." Acad. Bruxelles, t. xxiv.
| |
| | |
| (446) Ed. Claparede. "Studien iiber Acarinen." Zeit.f. wiss. Zool., Bd.
| |
| xvm. 1868.
| |
| | |
| Formation of the layers and the embryonic envelopes in the
| |
| | |
| Tracheata.
| |
| | |
| There is a striking constancy in the mode of formation of
| |
| the layers throughout the group. In the first place the hypoblast is not formed by a process which can be reduced to
| |
| invagination : in other words, there is no gastrula stage.
| |
| | |
| | |
| | |
| TRACHEATA. 457
| |
| | |
| | |
| | |
| Efforts have been made to shew that the mesoblastic groove of Insects
| |
| implies a modified gastrula, but since it is the essence of a gastrula that it
| |
| should directly or indirectly give rise to the archenteron, the groove in
| |
| question cannot fall under this category. Although the mesoblastic groove
| |
| of Insects is not a gastrula, it is quite possible that it is the rudiment of a
| |
| blastopore, the gastrula corresponding to which has now vanished from
| |
| the development. It would thus be analogous to the primitive streak of
| |
| Vertebrates 1 .
| |
| | |
| The growth of the blastoderm over the yolk in Scorpions admits no
| |
| doubt of being regarded as an epibolic gastrula. The blastopore would
| |
| however be situated dorsally, a position which it does not occupy in any
| |
| gastrula type so far dealt with. This fact, coupled with the consideration
| |
| that the partial segmentation of Scorpio can be derived without difficulty
| |
| from the ordinary Arachnidan type (vide p. 120), seems to shew that there
| |
| is no true epibolic invagination in the development of Scorpio.
| |
| | |
| On the formation of the blastoderm traces of two embryonic
| |
| layers are established. The blastoderm itself is essentially the
| |
| epiblast, while the central yolk is the hypoblast. The formation
| |
| of the embryo commences in connection with a thickening of the
| |
| blastoderm, known as the ventral plate. The mesoblast is
| |
| formed as an unpaired plate split off from the epiblast of the
| |
| ventral plate. This process takes place in at any rate two ways.
| |
| In Insects a groove is formed, which becomes constricted off to
| |
| form the mesoblastic plate : in Spiders there is a keel-like
| |
| thickening of the blastoderm, which takes the place of the
| |
| groove.
| |
| | |
| The unpaired mesoblastic plate becomes in all forms very
| |
| soon divided into two mesoblastic bands.
| |
| | |
| The mesoblastic bands are very similar to, and probably
| |
| homologous with, those of Chaetopoda ; but the different modes
| |
| by which they arise in these two groups are very striking, and
| |
| probably indicate that profound modifications have taken place
| |
| in the early development of the Tracheata. In the Chaetopoda
| |
| the bands are from the first widely separated, and gradually
| |
| approach each other ventrally, though without meeting. In the
| |
| Tracheata they arise from the division of an unpaired ventral
| |
| plate.
| |
| | |
| The further history of the mesoblastic bands is nearly the
| |
| | |
| 1 The primitive streak of Vertebrates, as will appear in the sequel, has no connection with the medullary groove, and is the rudiment of the blastopore.
| |
| | |
| | |
| | |
| 458 TRACHEATA.
| |
| | |
| | |
| | |
| same for all the Tracheata so far investigated, and is also very
| |
| much the same as for the Chaetopoda. There is a division into
| |
| somites; each containing a section of the body cavity. In the
| |
| cephalic section of the mesoblastic bands a section of the body
| |
| cavity is also formed. In Arachnida, Myriapoda, and probably
| |
| also Insecta, the body cavity is primitively prolonged into the
| |
| limbs.
| |
| | |
| In Spiders at any rate, and very probably in the other groups
| |
| of the Tracheata, a large part of the mesoblast is not derived
| |
| from the mesoblastic plate, but is secondarily added from the
| |
| yolk-cells.
| |
| | |
| In all Tracheata the yolk-cells give rise to the mesenteron
| |
| which, in opposition, as will hereafter appear, to the mesenteron
| |
| of the Crustacea, forms the main section of the permanent
| |
| alimentary tract.
| |
| | |
| One of the points which is still most obscure in connection
| |
| with the embryology of the Tracheata is the origin of the
| |
| embryonic membranes. Amongst Insects, with the exception
| |
| of the Thysanura, such membranes are well developed. In the
| |
| other groups definite membranes like those of Insects are never
| |
| found, but in the Scorpion a cellular envelope appears to be
| |
| formed round the embryo from the cells of the blastoderm, and
| |
| more or less similar structures have been described in some
| |
| Myriapods (vide p. 390). These structures no doubt further
| |
| require investigation, but may provisionally be regarded as
| |
| homologous with the amnion and serous membrane of Insects.
| |
| In the present state of our knowledge it does not seem easy to
| |
| give any explanation of the origin of these membranes, but they
| |
| may be in some way derived from an early ecdysis.
| |
| | |
| | |
| | |
| CHAPTER XVIII
| |
| | |
| | |
| | |
| CRUSTACEA 1 .
| |
| | |
| | |
| | |
| History of the larval forms 1 '.
| |
| | |
| THE larval forms of the Crustacea appear to have more faithfully preserved their primitive characters than those of almost
| |
| any other group.
| |
| | |
| BRANCHIOPODA.
| |
| | |
| The Branchiopoda, comprising under that term the Phyllopoda and Cladocera, contain the Crustacea with the maximum
| |
| number of segments and the least differentiation of the separate
| |
| appendages. This and other considerations render it probable
| |
| that they are to be regarded as the most central group of the
| |
| Crustaceans, and as in many respects least modified from the
| |
| ancestral type from which all the groups have originated.
| |
| | |
| 1 The following is the classification of the Crustacea employed in the present
| |
| chapter.
| |
| | |
| i Phyllopoda. ( Natantia.
| |
| | |
| I. Branchiopoda. ciadocenu III. Copepoda. Euc P e P da Iparasita.
| |
| | |
| ( Branchiura
| |
| T Nebaliadse. jThoracica.
| |
| | |
| M f Sat- < v - wdi, p a minai ia
| |
| | |
| II. Malacostraca. ] Stomatopoda . ULocephaia.
| |
| | |
| I Cumacese. v. Ostracoda.
| |
| | |
| I Edriophthalmata.
| |
| | |
| 2 The importance of the larval history of the Crustacea, coupled with our comparative ignorance of the formation of the layers, has rendered it necessary for me to
| |
| diverge somewhat from the general plan of the work, and to defer the account of the
| |
| formation of the layers till after that of the larval forms.
| |
| | |
| | |
| | |
| 460 PHYLLOPODA.
| |
| | |
| | |
| | |
| The free larval stages when such exist commence with a
| |
| larval form known as the Nauplius.
| |
| | |
| The term Nauplius was applied by O. F. Muller to certain
| |
| larval forms of the Copepoda (fig. 229) in the belief that they
| |
| were adult.
| |
| | |
| The term has now been extended to a very large number of
| |
| larvae which have certain definite characters in common. They
| |
| are provided (fig. 208 A) with three pairs of appendages, the
| |
| future two pairs of antennae and mandibles. The first pair of
| |
| antennae (an 1 ) is uniramous and mainly sensory in function, the
| |
| second pair of antennae (an*) and mandibles (md) are biramous
| |
| | |
| | |
| | |
| A qn
| |
| | |
| | |
| | |
| | |
| | |
| FlG. 208. TWO STAGES IN THE DEVELOPMENT OF APUS CANCRIFORM1S.
| |
| | |
| (After Claus.)
| |
| | |
| A. Nauplius stage at the time of hatching.
| |
| | |
| B. Stage after first ecdysis.
| |
| | |
| an 1 , and a 2 . First and second antennae ; md. mandible ; MX. maxilla ; /. labrum;
| |
| fr. frontal sense organ ; /. caudal fork ; s. segments.
| |
| | |
| swimming appendages, and the mandibles are without the future
| |
| cutting blade. The Nauplius mandibles represent in fact the
| |
| palp. The two posterior appendages are both provided with
| |
| hook-like prominences on their basal joints, used in mastication.
| |
| The body in most cases is unsegmented, and bears anteriorly a
| |
| single median eye. There is a large upper lip, and an alimentary canal formed of cesophagus, stomach and rectum. The anus
| |
| opens near the hind end of the body. On the dorsal surface
| |
| small folds of skin frequently represent the commencement of a
| |
| dorsal shield. One very striking peculiarity of the Nauplius
| |
| according to Claus and Dohrn is the fact that the second pair
| |
| of antennae is innervated from a sub-oesophageal ganglion. A
| |
| larval form with the above characters occurs with more or less
| |
| frequency in all the Crustacean groups. In most instances it
| |
| | |
| | |
| | |
| CRUSTACEA. 461
| |
| | |
| | |
| | |
| does not exactly conform to the above type, and the divergences
| |
| are more considerable in the Phyllopods than in most other
| |
| groups. Its characters in each case are described in the sequel.
| |
| Phyllopoda. For the Phyllopoda the development of Apus
| |
| cancriformis may conveniently be taken as type (Claus, No. 454).
| |
| The embryo at the time it leaves the egg (fig. 208 A) is somewhat oval in outline, and narrowed posteriorly. There is a
| |
| slight V-shaped indentation behind, at the apex of which is
| |
| situated the anus. The body, unlike that of the typical
| |
| Nauplius, is already divided into two regions, a cephalic and
| |
| post-cephalic. On the ventral side of the cephalic region there
| |
| are present the three normal pairs of appendages. Foremost
| |
| there are the small anterior antennae (an 1 ), which are simple
| |
| unjointed rod-like bodies with two moveable hairs at their
| |
| extremities. They are inserted at the sides of the large upperlip or labrum (/). Behind these are the posterior antennae, which
| |
| are enormously developed and serve as the most important
| |
| larval organs of locomotion. They are biramous, being formed
| |
| of a basal portion with a strong hook-like bristle projecting
| |
| from its inner side, an inner unjointed branch with three bristles,
| |
| and an outer large imperfectly five-jointed branch with five long
| |
| lateral bristles. The hook-like organ attached to this pair of
| |
| appendages would seem to imply that it served in some ancestral
| |
| form as jaws (Claus). This character is apparently universal in
| |
| the embryos of true Phyllopods, and constantly occurs in the
| |
| Copepoda, etc.
| |
| | |
| The third pair of appendages or mandibles (md) is attached
| |
| close below the upper lip. They are as yet unprovided with
| |
| cutting blades, and terminate in two short branches, the inner
| |
| with two and the outer with three bristles.
| |
| | |
| At the front of the head there is the typical unpaired eye.
| |
| On the dorsal surface there is already present a rudiment of the
| |
| cephalic shield, continuous anteriorly with the labrum (/) or
| |
| upper lip, the extraordinary size of which is characteristic of the
| |
| larvae of Phyllopods. The post-cephalic region, which afterwards
| |
| becomes the thorax and abdomen, contains underneath the skin
| |
| rudiments of the five anterior thoracic segments and their
| |
| appendages, and presents in this respect an important variation
| |
| from the typical Nauplius form. After the first ecdysis the
| |
| | |
| | |
| | |
| 462 PHYLLOPODA.
| |
| | |
| | |
| | |
| larva (fig. 208 B) loses its oval form, mainly owing to the elongation of the hinder part of the body and the lateral extension of
| |
| the cephalic shield, which moreover now completely covers over
| |
| the head and has begun to grow backwards so as to cover over
| |
| the thoracic region. At the second ecdysis there appears at its
| |
| side a rudimentary shell gland. In the cephalic region two
| |
| small papillae (fr) are now present at the front of the head close
| |
| to the unpaired eye. They are of the nature of sense organs,
| |
| and may be called the frontal sense papillae. They have been
| |
| shewn by Claus to be of some phylogenetic importance. The
| |
| three pairs of Nauplius appendages have not altered much, but
| |
| a rudimentary cutting blade has grown out from the basal joint
| |
| of the mandible. A gland opening at the base of the antennae
| |
| is now present, which is probably equivalent to the green gland
| |
| often present in the Malacostraca. Behind the mandibles a pair
| |
| of simple processes has appeared, which forms the rudiment of
| |
| the first pair of maxillae (mx).
| |
| | |
| In the thoracic region more segments have been added
| |
| posteriorly, and the appendages of the three anterior segments
| |
| are very distinctly formed. The tail is distinctly forked. The
| |
| heart is formed at the second ecdysis, and then extends to the
| |
| sixth thoracic segment : the posterior chambers are successively
| |
| added from before backwards.
| |
| | |
| At the successive ecdyses which the larva undergoes new
| |
| segments continue to be formed at the posterior end of the body,
| |
| and limbs arise on the segments already formed. These limbs
| |
| probably represent the primitive form of an important type of
| |
| Crustacean appendage, which is of value for interpreting the
| |
| parts of the various malacostracan appendages. They consist
| |
| (fig. 209) of a basal portion (protopodite of Huxley) bearing two
| |
| rami. The basal portion has two projections on the inner side.
| |
| To the outer side of the basal portion there is attached a
| |
| dorsally directed branchial sack (br) (epipodite of Huxley). The
| |
| outer ramus (ex) (exopodite of Huxley) is formed of a single plate
| |
| with marginal setae. The inner one (en) (endopodite of Huxley)
| |
| is four-jointed, and a process similar to those of the basal joint
| |
| is given off from the inner side of the three proximal joints.
| |
| | |
| At the third ecdysis several new features appear in the
| |
| cephalic region, which becomes more prominent in the succeeding
| |
| | |
| | |
| | |
| CRUSTACEA. 463
| |
| | |
| | |
| | |
| stages. In the first place the paired eyes are formed at each side
| |
| | |
| of and behind the unpaired eye, second
| |
| ly the posterior pair of maxillae is
| |
| | |
| formed though it always remains very
| |
| | |
| rudimentary. The shell gland becomes
| |
| | |
| fully developed opening at the base of
| |
| | |
| the first pair of maxillae. The dorsal
| |
| | |
| shield gradually grows backwards till it
| |
| | |
| covers its full complement of segments.
| |
| | |
| After the fifth ecdysis the Nauplius FIG. 209. TYPICAL PHYL
| |
| . , . , , LOPOD APPENDAGE. (Copied
| |
| | |
| appendages undergo a rapid atrophy. f rom ciaus.)
| |
| | |
| The second pair of antennae especially ex. exopodite ; en. endo
| |
| becomes reduced in size, and the man
| |
| | |
| | |
| | |
| dibular palp the primitive Nauplius portion bearing the two proxir . ..... , mal projections is not sharply
| |
| | |
| portion of the mandible is contracted separated from the endopoto a mere rudiment, which eventually dite completely disappears, while the blade is correspondingly enlarged and also becomes toothed. The adult condition is only
| |
| gradually attained after a very large number of successive changes
| |
| of skin.
| |
| | |
| The chief point of interest in the above development is the
| |
| fact of the primitive Nauplius form becoming gradually converted without any special metamorphosis into the adult condition 1 .
| |
| | |
| Branchipus like Apus is hatched as a somewhat modified Nauplius,
| |
| which however differs from that of Apus in the hinder region of the body
| |
| having no indications of segments. It goes through a very similar metamorphosis, but is at no period of its metamorphosis provided with a dorsal
| |
| shield : the second pair of antennae does not abort, and in the male is provided with clasping organs, which are perhaps remnants of the embryonic
| |
| hooks so characteristic of this pair of antennas.
| |
| | |
| The larva of Estheria when hatched has a Nauplius form, a large
| |
| upper lip, caudal fork and single eye. There are two functional pairs of
| |
| swimming appendages the second pair of antennae and mandibles. The
| |
| first pair of antennae has not been detected, and a dorsal mantle to form
| |
| the shell is not developed. At the first moult the anterior pair of
| |
| antennae arises as small stump-like structures, and a small dorsal shield
| |
| is also formed. Rudiments of six or seven pairs of appendages sprout
| |
| | |
| 1 Nothing appears to be known with reference to the manner in which it comes
| |
| about that more than one appendage is borne on each of the segments from the
| |
| eleventh to the twentieth. An investigation of this point would be of some interest
| |
| with reference to the meaning of segmentation
| |
| | |
| | |
| 464
| |
| | |
| | |
| | |
| CLADOCERA.
| |
| | |
| | |
| | |
| out in the usual way, and continue to increase in number at successive
| |
| moults : the shell is rapidly developed. The chief point of interest in
| |
| the development of this form is the close resemblance of the young larva
| |
| to a typical adult Cladocera (Claus). This is shewn in the form of the
| |
| shell, which has not reached its full anterior extension, the rudimentary
| |
| anterior antennae, the large locomotor second pair of antennas, which differ
| |
| however from the corresponding organs in the Cladocera in the presence
| |
| of typical larval hooks. Even the abdomen resembles that of Daphnia.
| |
| These features perhaps indicate that the Cladocera are to be derived
| |
| from some Phyllopod form like Estheria by a process of retrogressive
| |
| metamorphosis. The posterior antennas in the adult Estheria are large
| |
| biramous appendages, and are used for swimming ; and though they
| |
| have lost the embryonic hook, they still retain to a larger extent than
| |
| in other Phyllopod families their Nauplius characteristics.
| |
| | |
| The Nauplius form of the Phyllopods is marked by several
| |
| definite peculiarities. Its body is distinctly divided into a cephalic and post-cephalic region. The upper lip is extraordinarily
| |
| large, relatively very much more so than at the later stages.
| |
| The first pair of antennae is usually rudimentary and sometimes
| |
| even absent ; while the second pair is exceptionally large, and
| |
| would seem to be capable of functioning not only as a swimming
| |
| organ, but even as a masticating organ. A dorsal shield is
| |
| nearly or quite absent.
| |
| | |
| Cladocera. The probable derivation of the Cladocera from a form
| |
| similar to Estheria has already been mentioned, and it might have been
| |
| anticipated that the development would be similar
| |
| to that of the Phyllopods.
| |
| The development of the majority of the Cladocera takes
| |
| place however in the egg,
| |
| and the young when hatched
| |
| closely resembles their parents, though in the egg they
| |
| pass through a Nauplius
| |
| stage (Dohrn). An exception to the general rule is
| |
| however offered by the case
| |
| of the winter eggs of Leptodora, one of the most primitive of the Cladoceran
| |
| | |
| | |
| | |
| | |
| families. The summer eggs after Sars.)
| |
| | |
| | |
| | |
| FIG. 709 A. NAUPLIUS LARVA OF LEPTODORA
| |
| IIYAI.INA FROM wiNTKR EGG. (Copied from Bronn ;
| |
| | |
| | |
| | |
| develop without metamor
| |
| | |
| | |
| ;/'. antenna of first pair; an*, antenna of
| |
| | |
| | |
| | |
| phosis, but Sars (No. 461) second pair; ntd. mandible;/ caudal fork.
| |
| | |
| | |
| | |
| CRUSTACEA.
| |
| | |
| | |
| | |
| 465
| |
| | |
| | |
| | |
| has discovered that the larva leaves the winter eggs in the form of a
| |
| Nauplius (fig. 209). This Nauplius closely resembles that of the Phyllopods.
| |
| The body is elongated and in addition to normal Nauplius appendages
| |
| is marked by six pairs of ridges the indications of the future feet. The
| |
| anterior antennae are as usual small ; the second large and biramous,
| |
| but the masticatory bristle characteristic of the Phyllopods is not present.
| |
| The mandibles are without a cutting blade. A large upper lip and unpaired
| |
| eye are present.
| |
| | |
| The adult form is attained in the same manner as amongst the Phyllopods after the third moult.
| |
| | |
| | |
| | |
| MALACOSTRACA.
| |
| | |
| Owing to the size and importance of the various forms
| |
| included in the Malacostraca, greater attention has been paid to
| |
| their embryology than to that of any other division of the
| |
| Crustacea ; and the proper interpretation of their larval forms
| |
| involves some of the most interesting problems in the whole
| |
| range of Embryology.
| |
| | |
| The majority of Malacostraca pass through a more or less
| |
| complicated metamorphosis, though in the Nebaliadae, the
| |
| Cumaceae, some of the Schizopoda, a few Decapoda (Astacus,
| |
| Gecarcinus, etc.), and in the Edriophthalmata, the larva on
| |
| leaving the egg has nearly the form of the adult. In contradistinction to the lower groups of Crustacea the Nauplius form of
| |
| larva is rare, though it occurs in the case of one of the Schizopods
| |
| (Euphausia, fig. 212), in some of the lower forms of the Decapods
| |
| (Penaeus, fig. 214), and
| |
| perhaps also, though this
| |
| has not been made out, in
| |
| some of the Stomatopoda.
| |
| | |
| In the majority of the
| |
| Decapoda the larva leaves
| |
| the egg in a form known
| |
| as the Zoaea (fig. 210).
| |
| This larval form is
| |
| characterised by the presence of a large cephalo
| |
| thoracic t shield usually FIG. 210. ZO^EAOFTHIAPOLITA. (After'Claus.)
| |
| , ., , , , , mxp*. second maxillipede.
| |
| | |
| armed with lateral, anterior, and dorsal spines. The caudal segments are well de
| |
| B. II. 30
| |
| | |
| | |
| | |
| | |
| 466 SCHIZOPODA.
| |
| | |
| | |
| | |
| veloped, though wit/tout appendages, and the tail, which functions
| |
| in swimming, is usually forked. The six posterior thoracic segments are, on the other hand, rudimentary or non-existent. There
| |
| are seven anterior pairs of appendages shewn in detail in fig. 21 1,
| |
| viz. the two pairs of antennae (At. I. and At. II.), neither of them
| |
| used as swimming organs, the mandibles without a palp (ma 7 ),
| |
| well-developed maxillae (two pairs, mx I and mx 2), and two or
| |
| sometimes (Macrura) three pairs of biramous natatory maxillipeds (mxp I and mxp 2). Two lateral compound stalked eyes
| |
| are present, together with a median Nauplius eye. The heart
| |
| has in the majority of cases only one or two (Brachyura) pairs of
| |
| ostia.
| |
| | |
| The Zoaea larva, though typically developed in the Decapoda,
| |
| is not always present (e.g. Astacus and Homarus), and some
| |
| | |
| | |
| | |
| FIG. 211. THE APPENDAGES OF A CRAB Z<VEA.
| |
| | |
| .-//./. first antenna ; At. I I. second antenna ; md. mandible (without a palp); mx.
| |
| \. first maxilla; mx. i. second maxilla; mxp. \. first maxilliped ; mxp. i. second
| |
| maxilliped.
| |
| | |
| ex. exopodite ; en. endopodite.
| |
| | |
| times occurs in a very modified form. It makes its appearance
| |
| in an altered garb in the ontogeny of some of the other groups.
| |
| | |
| The two Malacostracan forms, amongst those so far studied,
| |
| in which the phylogenetic record is most fully preserved in the
| |
| ontogeny, are Euphausia amongst the Schizopods and Penaeus
| |
| amongst the Decapods.
| |
| | |
| Schizopoda. Euphausia leaves the egg (MetschnikofT, No. 4689)
| |
| as a true Nauplius with only three pairs of appendages, the two hinder
| |
| | |
| | |
| | |
| CRUSTACEA.
| |
| | |
| | |
| | |
| 467
| |
| | |
| | |
| | |
| biramous, and an unsegmented body. The second pair of antennae has not
| |
| however the colossal dimensions so common in the lower types. A mouth is
| |
| present, but the anus is undeveloped.
| |
| | |
| After the first moult three pairs of prominences the rudiments of the
| |
| two maxillae and ist maxillipeds arise behind the Nauplius appendages
| |
| (fig. 212). At the same time an anus appears between the two limbs of
| |
| a rudimentary caudal fork ; and an unpaired eye and upper lip appear in
| |
| front. After another moult (fig. 212) a lower lip is formed (UL) as a
| |
| pair of prominences very similar to true appendages ; and a delicate
| |
| cephalo-thoracic shield also becomes developed. Still later the cutting blade
| |
| of the mandible is formed, and the palp (Nauplius appendage) is greatly
| |
| | |
| | |
| | |
| | |
| FIG. 212. NAUPLIUS OF EUPHAUSIA. (From Glaus; after Metschnikoff.)
| |
| The Nauplius is represented shortly before an ecdysis, and in addition to the
| |
| | |
| proper appendages rudiments of the three following pairs are present.
| |
| | |
| OL. upper lip ; UL. lower lip ; Md. mandible ; MX', and MX", two pairs of
| |
| | |
| maxillae ; mf . maxilliped i .
| |
| | |
| reduced. The cephalo-thoracic shield grows over the front part of the
| |
| embryo, and becomes characteristically toothed at its edge. There are also
| |
| | |
| 302
| |
| | |
| | |
| | |
| 468 SCHIZOPODA.
| |
| | |
| | |
| | |
| two frontal papillae very similar to those already described in the Phyllopod
| |
| larvae. Rudiments of the compound eyes make their appearance, and
| |
| though no new appendages are added, those already present undergo further
| |
| differentiations. They remain however very simple ; the maxillipeds
| |
| especially are very short and resemble somewhat Phyllopod appendages.
| |
| | |
| Up to this stage the tail has remained rudimentary and short, but
| |
| after a further ecdysis (Claus) it grows greatly in length. At the same
| |
| time the cephalo-thoracic shield acquires a short spine directed backwards.
| |
| The larva is now in a condition to which Claus has given the name of
| |
| Protozoasa (fig. 213 A).
| |
| | |
| Very shortly afterwards the region immediately following the segments
| |
| already formed becomes indistinctly segmented, while the tail is still without a trace of segmentation. The region of the thorax proper soon becomes distinctly divided into seven very short segments, while at the same
| |
| time the now elongated caudal region has become divided into its normal
| |
| number of segments (fig. 213 B). By this stage the larva has become
| |
| | |
| | |
| | |
| | |
| FIG. 213. LARVAE OF EUPHAUSIA. (After Claus.) From the side.
| |
| | |
| A. Protozorea larva. B. Zonea larva.
| |
| | |
| mx'. and tux", maxillre I and 2 ; mxp^. maxilliped r.
| |
| | |
| a true Zoaea though differing from the normal Zoaea in the fact that
| |
| the thoracic region is segmented, and in the absence of a second pair of
| |
| maxillipeds.
| |
| | |
| The adult characters are very gradually acquired in a series of successive moults ; the later development of Euphausia resembling in this
| |
| respect that of the Phyllopods. On the other hand Euphausia differs from
| |
| that group in the fact that the abdominal (caudal) and thoracic appendages
| |
| develop as two independent series from before backwards, of which the
| |
| abdominal series is the earliest to attain maturity.
| |
| | |
| | |
| | |
| CRUSTACEA.
| |
| | |
| | |
| | |
| 469
| |
| | |
| | |
| | |
| This is shewn in the following table compiled from Claus' observations.
| |
| | |
| | |
| | |
| LENGTH OF LARVA.
| |
| | |
| | |
| APPENDAGES OF THORACIC
| |
| REGION ; viz. the 2nd and
| |
| 3rd maxilliped and 5 ambu
| |
| latory appendages.
| |
| | |
| | |
| APPENDAGES OF ABDOMEN.
| |
| | |
| | |
| 3 3^ mm.
| |
| | |
| | |
| 2nd maxilliped, rudimentary.
| |
| | |
| | |
| ist abdominal appendage.
| |
| | |
| | |
| 3 4 mm.
| |
| | |
| | |
| 2nd maxilliped, biramous.
| |
| 3rd rudimentary,
| |
| ist and 2nd ambulatory appendages, rudimentary.
| |
| | |
| | |
| 2nd and 3rd abdominal appendages.
| |
| 4th and 5th rudimentary.
| |
| | |
| | |
| 4^ 5 mm.
| |
| | |
| | |
| 3rd maxilliped, biramous.
| |
| | |
| | |
| 4 th, 5th, and 6th fully developed.
| |
| | |
| | |
| 55^ mm.
| |
| | |
| | |
| 3rd and 4th ambulatory appendages.
| |
| | |
| | |
| | |
| | |
| 6 mm.
| |
| | |
| | |
| 5th ambulatory appendage.
| |
| | |
| | |
| | |
| | |
| | |
| All the appendages following the second pair of maxillas are biramous,
| |
| and the first eight of these bear branched gills as their epipodites. It is
| |
| remarkable that the epipodite is developed on all the appendages anteriorly
| |
| in point of time to the outer ramus (exopodite).
| |
| | |
| Although in Mysis there is no free larval stage, and the development
| |
| takes place in a maternal incubatory pouch, yet a stage may be detected
| |
| which clearly corresponds with the Nauplius stage of Euphausia (E. van
| |
| Beneden, No. 465). At this stage, in which only the three Nauplius
| |
| appendages are developed, the Mysis embryo is hatched. An ecdysis
| |
| takes place, but the Nauplius skin is not completely thrown off, and
| |
| remains as an envelope surrounding the larva during its later development.
| |
| | |
| Decapoda. Amongst the Decapoda the larva usually leaves
| |
| the egg in the Zoaea form, but a remarkable exception to this
| |
| general rule is afforded by the case of one or more species of
| |
| Penseus. Fritz M tiller was the first to shew that the larva of
| |
| these forms leaves the egg as a typical Nauplius, and it is
| |
| probable that in the successive larval stages of these forms the
| |
| ancestral history of the Decapoda is most fully preserved 1 .
| |
| | |
| The youngest known larva of Penaeus (fig. 214) has a somewhat oval unsegmented body. There spring from it the three
| |
| typical pairs of Nauplius appendages. The first is uniramous,
| |
| the second and third are biramous, and both of them adapted
| |
| | |
| | |
| | |
| 1 The doubts which have been thrown upon Miiller's observations appear to be
| |
| quite unfounded.
| |
| | |
| | |
| | |
| 470 DECAPODA.
| |
| | |
| | |
| | |
| for swimming, and the third of them (mandibles) is without a
| |
| trace of the future blade. The body has no carapace, and bears
| |
| anteriorly a single median simple eye. Posteriorly it is produced
| |
| into two bristles.
| |
| | |
| After the first moult the larva has a rudiment of a forked
| |
| tail, while a dorsal fold of skin indicates the commencement of
| |
| | |
| | |
| | |
| | |
| FIG. 214. NAUPLIUS STAGE OF PEN^EUS. (After Fritz Miiller.)
| |
| | |
| the cephalo-thoracic shield. A large provisional helmet-shaped
| |
| upper lip like that in Phyllopods has also appeared. Behind
| |
| the appendages already formed there are stump-like rudiments
| |
| of the four succeeding pairs (two pairs of maxillae and two pairs
| |
| of maxillipeds) ; and in a slightly older larva the formation of
| |
| the mandibular blade has commenced, together with the atrophy
| |
| of the palp or Nauplius appendage.
| |
| | |
| Between this and the next observed stage there is possibly a
| |
| slight lacuna. The next stage (fig. 215) at any rate represents
| |
| the commencement of the Zoaea series. The cephalo-thoracic
| |
| shield has greatly grown, and eventually acquires the usual
| |
| dorsal spine. The posterior region of the body is prolonged
| |
| into a tail, which is quite as long as the whole of the remainder
| |
| of the body. The four appendages which were quite functionless
| |
| at the last stage have now sprouted into full activity. The
| |
| | |
| | |
| | |
| CRUSTACEA.
| |
| | |
| | |
| | |
| 471
| |
| | |
| | |
| | |
| region immediately behind them is divided (fig.
| |
| 215) into six segments
| |
| (the six thoracic segments) without appendages, while somewhat
| |
| later the five anterior
| |
| abdominal segments become indicated, but are
| |
| equally with the thoracic
| |
| segments without feet.
| |
| The mode of appearance
| |
| of these segments shews
| |
| that the thoracic and
| |
| abdominal segments develop in regular succession from before backwards (Claus). Of the
| |
| palp of the mandibles,
| |
| as is usual amongst Zosea
| |
| forms, not a trace remains,
| |
| though in the youngest
| |
| Zoaea caught by Fritz
| |
| Miiller a very small rudiment of the palp was present. The
| |
| first pair of antennae is unusually long, and the second pair
| |
| continues to function as a biramous swimming organ ; the
| |
| outer ramus is multiarticulate. The other appendages are fully
| |
| jointed, and the two maxillipeds biramous. On the dorsal
| |
| surface of the body the unpaired eye is still present, but on each
| |
| side of it traces of the stalked eyes have appeared. Frontal
| |
| sense organs like those of Phyllopods are also present.
| |
| | |
| From the Protozoaea form the larva passes into that of a true
| |
| Zoaea with the usual appendages and spines, characterised however by certain remarkable peculiarities. Of these the most
| |
| important are (i) the large size of the two pairs of antennae and
| |
| the retention of its Nauplius function by the second of them ;
| |
| (2) the fact that the appendages of the six thoracic segments
| |
| appear as small biramous Schizopod legs, while the abdominal
| |
| appendages, with the exception of the sixth, are still without
| |
| | |
| | |
| | |
| | |
| FIG.
| |
| | |
| | |
| | |
| 215.
| |
| | |
| | |
| | |
| PROTOZO^EA STAGE OF PEN/EUS.
| |
| (After Fritz Miiller.)
| |
| | |
| | |
| | |
| 472 DECAPODA.
| |
| | |
| | |
| | |
| their swimming feet. The early appearance of the appendages
| |
| of the sixth abdominal segment is probably correlated with
| |
| their natatory function in connection with the tail. As a point
| |
| of smaller importance which may be mentioned is the fact that
| |
| both pairs of maxillae are provided with small respiratory plates
| |
| (exopodites) for regulating the flow of water under the dorsal
| |
| shield. From the Zoaea form the larva passes into a Mysis or
| |
| Schizopod stage (fig. 216), characterised by the thoracic feet and
| |
| maxillipeds resembling in form and function the biramous feet
| |
| of Mysis, the outer ramus being at first in many cases much
| |
| larger than the inner. The gill pouches appear at the base of
| |
| these feet nearly at the same time as the endopodites become
| |
| functional. At the same time the antennae become profoundly
| |
| modified. The anterior antennae shed their long hairs, and from
| |
| the inner side of the fourth joint there springs a new process,
| |
| | |
| | |
| | |
| | |
| FIG. 216. PEN^EUS LARVA IN THE MYSIS STAGE. (After Claus.)
| |
| | |
| which eventually elongates and becomes the inner flagellum.
| |
| The outer ramus of the posterior antennae is reduced to a scale,
| |
| while the flagellum is developed from a stump-like rudiment of
| |
| the inner ramus (Claus). A palp sprouts on the mandible and
| |
| the median eye disappears.
| |
| | |
| The abdominal feet do not appear till the commencement of
| |
| the Mysis stage, and hardly become functional till its close.
| |
| | |
| From the Mysis stage the larva passes quite simply into the
| |
| adult form. The outer ramus of the thoracic feet is more or less
| |
| completely lost. The maxillipeds, or the two anterior pairs at
| |
| any rate, lose their ambulatory function, cutting plates develop
| |
| on the inner side of their basal joints, and the two rami persist
| |
| | |
| | |
| | |
| CRUSTACEA.
| |
| | |
| | |
| | |
| 473
| |
| | |
| | |
| | |
| as small appendages on their outer side. Gill pouches also
| |
| sprout from their outer side.
| |
| | |
| The respiratory plate of the second maxilla attains its full
| |
| development and that on the first maxilla disappears 1 . The
| |
| Nauplius, so far as is known, does not occur in any other
| |
| Decapod form except Penaeus.
| |
| | |
| | |
| | |
| The next most primitive
| |
| larval history known is
| |
| that which appears in the
| |
| Sergestidae. The larval
| |
| history, which has been
| |
| fully elucidated by Claus,
| |
| commences with a Protozoaea form (fig. 217), which
| |
| develops into a remarkable
| |
| Zoaea first described by
| |
| Dohrn as Elaphocaris.
| |
| This develops into a form
| |
| originally described by
| |
| Claus as Acanthosoma,
| |
| and this into a form known
| |
| as Mastigopus (fig. 218)
| |
| from which it is easy to
| |
| pass to the adult.
| |
| | |
| The remarkable Protozoaea (fig. 217) is characterised by the presence on
| |
| the dorsal shield of a frontal, dorsal and two lateral
| |
| spikes, each richly armed
| |
| with long side spines. The
| |
| | |
| | |
| | |
| | |
| FIG. 217. LATEST PROTOZO^A STAGE OF SEK
| |
| GESTES LARVA (ELAPHOCARIS). (After Claus.)
| |
| | |
| | |
| | |
| mxp'" '. third pair of maxillipeds.
| |
| | |
| normal Zoasa appendages are present, and in addition to them a small third
| |
| pair of maxillipeds. The thoracic region is divided into five short rings, but
| |
| the abdomen is unsegmented. The tail is forked and provided with long
| |
| spines. The antennae, like those of Penasus, are long the second pair
| |
| biramous ; the mandibles unpalped. Both pairs of maxillae are provided
| |
| with respiratory plates ; the second pair is footlike, and has at its base a
| |
| glandular mass believed by Claus to be the equivalent of the Entomostracan
| |
| shell-gland. The maxillipeds have the usual biramous characters. A
| |
| | |
| | |
| | |
| 1 From Claus' observations (No. 448) it would appear that the respiratory plate
| |
| is only the exopodite and not, as is usually stated, the coalesced exopodite and
| |
| epipodite. Huxley in his Comparative Anatomy reserves this point for embryological
| |
| elucidation.
| |
| | |
| | |
| | |
| 474
| |
| | |
| | |
| | |
| DECAPOD A.
| |
| | |
| | |
| | |
| | |
| FIG. 218. MASTIGOPUS STAGE
| |
| OF SERGESTES. (From Claus.)
| |
| Mf". maxilliped 3.
| |
| | |
| helmet-shaped upper lip like
| |
| that of a typical Nauplius is
| |
| present, and the eyes are situated on very long stalks.
| |
| | |
| In the true Zoaea stage there
| |
| appear on the five thoracic
| |
| | |
| segments pouch-like biramous rudiments of the limbs. The
| |
| tail becomes segmented; but the segments, with the
| |
| exception of the sixth, remain without appendages. On
| |
| the sixth a very long bilobed pouch appears as the commencement of the swimming feet of this segment. The
| |
| segments of the abdomen are armed with lateral spines.
| |
| | |
| From the Zoaea stage the larva passes into the form
| |
| known as Acanthosoma, which represents the Mysis stage
| |
| of Penaeus. The complex spikes on the dorsal shield of
| |
| the Zoaea stage are reduced to simple spines, but the
| |
| spines of the tail still retain their full size. In the appendages the chief
| |
| changes consist (i) in the reduction of the jointed outer ramus of the
| |
| second pair of antennae to a stump representing the scale, and the elongation of the inner one to the flagellum ; (2) in the elongation of the five
| |
| ambulatory thoracic appendages into biramous feet, like the maxillipeds,
| |
| and in the sprouting forth of rudimentary abdominal feet.
| |
| | |
| | |
| | |
| CRUSTACEA.
| |
| | |
| | |
| | |
| 475
| |
| | |
| | |
| | |
| The most obvious external indications of the passage from the Acanthosoma to the Mastigopus stage (fig. 218)
| |
| are to be found in the elongation of the
| |
| abdomen, the reduction and flattening
| |
| of the cephalo-thoracic shield, and the
| |
| nearly complete obliteration of all the
| |
| spines but the anterior. The eyes on
| |
| their elongated stalks are still very
| |
| characteristic, and the elongation of
| |
| the flagellum of the second pair of
| |
| antennae is very striking.
| |
| | |
| The maxillae and maxillipeds undergo considerable metamorphosis, the
| |
| abdominal feet attain their adult form,
| |
| and the three anterior thoracic ambulatory legs lose their outer rami. The
| |
| most remarkable change of all concerns
| |
| the two last pairs of thoracic appendages, which, instead of being metamorphosed like the preceding ones, are
| |
| completely or nearly completely thrown
| |
| off in the moult which inaugurates the
| |
| Mastigopus stage, and are subsequently
| |
| redeveloped. With the reappearance
| |
| of these appendages, and the changes
| |
| in the other appendages already indicated, the adult form is practically
| |
| attained.
| |
| | |
| | |
| | |
| | |
| FIG. 219. LARVA OF HIPPOLYTE
| |
| IN ZO/EA STAGE. (From Claus.)
| |
| | |
| MX', and MX", maxillae i and 2 ;
| |
| Mf. Mf. Mf". maxillipeds.
| |
| | |
| | |
| | |
| | |
| FIG. 220.
| |
| | |
| | |
| | |
| OLDER LARVA OF HIPPOLYTE AFTER THE THORACIC APPENDAGES HAVE
| |
| BECOME FORMED. (From Claus.)
| |
| | |
| | |
| | |
| 476 DECAPODA.
| |
| | |
| | |
| | |
| With reference to the development of the majority of the
| |
| Carabidae, Penaeinae, Palaemoninae, Crangoninae, it may be stated
| |
| generally that they leave the egg in the Zoaea stage (fig. 219)
| |
| with anterior appendages up to the third pair of maxillipeds.
| |
| The thorax is unsegmented and indeed almost unrepresented,
| |
| but the abdomen is long and divided into distinct segments.
| |
| Both thoracic and abdominal appendages are absent, and the
| |
| tail is formed by a simple plate with numerous bristles, not
| |
| forked, as in the case of the Zoaea of Fritz M tiller's Penaeus and
| |
| Sergestes. A dorsal spine is frequently found on the second
| |
| abdominal segment. From the Zoaea form the embryo passes
| |
| into a Mysis stage (fig. 220), during which the thoracic appendages gradually appear as biramous swimming feet; they
| |
| | |
| | |
| | |
| | |
| FIG. 221. NEWLY-HATCHED LARVA OF THE AMERICAN LOBSTER. (After Smith.)
| |
| are all developed before any of the abdominal appendages,
| |
| except the last. In some cases the development is still further
| |
| abbreviated. Thus the larvae of Crangon and Palaemonetes
| |
| (Faxon, No. 476) possess at hatching the rudiments of the two
| |
| anterior pairs of thoracic feet, and Palaemon of three pairs'.
| |
| | |
| Amongst the other Macrura the larva generally leaves the
| |
| egg as a Zoaea similar to that of the prawns. In the case of the
| |
| | |
| 1 Fritz Miiller has recently (Zoologisrher Anzeiger^ No. 52) described a still more
| |
| abbreviated development of a Pala-mon living in brooks near Blumenau.
| |
| | |
| | |
| | |
| CRUSTACEA. 477
| |
| | |
| | |
| | |
| Thalassinidae and Paguridae a Mysis stage has disappeared.
| |
| The most remarkable abbreviations of the typical development
| |
| are presented on the one hand by Homarus and Astacus, and on
| |
| the other by the Loricata.
| |
| | |
| The development of Homarus has been fully worked out by S. J. Smith
| |
| (No. 491) for the American lobster (Homarus americanus). The larva (fig.
| |
| 221) leaves the egg in an advanced Mysis stage. The cephalo-thoracic
| |
| shield is fully developed, and armed with a rostrum in front. The first pair
| |
| of antennae is unjointed but the second is biramous, the outer ramus forming
| |
| a large Mysis-like scale. The mandibles, which are palped, the maxillae,
| |
| and the two anterior maxillipeds differ only in minor details from the same
| |
| appendages of the adult. The third pair of maxillipeds is Mysis-like and
| |
| biramous, and the five ambulatory legs closely resemble them, the endopodite of the first being imperfectly chelate. The abdomen is well developed
| |
| but without appendages. The second, third, fourth and fifth segments are
| |
| armed with dorsal and lateral spines.
| |
| | |
| In the next stage swimming feet have appeared on the second, third,
| |
| fourth and fifth abdominal segments, and the appendages already present
| |
| have approached their adult form. Still later, when the larva is about half
| |
| an inch in length, the approach to the adult form is more marked, and the
| |
| exopodites of the ambulatory legs though present are relatively much
| |
| reduced in size. The swimmerets of the sixth abdominal segment are
| |
| formed. In the next stage observed the larva has entirely lost its Schizopod
| |
| characters, and though still retaining its free swimming habits differs from
| |
| the adult form only in generic characters.
| |
| | |
| As has been already stated, no free larval stages occur in the development of Astacus, but the young is hatched in a form in which it differs only
| |
| in unimportant details from the adult.
| |
| | |
| The peculiar larval form of the Loricata (Scyllarus, Palinurus) has long
| |
| been known under the name Phyllosoma (fig. 222 C), but its true nature was
| |
| first shewn by Couch (No. 474) [Couch did not however recognise the
| |
| identity of his larva with Phyllosoma ; this was first done by Gerstacker]
| |
| and shortly afterwards by Gerbe and Coste. These observations were
| |
| however for a long time not generally accepted, till Dohrn (No. 477)
| |
| published his valuable memoir giving an account of how he succeeded in
| |
| actually rearing Phyllosoma from the eggs of Scyllarus and Palinurus, and
| |
| shewing that some of the most remarkable features of the metamorphosis of
| |
| the Loricata occur before the larva is hatched.
| |
| | |
| The embryo of Scyllarus in the egg first of all passes through the usual
| |
| Nauplius stage, and then after the formation of a cuticle develops an
| |
| elongated thoracico-abdominal region bent completely over the anterior
| |
| part of the body. There appear moreover a number of appendages and the
| |
| rudiments of various organs ; and the embryo passes into a form which may
| |
| be described as the embryonic Phyllosoma stage. In this stage there are
| |
| present on the anterior part of the body, in front of the ventral flexure, two
| |
| | |
| | |
| | |
| 478 DECAPODA.
| |
| | |
| | |
| | |
| pairs of antennae, mandibles, two pairs of maxillae, the second commencing
| |
| to be biramous, and a small stump representing the first pair of maxillipeds.
| |
| The part of the body bent over consists of a small quadrate caudal plate,
| |
| and an appendage-bearing region to which are attached anteriorly three
| |
| pairs of biramous appendages the second and third maxillipeds, and the
| |
| anterior pair of ambulatory legs and two pairs of undivided appendages
| |
| the second and third pairs of ambulatory legs. In a slightly later stage the
| |
| first pair of maxillae becomes biramous, as also does the first pair of maxillipeds in a very rudimentary fashion. The second and third pairs of ambulatory legs become biramous, while the second and third maxilliped nearly
| |
| completely lose their outer ramus. Very small rudiments of the two hinder
| |
| ambulatory legs become formed. If the embryo is taken at this stage (vide
| |
| fig. 222 A, which represents a nearly similar larva of Palinurus) out of the
| |
| egg, it is seen to consist of (i) an anterior enlargement with a vaulted dorsal
| |
| shield enclosing the yolk, two stalked eyes, and a median eye ; (2) a thoracic
| |
| region in which the indications of segmentation are visible with the two
| |
| | |
| | |
| | |
| | |
| FIG. 222. LARWE OF THE LORICATA. (After Claus.)
| |
| | |
| A. Embryo of Palinurus shortly before hatching.
| |
| | |
| B. Young Phyllosoma larva of Scyllarus, without the first maxilliped, the two
| |
| last thoracic appendages, or the abdominal appendages.
| |
| | |
| C. Fully-grown Phyllosoma with all the Decapod appendages.
| |
| | |
| at*, antenna of first pair ; at*, antenna of second pair ; md. mandible ; ntx 1 . first
| |
| maxilla; mx 1 . second maxilla; mx^mxf. maxillipeds; / 1 / 3 . thoracic
| |
| appendages.
| |
| | |
| posterior pairs of maxillipeds (mxfp and wr/ 3 ) and the ambulatory legs (/ l );
| |
| (3) an abdominal region distinctly divided into segments and ending in a fork.
| |
| Before the embryo becomes hatched the first pair of maxillipeds becomes
| |
| reduced in size and finally vanishes. The second pair of maxillae becomes
| |
| reduced to simple stumps with a few bristles, the second pair of antennae
| |
| | |
| | |
| | |
| CRUSTACEA. 479
| |
| | |
| | |
| | |
| also appears to undergo a retrogressive change, while the two last thoracic
| |
| segments cease to be distinguishable. It thus appears that during embryonic
| |
| life the second pair of antennae, the second pair of maxillae, and the second
| |
| and third pair of maxillipeds and the two hinder ambulatory appendages
| |
| undergo retrogressive changes, while the first pair of maxillipeds is completely
| |
| obliterated !
| |
| | |
| The general form of the larva when hatched (fig. 222 B) is not very
| |
| different from that which it has during the later stages within the egg. The
| |
| body is divided into three regions: (i) an anterior cephalic; (2) a middle
| |
| thoracic, and (3) a small posterior abdominal portion ; and all of them are
| |
| characterised by their extreme dorso-ventral compression, so that the whole
| |
| animal has the form of a three-lobed disc, the strange appearance of which
| |
| is much increased by its glass-like transparency.
| |
| | |
| The cephalic portion is oval and projects slightly behind so as to overlap
| |
| the thorax. Its upper surface constitutes the dorsal shield, from which there
| |
| spring anteriorly the two compound eyes on long stalks, between which is a
| |
| median Nauplius eye. The mouth is situated about the middle of the under
| |
| surface of the anterior disc. It leads into a stomach from which an anterior
| |
| and a lateral hepatic diverticulum springs out on each side. The former
| |
| remains as a simple diverticulum through larval life, but the latter becomes
| |
| an extremely complicated glandular structure.
| |
| | |
| At the front border of the disc is placed the unjointed but elongated
| |
| first pair of antennae (rt/ 1 ). Externally to and behind these there spring the
| |
| short posterior antennae (at'*}. At the base of which the green gland is
| |
| already formed. Surrounding the mouth are the mandibles (md) and anterior
| |
| pair of maxillae (mx 1 ), and some distance behind the second pair of maxillae
| |
| (mx*), consisting of a cylindrical basal joint and short terminal joint armed
| |
| with bristles. The first pair of maxillipeds is absent.
| |
| | |
| The thoracic region is formed of an oval segmented disc attached to the
| |
| under surface of the cephalic disc. From its front segment arises the second
| |
| pair of maxillipeds (inxp l } as single five-jointed appendages, and from the
| |
| next segment springs the five-jointed elongated but uniramous third pair
| |
| of maxillipeds (mxfl 3 }, and behind this there arise three pairs of six-jointed
| |
| ambulatory appendages (p\ / 2 , p 3 , of which only the basal joint is represented
| |
| in the figure) with an exopodite springing from their second joint. The two
| |
| posterior thoracic rings and their appendages cannot be made out.
| |
| | |
| The abdomen is reduced to a short imperfectly segmented stump, ending
| |
| in a fork, between the prongs of which the anus opens. Even the youngest
| |
| larval Phyllosoma, such as has just been described, cannot be compared with a
| |
| Zoaea, but belongs rather, in the possession of biramous thoracic feet, to a
| |
| Mysis stage. In the forked tail and Nauplius eye there appear however to
| |
| be certain very primitive characters carried on to this stage.
| |
| | |
| The passage of this young larva to the fully formed Phyllosoma (fig.
| |
| 222 C) is very simple. It consists essentially in the fresh development of
| |
| the first pair of maxillipeds and the two last ambulatory appendages, the
| |
| growth and segmentation of the abdomen, and the sprouting on it of biramous
| |
| | |
| | |
| | |
| 480 DECAPODA.
| |
| | |
| | |
| | |
| swimming feet. In the course of these changes the larva becomes a true
| |
| Decapod in the arrangement and number of its appendages ; and indeed it
| |
| was united with this group before its larval character was made out. In
| |
| addition to the appearance of new appendages certain changes take place in
| |
| those already present. The two posterior maxillipeds, in the Palinurus
| |
| Phyllosoma at any rate, acquire again an exopodite, and together with the
| |
| biramous ambulatory feet develop epipodites in the form of gill pouches.
| |
| | |
| The mode of passage of the Phyllosoma to the adult is not known, but
| |
| it can easily be seen from the oldest Phyllosoma forms that the dorsal
| |
| cephalic plate grows over the thorax, and gives rise to the cephalo-thoracic
| |
| shield of the adult.
| |
| | |
| There are slight structural differences, especially in the antennae, between
| |
| the Phyllosoma of Scyllarus and that of Palinurus, but the chief difference
| |
| in development is that the first pair of maxillipeds of the Palinurus embryo,
| |
| though reduced in the embryonic state, does not completely vanish, at any
| |
| rate till after the free larval state has commenced ; and it is doubtful if
| |
| it does so even then. The freshly hatched Palinurus Phyllosoma is very
| |
| considerably more developed than that of Scyllarus.
| |
| | |
| Brachyura. All the Brachyura, with the exception of one or
| |
| more species of land crabs 1 , leave the egg in the Zoaia condition,
| |
| and though there are slight variations of structure, yet on the
| |
| | |
| | |
| | |
| | |
| FIG. 223. THE APPENDAGES OF A CRAB ZOJEA.
| |
| | |
| At. I. first antenna ; At. //. second antenna ; md. mandible (without a palp) ; mx.
| |
| i. first maxilla ; mx. i. second maxilla ; w.r. 3. third maxilla ; mxp. i. first maxilliped ;
| |
| mxp. i. second maxilliped.
| |
| | |
| ex. exopodite ; ett. cndopodite.
| |
| | |
| whole the Crab Zoaea is a very well marked form. Immediately
| |
| after leaving the egg (fig. 210) it has a somewhat oval shape
| |
| | |
| 1 It has been clearly demonstrated that the majority of land-crabs leave the egg in
| |
| the 7.oxa. form.
| |
| | |
| | |
| | |
| CRUSTACEA. 481
| |
| | |
| | |
| | |
| with a long distinctly-segmented abdomen bent underneath the
| |
| thorax. The cephalo-thoracic shield covers over the front part
| |
| of the body, and is prolonged into a long frontal spine pointing
| |
| forwards, and springing from the region between the two eyes ;
| |
| a long dorsal spine pointing backwards ; and two lateral spines.
| |
| | |
| To the under surface of the body are attached the anterior
| |
| appendages up to the second maxilliped, while the six following
| |
| pairs of thoracic appendages are either absent or represented
| |
| only in a very rudimentary form. The abdomen is without
| |
| appendages.
| |
| | |
| The anterior antennae are single and unjointed, but provided
| |
| at their extremity with a few olfactory hairs (only two in
| |
| Carcinus Mcenas) and one or two bristles. The rudiment of the
| |
| secondary flageltum appears in very young Zoaeae on the inner
| |
| side of the antennules (fig. 223 At. /.). The posterior antennae
| |
| are without the flagellum, but are provided with a scale representing the exopodite (fig. 223 At. II. ex] and usually a spinous
| |
| | |
| | |
| | |
| | |
| FIG. 224. CRAB ZO^EA AFTER TH.. THIRD PAIR OF MAXILLIPEDS AND THE
| |
| | |
| THORACIC AND ABDOMINAL APPENDAGES HAVE BECOME DEVELOPED.
| |
| | |
| at 1 , antenna of first pair ; at z . antenna of second pair ; mx l . first maxilla ; mop.
| |
| second maxilla ; mxp 1 . first maxilliped ; mxjP. second maxilliped ; mxf. third maxilliped ; oc. eye ; ht. heart.
| |
| | |
| process. The flagellum is very early developed and is represented in fig. 223, At. II. en. The mandibles (md) are large but
| |
| without a palp. The anterior maxillae (mx i) have a short twojointed endopodite (palp) with a few hairs, and a basal portion
| |
| B. II. 31
| |
| | |
| | |
| | |
| -
| |
| | |
| | |
| 482 DECAPODA.
| |
| | |
| | |
| | |
| with two blades, of which the distal is the largest, both armed
| |
| with stiff bristles. The posterior maxillae have a small respiratory plate (exopodite), an endopodite (palp) shaped like a
| |
| double blade, and two basal joints each continued into a double
| |
| blade. The two maxillipeds (inxp i and mxp 2) have the form
| |
| and function of biramous swimming feet. The exopodite of
| |
| both is two-jointed and bears long bristles at its extremity ; the
| |
| endopodite of the anterior is five-jointed and long, that of the
| |
| second is three-jointed and comparatively short.
| |
| | |
| In the six-jointed tail the second segment has usually two
| |
| dorsally directed spines, and the three succeeding segments each
| |
| of them two posteriorly directed. The telson or swimming plate
| |
| is not at first separated from the sixth segment ; on each side it
| |
| is prolonged into two well-marked prongs ; and to each prong
| |
| three bristles are usually attached (fig. 224). The heart (fig.
| |
| 224 ht) lies under the dorsal spine and is prolonged into an
| |
| anterior, posterior, and dorsal aorta. It has only two pairs of
| |
| venous ostia.
| |
| | |
| During the Zoaea stage the larva rapidly grows in size, and
| |
| undergoes considerable changes in its appendages which reach
| |
| the full Decapod number (fig. 224). On both pairs of antennae
| |
| a flagellum becomes developed and grows considerably in length.
| |
| Before the close of the Zoaea condition a small and unjointed
| |
| palp appears on the mandible. Behind the second maxilliped
| |
| the third maxilliped (inxp*} early appears as a small biramous
| |
| appendage, and the five ambulatory feet become distinctly
| |
| formed as uniramous appendages the exopodites not being
| |
| present. The third pair of maxillipeds and three following
| |
| ambulatory appendages develop gill pouches. The abdominal
| |
| feet are formed on the second to the sixth segments of the tail
| |
| as simple pouches.
| |
| | |
| The oldest Zoaea is transmuted at its moult into a form
| |
| known as Megalopa, which is really almost identical with an
| |
| anomurous Decapod. No Schizopod stage is intercalated, which
| |
| shews that the development is in many respects greatly abbreviated. The essential characters of the Megalopa are to be found
| |
| in (i) the reduction of the two anterior maxillipeds, which
| |
| cease to function as swimming feet, and together with the
| |
| appendages in front of them assume the adult form ; (2) the full
| |
| | |
| | |
| | |
| CRUSTACEA. 483
| |
| | |
| | |
| | |
| functional development of the five ambulatory appendages ;
| |
| (3) the reduction of the forked telson to an oval swimming
| |
| plate, and the growth in size of the abdominal feet, which
| |
| become large swimming plates and are at the same time
| |
| provided with short endopodites which serve to lock the feet of
| |
| the two sides.
| |
| | |
| With these essential characters the form of the Megalopa differs considerably in different cases. In some instances (e.g. Carcinus mcenas) the
| |
| Zoasa spines of the youngest Megalopa are so large that the larva appears
| |
| almost more like a Zoasa than a Megalopa (Spence Bate, No. 470). In other
| |
| cases, e.g. that represented on fig. 225, the Zoasa spines are still present but
| |
| much reduced; and the cephalo-thoracic shield has very much the adult
| |
| form. In other cases again (e.g. Portunus) the Zoasa spines are completely
| |
| thrown off at the youngest Megalopa stage.
| |
| | |
| There is a gradual passage from the youngest Megalopa to
| |
| the adult form by a series of moults.
| |
| | |
| Some of the brachyurous Zoasa forms exhibit
| |
| considerable divergences
| |
| from the described type,
| |
| more espcially in the armature of the shield. In
| |
| some forms the spines are
| |
| altogether absent, e.g. Maja
| |
| (Couch, No. 474) and Eurynome. In other forms
| |
| the frontal spine may be
| |
| much reduced or absent
| |
| (Inachus and Achasus).
| |
| The dorsal spine may also
| |
| be absent, and in one form
| |
| described by Dohrn (No.
| |
| 478) there is a long frontal
| |
| spine and two pairs of
| |
| lateral spines, but no dorsal ^ MEGALOPA STAGE OF CRAB LARVA.
| |
| | |
| spine. Both dorsal and
| |
| | |
| frontal spines may attain enormous dimensions and be swollen at their extremities (Dohrn). A form has been described by Claus as Pterocaris in which
| |
| the cephalo-thoracic shield is laterally expanded into two wing-like processes.
| |
| | |
| The Zoasa of Porcellana presents on the whole the most remarkable
| |
| peculiarities and, as might be anticipated from the systematic position of the
| |
| adult, is in some respects intermediate between the macrurous Zoasa and that
| |
| of the Brachyura. It is characterized by the oval form of the body, and by
| |
| | |
| 31-2
| |
| | |
| | |
| | |
| | |
| 484 STOMATOPODA.
| |
| | |
| | |
| | |
| the presence of one enormously long frontal spine and two posterior spines.
| |
| The usual dorsal spine is absent. The tail plate is rounded and has the
| |
| character of the tail of a macrurous Zoaea, but in the young Zoasa the third
| |
| pair of maxillipeds is absent and the appendages generally have a brachyurous character. A Megalopa stage is hardly represented, since the adult
| |
| may almost be regarded as a permanent Megalopa.
| |
| | |
| Stomatopoda. The history of the larval forms of the Stomatopoda
| |
| (Squilla etc.) has not unfortunately been thoroughly worked out, but what is
| |
| known from the researches of Fritz Miiller (No. 495) and Claus (No. 494) is
| |
| of very great importance. There are it appears two types, both of which
| |
| used to be described as adult forms under the respective names Erichthus
| |
| and Alima.
| |
| | |
| The youngest known Erichthus form is about two millimetres in length,
| |
| and has the characters of a modified Zoaea (fig. 226). The body is divided
| |
| into three regions, an anterior unsegmented region to which are attached
| |
| two pairs of antennas, mandibles, and maxillae (two pairs). This portion
| |
| has a dorsal shield covering the next or middle region, which consists of
| |
| five segments each with a pair of biramous appendages. These appendages
| |
| represent the five maxillipeds of the adult 1 . The portion of the body
| |
| behind this is without appendages. It consists of three short anterior
| |
| segments, the three posterior thoracic segments of the adult, and a long
| |
| unsegmented tail. The three footless thoracic segments are covered by the
| |
| dorsal shield. Both pairs of antennae are uniramous and comparatively
| |
| short. The mandibles, like those of Phyllopods, are without palps, and the
| |
| two following pairs of maxillae are small. The five maxillipeds have the
| |
| characters of normal biramous Zoaea feet. From the front of the head
| |
| spring a pair of compound eyes with short stalks, which grow longer
| |
| in the succeeding stages ; between them is a median eye. The dorsal
| |
| shield is attached just behind this eye, and is provided, as in the typical
| |
| Zoaea, with a frontal spike while its hinder border is produced into two
| |
| lateral spikes and one median. In a larva of about three millimetres a pair
| |
| of biramous appendages arises behind the three footless thoracic segments.
| |
| It is the anterior pair of abdominal feet (fig. 226). The
| |
| inner ramus of the second pair
| |
| of maxillipeds soon grows
| |
| greatly in length, indicating
| |
| its subsequent larger size and
| |
| prehensile form (fig. 227 g).
| |
| When the larva after one or
| |
| | |
| two moults attains a length FlG - 6 - SECOND STAGE OF ERICHTHUS
| |
| of six millimetres Cfitr 227 1 LARVA OFSQUII.LA WITH FIVE MAXILLIPEDS AND
| |
| | |
| (tig. 227) THE FIRST PAIR OF ABDOMINAL APPENDAGES.
| |
| | |
| the abdomen has six segments (From Claus.)
| |
| | |
| 1 These five maxillipeds correspond with the three maxillipeds and two anterior
| |
| ambulatory appendages of the Decapoda.
| |
| | |
| | |
| | |
| | |
| CRUSTACEA.
| |
| | |
| | |
| | |
| 485
| |
| | |
| | |
| | |
| (the sixth hardly differentiated), each with a pair of appendages (the two
| |
| hindermost still rudimentary) which have become gradually developed from
| |
| before backwards. The three hindermost thoracic segments are still without
| |
| appendages.
| |
| | |
| Some changes of importance have occurred in the other parts. Both
| |
| antennas have acquired a second flagellum, but the mandible is still without
| |
| | |
| | |
| | |
| | |
| FIG. 227. ADVANCED ERICHTHUS LARVA OF SQUILLA WITH FIVE PAIRS OF
| |
| | |
| ABDOMINAL APPENDAGES. (From Claus.)
| |
| | |
| f. first maxilliped ; g. second maxilliped.
| |
| | |
| a palp. The first and second pair of maxillipeds have both undergone
| |
| important modifications. Their outer ramus (exopodite) has been thrown
| |
| off, and a gill-plate (epipodite) has appeared as an outgrowth from their
| |
| basal joint. Each of them is composed of six joints. The three following
| |
| biramous appendages have retained their earlier characters but have become
| |
| much reduced in size. In the subsequent moults the most remarkable new
| |
| features concern the three posterior maxillipeds, which undergo atrophy, and
| |
| are either completely lost or reduced to mere unjointed sacks (fig. 228). In
| |
| | |
| | |
| | |
| | |
| FIG. 228. ADVANCED ERICHTHUS LARVA OF SQUILLA WHEN THE THREE
| |
| POSTERIOR MAXILLIPEDS HAVE BECOME REDUCED TO MINUTE POUCHES.
| |
| | |
| (From Claus.)
| |
| | |
| the stage where the complete Erichthus type has been reached, these
| |
| three appendages have again sprouted forth in their permanent form and
| |
| each of them is provided with a gill-sack on its coxal joint. Behind them
| |
| the three ambulatory appendages of the thorax have also appeared, first
| |
| as simple buds, which subsequently however become biramous. On their
| |
| development the full number of adult appendages is acquired.
| |
| | |
| The most noteworthy points in the developmental history detailed above
| |
| are the following :
| |
| | |
| (i) The thoracic and abdominal segments (apart from their appendages)
| |
| develop successively from before backwards.
| |
| | |
| | |
| | |
| 486 STOMATOPODA.
| |
| | |
| | |
| | |
| (2) The three last maxillipeds develop before the abdominal feet, as
| |
| biramous appendages, but subsequently completely atrophy, and then sprout
| |
| out again in their permanent form.
| |
| | |
| (3) The abdominal feet develop in succession from before backwards,
| |
| and the whole series of them is fully formed before a trace of the appendages
| |
| of the three hindermost thoracic segments has appeared. It may be
| |
| mentioned as a point of some importance that the Zoaea of Squilla has
| |
| an elongated many-chambered heart, and not the short compact heart
| |
| usually found in the Zoaea.
| |
| | |
| The younger stages of the Alima larva are not known 1 , but the earliest
| |
| stage observed is remarkable for presenting no trace of the three posterior
| |
| pairs of maxillipeds, or of the three following pairs of thoracic appendages.
| |
| The segments belonging to these appendages are however well developed.
| |
| The tail has its full complement of segments with the normal number of
| |
| well developed swimming feet. The larva represents in fact the stage of
| |
| the Erichthus larva when the three posterior pairs of maxillipeds have
| |
| undergone atrophy ; but it is probable that these appendages never become
| |
| developed in this form of larva.
| |
| | |
| Apart from the above peculiarities the Alima form of larva closely
| |
| resembles the Erichthus form.
| |
| | |
| Nebaliadae. The development of Nebalia is abbreviated, but from
| |
| MetschnikofFs figures 2 may be seen to resemble closely that of Mysis.
| |
| The abdomen has comparatively little yolk, and is bent over the ventral
| |
| surface of the thorax. There is in the egg a Nauplius stage with three
| |
| appendages, and subsequently a stage with the Zoaea appendages.
| |
| | |
| The larva when it leaves the egg has the majority of its appendages
| |
| formed, but is still enveloped in a larval skin, and like Mysis bends its
| |
| abdomen towards the dorsal side. When the larva is finally hatched it does
| |
| not differ greatly from the adult.
| |
| | |
| Cum ace ae. The development of the Cumaceae takes place for the
| |
| most part within the egg, and has been shewn by Dohrn (No. 496) to
| |
| resemble in many points that of the Isopods. A dorsal organ is present,
| |
| and a fold is formed immediately behind this which gives to the embryo a
| |
| dorsal flexure. Both of these features are eminently characteristic of the
| |
| Isopoda.
| |
| | |
| The formation of the two pairs of antennie, mandibles, and two pairs
| |
| of maxillae and the following seven pairs of appendages takes place very
| |
| early. The pair of appendages behind the second maxilku assumes an
| |
| ambulatory form, and exhibits a Schizopod character very early, differing
| |
| in both these respects from the homologous appendages in the Isopoda.
| |
| The cephalo-thoracic shield commences to be formed when the appendages
| |
| are still quite rudimentary as a pair of folds in the maxillary region. The
| |
| | |
| 1 The observations of Brooks (No. 493) render it probable that the Alima larva
| |
| leaves the egg in a form not very dissimilar to the youngest known larva.
| |
| 3 His paper is unfortunately in Russian.
| |
| | |
| | |
| | |
| CRUSTACEA. 487
| |
| | |
| | |
| | |
| eyes are formed slightly later on each side of the head, and only coalesce at
| |
| a subsequent period to form the peculiar median sessile eye of the adult.
| |
| | |
| The two pairs of appendages behind the second maxillae become converted into maxillipeds, and the exopodite of the first of them becomes the
| |
| main ramus, while in the externally similar second maxilliped the exopodite
| |
| atrophies and the endopodite alone remains.
| |
| | |
| The larva is hatched without the last pair of thoracic limbs or the
| |
| abdominal appendages (which are never developed in the female), but in
| |
| other respects closely resembles the adult. Before hatching the dorsal
| |
| flexure is exchanged for a ventral one, and the larva acquires a character
| |
| more like that of a Decapod.
| |
| | |
| COPEPODA.
| |
| | |
| Natantia. The free Copepoda are undoubtedly amongst
| |
| the lowest forms of those Crustacea which are free or do not
| |
| lead a parasitic existence. Although some features of their
| |
| anatomy, such for instance as the frequent absence of a heart,
| |
| may be put down to a retrogressive development, yet, from their
| |
| retention of the median frontal eye of the Nauplius as the sole
| |
| organ of vision 1 , their simple biramous swimming legs, and other
| |
| characters, they may claim to be very primitive forms, which
| |
| have diverged to no great extent from the main line of Crustacean development. They supply a long series of transitional
| |
| steps from the Nauplius stage to the adult condition.
| |
| | |
| While still within the egg-shell the embryo is divided by two
| |
| transverse constrictions into three segments, on which the three
| |
| Nauplius appendages are developed, viz. the two pairs of
| |
| antennae and the mandibles. When the embryo is hatched the
| |
| indication of a division into segments has vanished, but the
| |
| larva is in the fullest sense a typical Nauplius 2 . There are
| |
| slight variations in the shape of the Nauplius in different genera,
| |
| but its general form and character are very constant. It has
| |
| (fig. 229 A) an oval unsegmented body with three pairs of
| |
| appendages springing from the ventral surface. The anterior of
| |
| these (at i) is uniramous, and usually formed of three joints
| |
| which bear bristles on their under surface. The two posterior
| |
| | |
| 1 The Pontellidse form an exception to this statement, in that they are provided
| |
| with paired lateral eyes in addition to the median one.
| |
| | |
| 2 The term Nauplius was applied to the larva of Cyclops and allied organisms by
| |
| O. F. Muller under the impression that they were adult forms.
| |
| | |
| | |
| | |
| 488
| |
| | |
| | |
| | |
| COPEPODA.
| |
| | |
| | |
| | |
| pairs of appendages are both biramous. The second pair of
| |
| antennae (at 2) is the largest. Its basal portion (protopodite)
| |
| bears on its inner side a powerful hook-like bristle. The outer
| |
| ramus is the longest and many-jointed ; the inner ramus has
| |
| only two joints. The mandibles (md), though smaller than the
| |
| second pair of antennae, have a nearly identical structure. No
| |
| blade-like projection is as yet developed on their protopodite.
| |
| Between the points of insertion of the first pair of antennae is
| |
| the median eye (oc), which originates by the coalescence of two
| |
| distinct parts. The mouth is ventral, and placed in the middle
| |
| line between the second pair of antennae and the mandibles : it
| |
| | |
| | |
| | |
| | |
| FIG. 229. SUCCESSIVE STAGES IN THE DEVELOPMENT OF CYCLOPS TENUICORMS.
| |
| | |
| (Copied from Bronn ; after Claus.)
| |
| | |
| A. B. and C. Nauplius stages. D. Youngest Copepod stage. In this figure maxillae
| |
| and the two rami of the maxilliped are seen immediately behind the mandible md.
| |
| | |
| oc. eye ; at 1 , first pair of antennae ; a/ 8 , second pair of antennre ; md. mandible ;
| |
| /*. first pair of feet ; / 2 . second pair of feet ; f. third pair of feet ; //. excretory concretions in the intestine.
| |
| | |
| is provided with an unpaired upper lip. There are two bristles
| |
| at the hind end of the embryo between which the anus is placed,
| |
| and in some cases there is at this part a slight indication of the
| |
| future caudal fork.
| |
| | |
| The larva undergoes a number of successive ecdyses, at each
| |
| of which the body becomes more elongated, and certain other
| |
| | |
| | |
| | |
| CRUSTACEA. 489
| |
| | |
| | |
| | |
| changes take place. First of all a pair of appendages arises
| |
| behind the mandibles, which form the maxillae (fig. 229 B) ; at
| |
| the same time the basal joint of the maxillae develops a cuttingblade. Three successive pairs of appendages (fig. 229 C) next
| |
| become formed the so-called maxillipeds (the homologues of
| |
| the second pair of maxillae), and the two first thoracic limbs.
| |
| Each of these though very rudimentary is nevertheless bifid.
| |
| The body becomes greatly elongated, and the caudal fork more
| |
| developed.
| |
| | |
| Up to this stage of development the Nauplius appendages
| |
| have retained their primitive character almost unaltered ; but
| |
| after a few more ecdyses a sudden change takes place ; a cephalothoracic shield becomes fully developed, and the larva comes to
| |
| resemble in character an adult Copepod, from which it mainly
| |
| differs in the smaller number of segments and appendages. In
| |
| the earliest 'Cyclops' stage the same number of appendages are
| |
| present as in the last Nauplius stage. There (fig. 229 D) is a
| |
| well developed cephalo-thorax, and four free segments behind it.
| |
| To the cephalo-thoracic region the antennae, mandibles, maxillae,
| |
| the now double pair of maxillipeds (derived from the original
| |
| single pair of appendages), and first pair of thoracic appendages
| |
| (p l ) are attached. The second pair of thoracic appendages (/ 2 )
| |
| is fixed to the first free segment, and the rudiment of a third
| |
| pair (/ 3 ) projects from the second free segment. The first pair
| |
| of antennae has grown longer by the addition of new joints, and
| |
| continues to increase in length in the following ecdyses till it
| |
| attains its full adult development, and then forms the chief
| |
| organ of locomotion. The second pair of antennae is much
| |
| reduced and has lost one of its rami. The two rami of the
| |
| mandibles are reduced to a simple palp, while the blade has
| |
| assumed its full importance. The maxillae and following appendages have greatly increased in size. They are all biramous,
| |
| though the two rami are not as yet jointed. The adult state is
| |
| gradually attained after a number of successive ecdyses, at
| |
| which new segments and appendages are added, while new
| |
| joints are formed for those already present.
| |
| | |
| Parasita. The earliest developmental stages of the parasitic types
| |
| of Copepoda closely resemble those of the free forms, but, as might be
| |
| expected from the peculiarly modified forms of the adult, they present a
| |
| | |
| | |
| | |
| 490
| |
| | |
| | |
| | |
| COPEPODA.
| |
| | |
| | |
| | |
| large number of secondary characters. So far as is known a more or less
| |
| modified Nauplius larva is usually preserved.
| |
| | |
| The development of Achtheres percarum, one of the Lernaeopoda parasitic
| |
| in the mouth, etc. of the common Perch, may be selected to illustrate the
| |
| mode of development of these forms. The larva leaves the egg as a much
| |
| simplified Nauplius (fig. 230 A). It has an oval body with only the two
| |
| anterior pairs of Nauplius appendages ; both of them in the rudimentary
| |
| condition of unjointed rods. The usual median eye is present, and there is
| |
| also found a peculiar sternal papilla, on which opens a spiral canal filled
| |
| with a glutinous material, which is probably derived from a gland which
| |
| disappears on the completion of the duct. The probable function of this
| |
| | |
| | |
| | |
| | |
| FIG. 330. SUCCESSIVE STAGES IN THE DEVELOPMENT OF ACHTHERES PERCARUM.
| |
| (Copied from Bronn ; after Claus. )
| |
| | |
| A. Modified Nauplius stage. B. Cyclops stage. C. Late stage of male
| |
| embryo. D. Sexually mature female. E. Sexually mature male.
| |
| | |
| at 1 , first pair of antennae; at 3 , second pair of antennae; tnd. mandible; tnx.
| |
| maxillae ; ptn 1 . outer pair of maxillipeds ; ftn^. inner pair of maxillipeds ; J> 1 . first pair
| |
| of legs ; /*. second pair of legs ; z. frontal organ ; i. intestine ; o. larval eye ; b.
| |
| glandular body ; t. organ of touch ; ov. ovary ; /. rod projecting from coalesced maxillipeds ; g. cement gland ; rs. receptaculurn seminis ; n. nervous system ; te. testis ; v.
| |
| vas deferens.
| |
| | |
| organ is to assist at a later period in the attachment of the parasite to its
| |
| host. Underneath the Nauplius skin a number of appendages are visible,
| |
| which become functional after the first ecdysis. This takes place within a
| |
| few hours after the hatching of the Nauplius, and the larva then passes from
| |
| | |
| | |
| | |
| CRUSTACEA. 491
| |
| | |
| | |
| | |
| this rudimentary Nauplius stage into a stage corresponding with the Cyclops
| |
| stage of the free forms (fig. 230 B). In the Cyclops stage the larva has an
| |
| elongated body with a large cephalo-thoracic shield, and four free posterior
| |
| segments, the last of which bears a forked tail.
| |
| | |
| There are now present eight pairs of appendages, viz. antennae (two
| |
| pairs), mandibles, maxillae, maxillipeds, and three pairs of swimming feet.
| |
| The Nauplius appendages are greatly modified. The first pair of antennae is
| |
| three-jointed, and the second biramous. The outer ramus is the longest, and
| |
| bears a claw-like bristle at its extremity. This pair of appendages is used
| |
| by the larva for fixing itself. The mandibles are small and connected with
| |
| the proboscidiform mouth ; and the single pair of maxillae is small and palped.
| |
| The maxillipeds (pm* and flm 2 ) are believed by Claus to be primitively
| |
| a single biramous appendage, but early appear as two distinct structures 1 ,
| |
| the outer and larger of which becomes the main organ by which the larva is
| |
| fixed. Both are at this stage simple two-jointed appendages. The two
| |
| anterior pairs of swimming feet have the typical structure, and consist of a
| |
| protopodite bearing an unjointed exopodite and endopodite. The first pair
| |
| is attached to the cephalo-thorax and the second (p*} to the first free thoracic
| |
| segment. The third pair is very small and attached to the second free
| |
| segment. The mouth is situated at the end of a kind of proboscis formed
| |
| by prolongations of the upper and lower lips. The alimentary tract is fairly
| |
| simple, and the anus opens between the caudal forks.
| |
| | |
| Between this and the next known stage it is quite possible that one
| |
| or more may intervene. However this may be the larva in the next stage
| |
| observed (fig. 230 C) has already become parasitic in the mouth of the Perch,
| |
| and has acquired an elongated vermiform aspect. The body is divided into
| |
| two sections, an anterior unsegmented, and a posterior formed of five
| |
| segments, of which the foremost is the first thoracic segment which in the
| |
| earlier stage was fused with the cephalo-thorax. The tail bears a rudimentary fork between the prongs of which the anus opens. The swimming feet
| |
| have disappeared, so also has the eye and the spiral duct of the embryonic
| |
| frontal organ. The outer of the two divisions of the maxilliped have undergone the most important modification, in that they have become united at
| |
| their ends, where they form an organ from which an elongated rod (_/)
| |
| projects, and attaches the larva to the mouth or gills of its host. The
| |
| antennae and jaws have nearly acquired their adult form. The nervous
| |
| system consists of supra- and infra-cesophageal ganglia and two lateral
| |
| trunks given off from the latter. At this stage the males and females can
| |
| already be distinguished, not only by certain differences in the rudimentary generative organs, but also by the fact that the outer branch of the
| |
| maxillipeds is much longer in the female than in the male, and projects
| |
| beyond the head.
| |
| | |
| In the next ecdysis the adult condition is reached. The outer maxilli
| |
| 1 Van Beneden (No. 506) in the genera investigated by him finds that the two
| |
| maxillipeds are really distinct pairs of appendages.
| |
| | |
| | |
| | |
| 492 CIRRIPEDIA.
| |
| | |
| | |
| | |
| peds of the male (fig. 230 ,/>#*) separate again ; while in the female (fig.
| |
| 230 D) they remain fused and develop a sucker. The male is only about
| |
| one-fifth the length of the female. In both sexes the abdomen is much
| |
| reduced.
| |
| | |
| In the genera Anchorella, Lernaeopoda, Brachiella and Hessia, Ed. van
| |
| BenecUn (No. 506) has shewn that the embryo, although it passes through
| |
| a crypto-Nauplius stage in the egg, is when hatched already in the Cyclops
| |
| stage.
| |
| | |
| Branchiura. The peculiar parasite Argulus, the affinities of which
| |
| with the Copepoda have been demonstrated by Claus (No. 511), is hatched
| |
| in a Cyclops stage, and has no Nauplius stage. At the time of hatching it
| |
| closely resembles the adult in general form. Its appendages are however
| |
| very nearly those of a typical larval Copepod. The body is composed of
| |
| a cephalo-thorax and free region behind this. The cephalo-thorax bears
| |
| on its under surface antennae (two pairs), mandibles, maxillipeds, and the
| |
| first pair of thoracic feet.
| |
| | |
| The first pair of antennae is three-jointed, but the basal joint bears a
| |
| hook. The second pair is biramous, the inner ramus terminating in a hook.
| |
| The mandible is palped, but the palp is completely separated from the
| |
| cutting blade 1 . The maxilla would, according to Claus, appear to be
| |
| absent.
| |
| | |
| The two typical divisions of the Copepod maxillipeds are present, viz. an
| |
| outer and anterior larger division, and an inner and posterior smaller one.
| |
| The first pair of thoracic feet, as is usual amongst Copepoda, is attached
| |
| to the cephalo-thorax. It has not the typical biramous Copepod character.
| |
| There are four free segments behind the cephalo-thorax, the last of which
| |
| ends in a fork. Three of them bear appendages, which are rudimentary in
| |
| this early larval stage. On the dorsal surface are present paired eyes as
| |
| well as an unpaired median eye.
| |
| | |
| Between the larval condition and that of the adult a number of ecdyses
| |
| intervene.
| |
| | |
| CIRRIPEDIA.
| |
| | |
| The larvae of all the Cirripedia, with one or two exceptions,
| |
| leave the egg in the Nauplius condition. The Nauplii differ
| |
| somewhat in the separate groups, and the post-nauplial stages
| |
| vary not inconsiderably.
| |
| | |
| It will be most convenient to treat successively the larval
| |
| | |
| 1 It seems not impossible that the appendage regarded by Claus as the mandibular
| |
| palp may really represent the maxilla, which would otherwise seem to be absent.
| |
| This mode of interpretation would bring the appendages of Argulus into a much
| |
| closer agreement with those of the parasitic Copepoda. It does not seem incompatible
| |
| with the existence of the stylet-like maxillse detected by Claus in the adult.
| |
| | |
| | |
| | |
| CRUSTACEA. 493
| |
| | |
| | |
| | |
| history of the four sub-orders, viz. Thoracica, Abdominalia,
| |
| Apoda, and Rhizocephala.
| |
| | |
| Thoracica. The just hatched larvae at once leave the egg
| |
| lamellae of their parent. They pass out through an opening in
| |
| the mantle near the mouth, and during this passage the shell of
| |
| the parent is opened and the movements of the cirriform feet
| |
| cease.
| |
| | |
| The larval stages commence with a Nauplius 1 which, though
| |
| regarded by Claus as closely resembling the Copepod Nauplius
| |
| (figs. 231 and 232 A), certainly has very marked pecularities of
| |
| its own, and in some respects approaches the Phyllopod
| |
| Nauplius. It is in the youngest stage somewhat triangular in
| |
| form, and covered on the dorsal side by a very delicate and
| |
| hardly perceptible dorsal shield, which is prolonged laterally
| |
| into two very peculiar conical horns (fig. 231 Ik), which are the
| |
| most characteristic structures of the Cirriped Nauplius. They
| |
| are connected with a glandular mass, the secretion from which
| |
| passes out at their apex. Anteriorly the dorsal shield has the
| |
| same extension as the body, but posteriorly it projects slightly.
| |
| | |
| An unpaired eye is situated on the ventral surface of the
| |
| head, and immediately behind it there springs a more or less
| |
| considerable upper lip (Ib), which resembles the Phyllopod
| |
| labrum rather than that of the Copepoda. Both mouth and
| |
| anus are present, and the hind end of the body is slightly forked
| |
| in some forms, but ends in others, e.g. Lepas fascicularis, in an
| |
| elongated spine. The anterior of the three pairs of Nauplius
| |
| appendages (At*) is uniramous, and the two posterior (Af and
| |
| md) are biramous. From the protopodites of both the latter
| |
| spring strong hooks like those of the Copepod and Phyllopod
| |
| Nauplii. In some Nauplii, e.g. that of Balanus, the appendages
| |
| are at first not jointed, but in other Nauplii, e.g. that of Lepas
| |
| fascicularis, the jointing is well marked. In Lepas fascicularis
| |
| the earliest free Nauplius is enveloped in a larval skin, which is
| |
| thrown off after a few hours. The Nauplii of all the Thoracica
| |
| undergo a considerable number of moults before their appendages
| |
| increase in number or segmentation of the body appears. During
| |
| these moults they grow larger, and the posterior part of the
| |
| | |
| 1 Alepas squalicola is stated by Koren and Danielssen to form an exception to
| |
| this rule, and to leave the egg with six pairs of appendages.
| |
| | |
| | |
| | |
| 494
| |
| | |
| | |
| | |
| CIRRIPEDIA.
| |
| | |
| | |
| | |
| body the future thoracic and abdominal region grows relatively in length. There also appear close to the sides of the
| |
| unpaired eye two conical bodies, which correspond with the
| |
| frontal sense organs of the Phyllopods. During their growth
| |
| the different larvae undergo changes varying greatly in degree.
| |
| | |
| In Balanus the changes consist for the most part in the full
| |
| segmentation of the appendages and the growth and distinctness
| |
| | |
| | |
| | |
| | |
| FIG. 231. NAUPLIUS LARVA OF LEPAS FASCICULARIS VIEWED FROM THE SIDE.
| |
| oc. eye ; At. i. antenna of first pair ; At. 2. antenna of second pair ; md. mandible ;
| |
| Ib. labrum ; an. anus; me. mesenteron; d.sp. dorsal spine; c.sp. caudal spine;
| |
| Vp. ventral spine ; Ih. lateral horns.
| |
| | |
| of the dorsal shield, which forms a somewhat blunt triangular
| |
| plate, broadest in front, with the anterior horns very long, and
| |
| two short posterior spines. The tail also becomes produced into
| |
| a long spine.
| |
| | |
| In Lepas fascicularis the changes in appearance of the
| |
| Nauplius, owing to a great spinous development on its shield,
| |
| are very considerable ; and, together with its enormous size,
| |
| render it a very remarkable form. Dohrn (No. 520), who was
| |
| the first to describe it, named it Archizoaea gigas.
| |
| | |
| | |
| | |
| CRUSTACEA. 495
| |
| | |
| | |
| | |
| The dorsal shield of the Nauplius of Lepas fascicularis (fig. 231) becomes
| |
| somewhat hexagonal, and there springs from the middle of the dorsal surface
| |
| an enormously long spine (d,sp], like the dorsal spine of a Zoa^a. The hind
| |
| end of the shield is also produced into a long caudal spine (c.sfi] between
| |
| which and the dorsal spine are some feather-like processes. From its edge
| |
| there spring in addition to the primitive frontal horns three main pairs of
| |
| horns, one pair anterior, one lateral, and one posterior, and smaller ones in
| |
| addition. All these processes (with the exception of the dorsal and posterior
| |
| spines) are hollow and open at their extremities, and like the primitive
| |
| frontal horns contain the ducts of glands situated under the shield. On the
| |
| under surface of the larva is situated the unpaired eye (pc] on each side of
| |
| which spring the two-jointed frontal sense organs. Immediately behind
| |
| these is the enormous upper lip (lb] which covers the mouth 1 . At the sides
| |
| of the lip lie the three pairs of Nauplius appendages, which are very
| |
| characteristic but present no special peculiarities. Posteriorly the body is
| |
| produced into a long ventral spine-like process ( Vfi) homologous with that
| |
| of other more normal Nauplii. At the base of this process large moveable
| |
| paired spines appear at successive moults, six pairs being eventually formed.
| |
| These spines give to the region in which they are situated a segmented
| |
| appearance, and perhaps similar structures have given rise to the appearance of segmentation in Spence Bate's figures. The anus is situated on
| |
| the dorsal side of this ventral process, and between it and the caudal
| |
| spine of the shield above. The fact that the anus occupies this position
| |
| appears to indicate that the ventral process is homologous with the
| |
| caudal fork of the Copepoda, on the dorsal side of which the anus so
| |
| often opens 2 .
| |
| | |
| From the Nauplius condition the larvae pass at a single
| |
| moult into an entirely different condition known as the Cypris
| |
| stage. In preparation for this condition there appear, during
| |
| the last Nauplius moults, the rudiments of several fresh organs,
| |
| which are more or less developed in different types. In the
| |
| first place a compound eye is formed on each side of the
| |
| median eye. Secondly there appears behind the mandibles a
| |
| fourth pair of appendages the first pair of maxillae and
| |
| internal to these a pair of small prominences, which are perhaps
| |
| | |
| 1 Willemoes Suhm (No. 530) states that the mouth is situated at the free end of the
| |
| upper lip, and that the oesophagus passes through it. From an examination of some
| |
| specimens of this Nauplius, for which I am indebted to Moseley, I am inclined to
| |
| think that this is a mistake, and that a groove on the surface of the upper lip has been
| |
| taken by Suhm for the oesophagus.
| |
| | |
| 2 The enormous spinous development of the larva of Lepas fascicularis is probably
| |
| to be explained as a secondary protective adaptation, and has no genetic connection
| |
| with the somewhat similar spinous armature of the Zosea.
| |
| | |
| | |
| | |
| 496" CIRRIPEDIA.
| |
| | |
| | |
| | |
| equivalent to the second pair of maxillae, and give rise to the
| |
| third pair of jaws in the adult (sometimes spoken of as the
| |
| lower lip).
| |
| | |
| Behind these appendages there are moreover formed the rudiments of six pairs of feet. Under the cuticle of the first pair of
| |
| antennae there may be seen just before the final moult the fourjointed antennae of the Cypris stage with the rudiment of a disc
| |
| on the second joint by which the larvae eventually become
| |
| attached.
| |
| | |
| By the free Cypris stage, into which the larva next passes, a
| |
| very complete metamorphosis has been effected. The median
| |
| and paired eyes are present as before, but the dorsal shield has
| |
| become a bivalve shell, the two valves of which are united along
| |
| their dorsal, anterior, and posterior margins. The two valves
| |
| are further kept in place by an adductor muscle situated close
| |
| below the mouth. Remains of the lateral horns still persist. The
| |
| anterior antennae have undergone the metamorphosis already
| |
| indicated. They are four-jointed, the two basal joints being
| |
| long, and the second provided with a suctorial disc, in the centre
| |
| of which is the opening of the duct of the so-called antennary or
| |
| cement gland, which is a granular mass lying on the ventral
| |
| side of the anterior region of the body. The gland arises
| |
| (Willemoes Suhm) during the Nauplius stage in the large upper
| |
| lip. The two distal joints of the antennae are short, and the
| |
| last of them is provided with olfactory hairs. The great upper
| |
| lip and second pair of antennae and mandibles have disappeared,
| |
| but a small papilla, forming the commencement of the adult
| |
| mandibles, is perhaps developed in the base of the Nauplius
| |
| mandibles. The first pair of maxillae have become small papillae
| |
| and the second pair probably remain. The six posterior pairs
| |
| of appendages have grown out as functional biramous swimming
| |
| feet, which can project beyond the shell and are used in the
| |
| locomotion of the larva. They are composed of two basal
| |
| joints, and two rami with swimming hairs, each two-jointed.
| |
| These feet resemble Copepod feet, and form the main ground
| |
| for the views of Claus and others that the Copepoda and
| |
| Cirripedia are closely related. They are regarded by Claus as
| |
| representing the five pairs of natatory feet of Copepoda, and the
| |
| generative appendages of the segment behind these. Between
| |
| | |
| | |
| | |
| CRUSTACEA.
| |
| | |
| | |
| | |
| 497
| |
| | |
| | |
| | |
| the natatory feet are delicate chitinous lamellae, in the spaces
| |
| between which the cirriform feet of the adult become developed.
| |
| The ventral spinous process of the Nauplius stage is much reduced,
| |
| though usually three-jointed. It becomes completely aborted
| |
| after the larva is fixed.
| |
| | |
| In addition to the antennary gland there is present, near the
| |
| dorsal side of the body above the natatory feet, a peculiar paired
| |
| glandular mass, the origin of which has not been clearly made
| |
| out, but which is perhaps equivalent to the entomostracan shell
| |
| gland. It probably supplies the material for the shell in succeeding stages 1 .
| |
| | |
| The free Cypris stage is not of long duration ; and during it
| |
| the larva does not take food. It is succeeded by a stage known
| |
| as the pupa stage (fig. 232 B), in which the larva becomes fixed,
| |
| while underneath the larval skin the adult structures are developed. This stage fully deserves its name, since it is a quiescent stage during which no nutriment is taken. The attachment
| |
| takes place by the sucker of the antennae, and the cement gland
| |
| (/) supplies the cementing material for effecting it. A retrogressive metamorphosis of a large number of the organs sets in,
| |
| while at the same time the formation of new adult structures
| |
| is proceeded with. The eyes
| |
| become gradually lost, but the
| |
| Nauplius eye is retained,though
| |
| in a rudimentary state, and the
| |
| terminal joints of the antennae
| |
| with their olfactory hairs are
| |
| thrown off. The bivalve shell
| |
| is moulted about the same time
| |
| as the eyes, the skin below it
| |
| remaining as the mantle. The
| |
| caudal process becomes aborted. Underneath the natatory
| |
| | |
| | |
| | |
| | |
| | |
| FIG. 232. LARVAL FORMS OF THE
| |
| THORACICA. (From Huxley.)
| |
| | |
| A. Nauplius of Balanus balanoides.
| |
| (After Sp. Bate.) B. Pupa stage of Lepas
| |
| australis. (After Darwin.)
| |
| | |
| n. antennary apodemes ; /. cement
| |
| gland with duct to antenna.
| |
| | |
| | |
| | |
| 1 There is considerable confusion about the shell gland and antennary gland. In
| |
| my account Willemoes Suhm has been followed. Claus however regards what I have
| |
| called the antennary gland as the shell gland, and states that it does not open into the
| |
| antennae till a later period. He does not clearly describe its opening, nor the organ
| |
| which I have called the shell gland.
| |
| | |
| B. II. 32
| |
| | |
| | |
| | |
| 498 CIRRIPEDIA.
| |
| | |
| | |
| | |
| feet, and between the above-mentioned chitinous lamellae, the
| |
| cirriform feet are formed ; and on their completion the natatory
| |
| feet become thrown off and replaced by the permanent feet. In
| |
| the Lepadidae, in which the metamorphosis of the pupa stages
| |
| has been most fully studied, the anterior part of the body with
| |
| the antennae gradually grows out into an elongated stalk, into
| |
| which pass the ovaries, which are formed during the Cypris
| |
| stage. At the base of the stalk is the protuberant mouth, the
| |
| appendages of which soon attain a higher development than in
| |
| the Cypris stage. At the front part of it a large upper lip
| |
| becomes formed. Above the mantle and between it and the
| |
| shell there are formed in the Lepadidae the provisional valves of
| |
| the shell. These valves are chitinous, and have a fenestrated
| |
| structure, owing to the chitin being deposited round the margin
| |
| of the separate epidermis (hypodermis) cells. These valves in
| |
| the Lepadidae " prefigure in shape, size, and direction of growth,
| |
| the shelly valves to be formed under and around them" (Darwin,
| |
| No. 519, p. 129).
| |
| | |
| Whatever may be the number of valves in the adult the provisional
| |
| valves never exceed five, viz. the two scuta, the two terga and the carina.
| |
| They are relatively far smaller than the permanent valves and are therefore
| |
| separated by considerable membranous intervals. They are often preserved
| |
| for a long time on the permanent calcareous valves. In the Balanidce
| |
| the embryonic valves are membranous and do not overlap, but do not
| |
| present the peculiar fenestrated structure of the primordial valves of the
| |
| Lepadidae.
| |
| | |
| In connection with the moult of the pupa skin, and the
| |
| conversion of the pupa into the adult form, a remarkable change
| |
| in the position takes place. The pupa lies with the ventral side
| |
| parallel to and adjoining the surface of attachment, while the
| |
| long axis of the body of the young Cirriped is placed nearly at
| |
| right angles to the surface of attachment. This change is
| |
| connected with the ecdyses of the antennary apodemes (),
| |
| which leave a deep bay on the ventral surface behind the
| |
| peduncle. The chitinous skin of the Cirriped passes round
| |
| the head of this bay, but on the moult of the pupa skin
| |
| taking place becomes stretched out, owing to the posterior
| |
| part of the larva bending dorsalwards. It is this flexure which
| |
| causes the change in the position of the larva.
| |
| | |
| | |
| | |
| CRUSTACEA.
| |
| | |
| | |
| | |
| 499
| |
| | |
| | |
| | |
| In addition to the remarkable external metamorphosis
| |
| undergone during the pupa stage, a series of hardly less considerable internal changes take place, such as the atrophy of
| |
| the muscles of the antennae, a change in the position of the
| |
| stomach, etc.
| |
| | |
| Abdominalia. In the Alcippidae the larva leaves the egg as a
| |
| Nauplius, and this stage is eventually followed by a pupa stage closely
| |
| resembling that of the Thoracica. There are six pairs of thoracic natatory
| |
| legs (Darwin, No. 519). Of these only the first and the last three are preserved in the adult, the first being bent forward in connection with the
| |
| mouth. The body moreover partially preserves its segmentation, and the
| |
| mantle does not secrete calcareous valves.
| |
| | |
| The very remarkable genus Cryptophialus, the development of which is
| |
| described by Darwin (No. 519) in his classical memoir, is without a free
| |
| Nauplius stage. The embryo is at first oval but soon acquires two anterior
| |
| processes, apparently the first pair of antennae, and a posterior prominence,
| |
| the abdomen. In a later stage the abdominal prominence disappears, and
| |
| the antennary processes, within which the true antennas are now visible, are
| |
| carried more towards the ventral
| |
| surface. The larva next passes into
| |
| the free Cypris stage, during which it
| |
| creeps about the mantle cavity of its
| |
| parent. It is enveloped in a bivalve
| |
| shell, and the antennae have the normal cirriped structure. There are no
| |
| other true appendages, but posteriorly
| |
| three pairs of bristles are attached to
| |
| a rudimentary abdomen. Paired compound eyes are present. During the
| |
| succeeding pupa stage the metamorphosis into the adult form takes place,
| |
| but this has not been followed out in
| |
| detail.
| |
| | |
| In Kochlorine, a form discovered
| |
| by Noll (No. 526) and closely related
| |
| to Cryptophialus, the larvae found
| |
| within the mantle represent apparently two larval stages, similar to
| |
| two of the larval stages described by
| |
| Darwin.
| |
| | |
| Rhizocephala. The Rhizocephala, as might have been antici
| |
| | |
| | |
| | |
| FIG. 233. STAGES IN THE DEVELOPMENT OF THE RHIZOCEPHALA. (From
| |
| Huxley, after Fritz Miiller.)
| |
| | |
| A. Nauplius of Sacculina purpurea.
| |
| B. Cypris stage of Lernseodiscus porcellanae. C. Adult of Peltogaster paguri.
| |
| | |
| II, III. IV. Two pairs of antennae
| |
| and mandibles; cp. carapace; a. anterior
| |
| end of body; b. generative aperture; c.
| |
| root-like processes.
| |
| | |
| | |
| | |
| pated from their close relationship to Anelasma squalicola amongst the
| |
| Thoracica, undergo a development differing much less from the type of the
| |
| Thoracica than that of Cryptophialus and Kochlorine.
| |
| | |
| 322
| |
| | |
| | |
| | |
| 5oo
| |
| | |
| | |
| | |
| OSTRACODA.
| |
| | |
| | |
| | |
| Sacculina leaves the egg as a Nauplius (fig. 233 A), which differs from
| |
| the ordinary type mainly (i) in the large development of an oval dorsal
| |
| shield (eft] which projects far beyond the edge of the body, but is provided
| |
| with the typical sternal horns, etc. ; and (2) in the absence of a mouth.
| |
| The Cypris and pupa stages of Sacculina and other Rhizocephala (fig. 233 B)
| |
| are closely similar to those of the Thoracica, but the paired eye is absent.
| |
| The attachment takes place in the usual way, but the subsequent metamorphosis leads to the loss of the thoracic feet and generally to retrogressive
| |
| changes.
| |
| | |
| OSTRACODA.
| |
| | |
| Our knowledge of the development of this remarkable group is entirely
| |
| due to the investigations of Claus.
| |
| | |
| Some forms of Cythere are viviparous, and in the marine form Cypridina
| |
| the embryo develops within the valves of the shell. Cypris attaches its
| |
| eggs to water plants. The larvae of Cypris are free, and their development
| |
| is somewhat complicated. The whole development is completed in nine
| |
| ecdyses, each of them accompanied by more or less important changes in
| |
| the constitution of the larva.
| |
| | |
| In the earliest free stage the larva has the characters of a true Nauplius
| |
| with three pairs of appendages (fig. 234 A). The Nauplius presents howB A
| |
| | |
| | |
| | |
| -A'
| |
| | |
| | |
| | |
| | |
| MX SM
| |
| | |
| | |
| | |
| FlG. 234. TWO STAGES IN THE DEVELOPMENT OF CYPRIS. (From ChlUS.)
| |
| | |
| A. Earliest (Nauplius) stage. B. Second stage.
| |
| | |
| A'. A". First and second pairs of antennae ; Md. mandibles ; OL. labrum ;
| |
| MX,', first pair of maxilla; /". first pair of feet.
| |
| | |
| ever one or two very marked secondary characters. In the first place it is
| |
| completely enveloped in a fully formed bivalve shell, differing in unessential
| |
| points from the shell of the adult. An adductor muscle (SM] for the shell
| |
| is present. Again the second and third appendages, though locomotive in
| |
| function are neither of them biramous, and the third one already contains
| |
| a rudiment of the future mandibular blade, and terminates in an anteriorly
| |
| directed hook-like bristle. The first pair of antenna? is moreover very
| |
| similar to the second and is used in progression. Neither of the pairs of
| |
| | |
| | |
| | |
| CRUSTACEA.
| |
| | |
| | |
| | |
| 501
| |
| | |
| | |
| | |
| antennae become much modified in the subsequent metamorphosis. The
| |
| Nauplius has a single median eye, as in the adult Cypris, and a fully
| |
| developed alimentary tract.
| |
| | |
| The second stage (fig. 234 B), inaugurated by the first moult, is mainly
| |
| characterized by the appearance of two fresh pairs of appendages, viz. the
| |
| first pair of maxillae and the first pair of feet ; the second pair of maxillae
| |
| not being developed till later. The first pair appear as leaf-like curved
| |
| | |
| | |
| | |
| | |
| | |
| FIG. 235. STAGES IN THE DEVELOPMENT OF CYPRIS. (From Claus.)
| |
| | |
| A. Fourth stage. B. Fifth stage.
| |
| | |
| MX', first maxilla ; MX", and/', second maxilla ; /". first pair of feet ; L. liver.
| |
| | |
| plates (Mx'} more or less like Phyllopod appendages (Claus) though at this
| |
| stage without an exopodite. The first pair of feet (/"} terminates in a
| |
| curved claw and is used for adhering. The mandibles have by this stage
| |
| fully developed blades, and have practically attained their adult form, consisting of a powerful toothed blade and a four-jointed palp.
| |
| | |
| During the third and fourth stages the first pair of maxillae acquire
| |
| their pectinated gill plate (epipodite) and four blades ; and in the fourth
| |
| stage (fig. 235 A) the second pair of maxillae (Mx"} arises, as a pair of
| |
| curved plates, similar to the first pair of maxillae at their first appearance.
| |
| The forked tail is indicated during the fourth stage by two bristles. During
| |
| the fifth stage (fig. 235 B) the number of joints of the first pair of antennae
| |
| becomes increased, and the posterior maxillae develop a blade and become
| |
| | |
| | |
| | |
| 502
| |
| | |
| | |
| | |
| PHYLOGENY OF THE CRUSTACEA.
| |
| | |
| | |
| | |
| four-jointed ambulatory appendages terminating in a hook. The caudal fork
| |
| becomes more distinct.
| |
| | |
| In the sixth stage (fig. 236) the second and hindermost pair of feet becomes formed (/"') and the maxillae of the second pair lose their ambulatory
| |
| function, and begin to be converted into definite masticatory appendages by
| |
| the reduced jointing of their palp, and the increase of their cutting blades.
| |
| By the seventh stage all the appendages have practically attained their
| |
| | |
| | |
| | |
| Fu
| |
| | |
| | |
| | |
| | |
| FIG. 236. SIXTH STAGE IN THE DEVELOPMENT OF CYPRIS. (From Claus.)
| |
| MX!, first maxilla ; Mx".f. second maxilla; /'. and/"', first and second pair oi
| |
| feet ; Fu. caudal fork ; L. liver ; S.D. shell gland.
| |
| | |
| permanent form ; the second pair of maxillae has acquired small branchial
| |
| plates, and the two following feet have become jointed. In the eighth and
| |
| ninth stages the generative organs attain their mature form.
| |
| | |
| The larva of Cythere at the time of birth has rudiments of all the limbs,
| |
| but the mandibular palp still functions as a limb, and the three feet (2nd
| |
| pair of maxillae and two following appendages) are very rudimentary.
| |
| | |
| The larvae of Cypridina are hatched in a condition which to all intents
| |
| and purposes resembles the adult.
| |
| | |
| | |
| | |
| Phylogeny of the Crustacea.
| |
| | |
| The classical work of Fritz Miiller (No. 452) on the phylogeny of the
| |
| Crustacea has given a great impetus to the study of their larval forms, and
| |
| the interpretations of these forms which he has offered have been the subject
| |
| of a very large amount of criticism and discussion. A great step forward
| |
| in this discussion has been recently made by Claus in his memoir (No. 448).
| |
| | |
| The most fundamental question concerns the meaning of the Nauplius.
| |
| Is the Nauplius the ancestral form of the Crustacea, as is believed by Fritz
| |
| Miiller and Claus, or are its peculiarities and constant occurrence due to
| |
| some other cause ? The most plausible explanation on the second hypothesis
| |
| | |
| | |
| | |
| CRUSTACEA. 503
| |
| | |
| | |
| | |
| would seem to be the following. The segments with their appendages of
| |
| Arthropoda and Annelida are normally formed from before backwards,
| |
| therefore every member of these two groups with more than three segments
| |
| must necessarily pass through a stage with only three segments, and the fact
| |
| that in a particular group this stage is often reached on the larva being
| |
| hatched is in itself no proof that the ancestor of the group had only three
| |
| segments with their appendages. This explanation appears to me, so far
| |
| as it goes, quite valid ; but though it relieves us from the necessity of
| |
| supposing that the primitive Crustacea had only three pairs of appendages,
| |
| it does not explain several other peculiarities of the Nauplius 1 . The more
| |
| important of these are the following.
| |
| | |
| 1. That the mandibles have the form of biramous swimming feet and
| |
| are not provided with a cutting blade.
| |
| | |
| 2. That the second pair of antennae are biramous swimming feet with a
| |
| hook used in mastication, and are innervated (?) from the subcesophageal
| |
| ganglion.
| |
| | |
| 3. The absence of segmentation in the Nauplius body. An absence
| |
| which is the more striking in that before the Nauplius stage is fully reached
| |
| the body of the embryo is frequently divided into three segments, e.g.
| |
| Copepoda and Cirripedia
| |
| | |
| 4. The absence of a heart.
| |
| | |
| 5. The presence of a median single eye as the sole organ of vision.
| |
| | |
| Of these points the first, second, and fifth appear only to be capable of
| |
| being explained phylogenetically, while with reference to the absence of a
| |
| heart it appears very improbable that the ancestral Crustacea were without
| |
| a central organ of circulation. If the above positions are accepted the
| |
| conclusion would seem to follow that in a certain sense the Nauplius is
| |
| an ancestral form but that, while it no doubt had its three anterior pairs
| |
| of appendages similar to those of existing Nauplii, it may perhaps have
| |
| been provided with a segmented body behind provided with simple biramous
| |
| appendages. A heart and cephalo-thoracic shield may also have been
| |
| present, though the existence of the latter is perhaps doubtful. There was
| |
| no doubt a median single eye, but it is difficult to decide whether or no
| |
| paired compound eyes were also present. The tail ended in a fork between
| |
| the prongs of which the anus opened ; and the mouth was protected by a
| |
| large upper lip. In fact, it may very probably turn out that the most
| |
| primitive Crustacea more resembled an Apus larva at the moult immediately
| |
| before the appendages lose their Nauplius characters (fig. 208 B), or a
| |
| Cyclops larva just before the Cyclops stage (fig. 229), than the earliest
| |
| Nauplius of either of these forms.
| |
| | |
| If the Nauplius ancestor thus reconstructed is admitted to have existed,
| |
| the next question in the phylogeny of the Crustacea concerns the relations
| |
| of the various phyla to the Nauplius. Are the different phyla descended
| |
| from the Nauplius direct, or have they branched at a later period from
| |
| | |
| 1 For the characters of Nauplius vide p. 460.
| |
| | |
| | |
| | |
| 504 PHYLOGENY OF THE CRUSTACEA.
| |
| | |
| some central stem? It is perhaps hardly possible as yet to give a full and
| |
| satisfactory answer to this question, which requires to be dealt with for each
| |
| separate phylum ; but it may probably be safely maintained that the existing
| |
| Phyllopods are members of a group which was previously much larger, and
| |
| the most central of all the Crustacean groups; and which more nearly
| |
| retains in the characters of the second pair of antennae etc. the Nauplius
| |
| peculiarities. This view is shared both by Claus and Dohrn, and appears
| |
| to be in accordance with all the evidence we have both palaeontological and
| |
| morphological. Claus indeed carries this view still further, and believes
| |
| that the later Nauplius stages of the different Entomostracan groups and
| |
| the Malacostraca (Penaeus larva) exhibit undoubted Phyllopod affinities.
| |
| He therefore postulates the earlier existence of a Protophyllopod form, which
| |
| would correspond very closely with the Nauplius as reconstructed above,
| |
| from which he believes all the Crustacean groups to have diverged.
| |
| | |
| It is beyond the scope of this work to attempt to grapple with all the
| |
| difficulties which arise in connection with the origin and relationships of the
| |
| various phyla, but I confine myself to a few suggestions arising out of the
| |
| developmental histories recorded above.
| |
| | |
| Malacostraca. In attempting to reconstitute from the evidence in
| |
| our possession the ancestral history of the Malacostraca we may omit from
| |
| consideration the larval history of all those types which leave the egg in
| |
| nearly the adult form, and confine our attention to those types in which the
| |
| larval history is most completely preserved.
| |
| | |
| There are three forms which are of special value in this respect, viz.
| |
| Euphausia, Penaeus and Squilla. From the history of these which has
| |
| already been given it appears that in the case of the Decapoda four stages
| |
| (Claus) may be traced in the best preserved larval histories.
| |
| | |
| 1. A Nauplius stage with the usual Nauplius characters.
| |
| | |
| 2. A Protozoaea stage in which the maxillae and first pair of maxillipeds
| |
| are formed behind the Nauplius appendages ; but in which the tail is still
| |
| unsegmented. This stage is comparatively rarely preserved and usually not
| |
| very well marked.
| |
| | |
| 3. A Zoaea stage the chief features of which have already been fully
| |
| characterised (vide p. 465). Three more or less distinct types of Zosea are
| |
| distinguished by Claus. (a) That of Penaeus, in which the appendages up
| |
| to the third pair of maxillipeds are formed, and the thorax and abdomen are
| |
| segmented, the former being however very short. The heart is oval, with
| |
| one pair of ostia. From this type the Zoaea forms of the other Decapoda
| |
| are believed by Claus to be derived, (b} That of Euphausia, with but one
| |
| pair of maxillipeds and those short and Phyllopod-like. The heart oval
| |
| with one pair of ostia. (c) That of Squilla, with an elongated manychambered heart, two pairs of maxillipeds and the abdominal appendages in
| |
| full activity.
| |
| | |
| 4. A Mysis stage, which is only found in the macrurous Decapod
| |
| larvie.
| |
| | |
| The embryological questions requiring to be settled concern the value
| |
| | |
| | |
| | |
| CRUSTACEA. 50$
| |
| | |
| | |
| | |
| of the above stages. Do they represent stages in the actual evolution of
| |
| the present types, or have their characters been secondarily acquired in
| |
| larval life ?
| |
| | |
| With reference to the first stage this question has already been discussed,
| |
| and the conclusion arrived at, that the Nauplius does in a much modified
| |
| form represent an ancestral type. As to the fourth stage there can be no
| |
| doubt that it is also ancestral, considering that it is almost the repetition of
| |
| an actually existing form.
| |
| | |
| The second stage can clearly only be regarded as an embryonic preparation for the third ; and the great difficulty concerns the third stage.
| |
| | |
| The natural view is that this stage like the others has an ancestral value,
| |
| and this view was originally put forward by Fritz Miiller and has been
| |
| argued for also by Dohrn. On the other hand the opposite side has been
| |
| taken by Claus, who has dealt with the question very ably and at great
| |
| length, and has clearly shewn that some of Fritz Miiller's positions are
| |
| untenable. Though Claus' opinion is entitled to very great weight, an
| |
| answer can perhaps be given to some of his objections. The view adopted
| |
| in this section can best be explained by setting forth the chief points which
| |
| Claus urges against Fritz Miiller's view.
| |
| | |
| The primary question which needs to be settled is whether the Malacostraca have diverged very early from the Nauplius root, or later in the history
| |
| of the Crustacea from the Phyllopod stem. On this question Claus 1 brings
| |
| arguments, which appear to me very conclusive, to shew that the Malacostraca are derived from a late Protophyllopod type, and Claus' view on this
| |
| point is shared also by Dohrn. The Phyllopoda present so many characters
| |
| (not possessed by the Nauplius) in common with the Malacostraca or their
| |
| larval forms, that it is incredible that the whole of these should have
| |
| originated independently in the two groups. The more important of these
| |
| characters are the following.
| |
| | |
| 1. The compound eyes, so often stalked in both groups.
| |
| | |
| 2. The absence of a palp on the mandible, a very marked character of
| |
| the Zoasa as well as of the Phyllopoda.
| |
| | |
| 3. The presence of a pair of frontal sense knobs.
| |
| | |
| 4. The Phyllopod character of many of the appendages. Cf. first pair of
| |
| maxillipeds of the Euphausia Zosea.
| |
| | |
| 1 Claus speaks of the various Crustacean phyla as having sprung from a Protophyllopod form, and it might be supposed that he considered that they all diverged from
| |
| the same form. It is clear however from the context that he regards the Protophyllopod type from which the Malacostraca originated as far more like existing Phyllopods than that from which the Entomostracan groups have sprung. It is not quite
| |
| easy to get a consistent view of his position on the question, since he states (p. 77) that
| |
| the Malacostraca and the Copepods diverged from a similar form, which is represented
| |
| in their respective developments by the Protozosea and earliest Cyclops stage. Yet if
| |
| I understand him rightly, he does not consider the Protozosea stage to be the Protophyllopod stage from which the Malacostraca have diverged, but states on p. 71 that
| |
| it was not an ancestral form at all.
| |
| | |
| | |
| | |
| 506 PHYLOGENY OF THE CRUSTACEA.
| |
| | |
| 5. The presence of gill pouches (epipodites) on many of the appendages 1 .
| |
| | |
| In addition to these points, to which others might be added, Claus
| |
| attempts to shew that Nebalia must be regarded as a type intermediate
| |
| between the Phyllopods and Malacostraca. This view seems fairly established, and if true is conclusive in favour of the Phyllopod origin of the
| |
| Malacostraca. If the Protophyllopod origin of the Malacostraca is admitted,
| |
| it seems clear that the ancestral forms of the Malacostraca must have developed their segments regularly from before backwards, and been provided
| |
| with nearly similar appendages on all the segments. This however is far
| |
| from the case in existing Malacostraca, and Fritz Miiller commences his
| |
| summary of the characters of the Zoaea in the following words 2 . "The
| |
| middle body with its appendages, those five pairs of feet to which these
| |
| animals owe their name, is either entirely wanting or scarcely indicated."
| |
| This he regards as an ancestral character of the Malacostraca, and is of
| |
| opinion that their thorax is to be regarded as a later acquirement than the
| |
| head or abdomen. Claus' answer on this point is that in the most primitive
| |
| Zoasas, viz. those already spoken of as types, the thoracic and abdominal
| |
| segments actually develop, in regular succession from before backwards,
| |
| and he therefore concludes that the late development of the thorax in the
| |
| majority of Zoaea forms is secondary and not an ancestral Phyllopod
| |
| peculiarity.
| |
| | |
| This is the main argument used by Claus against the Zosea having any
| |
| ancestral meaning. His view as to the meaning of the Zoaea may be
| |
| gathered from the following passage. After assuming that none of the
| |
| existing Zoaea types could have been adult animals, he says" Much more
| |
| "probably the process of alteration of the metamorphosis, which the Mala" costracan phylum underwent in the course of time and in conjunction
| |
| " with the divergence of the later Malacostracan groups, led secondarily
| |
| " to the three different Zoaea configurations to which probably later modifica" tions were added, as for instance in the young form of the Cumaceae. We
| |
| "might with the same justice conclude that adult Insects existed as cater" pillars or pupae as that the primitive form of the Malacostraca was a
| |
| " Protozoaea or Zoaea."
| |
| | |
| Granting Claus' two main positions, viz. that the Malacostraca are
| |
| derived from Protophyllopods, and that the segments were in the primary
| |
| ancestral forms developed from before backwards, it does not appear impossible that a secondary and later ancestral form may have existed with a
| |
| reduced thorax. This reduction may only have been partial, so that the
| |
| Zoaea ancestor would have had the following form. A large cephalo-thorax
| |
| and well-developed tail (?) with swimming appendages. The appendages up
| |
| to the second pair of maxillipeds fully developed, but the thorax very
| |
| | |
| 1 Claus appears to consider it doubtful whether the Malacostracan gills can be
| |
| compared with the Phyllopod gill-pouches.
| |
| 3 Facts for Darwin, p. 49.
| |
| | |
| | |
| | |
| CRUSTACEA. 507
| |
| | |
| | |
| | |
| imperfect and provided only with delicate foliaceous appendages not projecting beyond the edge of the cephalo-thoracic shield.
| |
| | |
| Another hypothesis for which there is perhaps still more to be said is
| |
| that there was a true ancestral Zoaea stage in which the thoracic appendages
| |
| were completely aborted. Claus maintains that the Zoaea form with
| |
| aborted thorax is only a larval form ; but he would probably admit that its
| |
| larval characters were acquired to enable the larva to swim better. If this
| |
| much be admitted it is not easy to see why an actual member of the
| |
| ancestral series of Crustacea should not have developed the Zoaea peculiarities when the mud-dwelling habits of the Phyllopod ancestors were
| |
| abandoned, and a swimming mode of life adopted. This view, which
| |
| involves the supposition that the five (or six including the third maxillipeds)
| |
| thoracic appendages were lost in the adult (for they may be supposed to
| |
| have been retained in the larva) for a series of generations, and reappeared
| |
| again in the adult condition, at a later period, may at first sight appear very
| |
| improbable, but there are, especially in the larval history of the Stomatopoda,
| |
| some actual facts which receive their most plausible explanation on this
| |
| hypothesis.
| |
| | |
| These facts consist in cases of the actual loss of appendages during
| |
| development, and their subsequent reappearance. The two most striking
| |
| cases are the following.
| |
| | |
| 1. In the Erichthus form of the Squilla larva the appendages corresponding to the third pair of maxillipeds and first two pairs of ambulatory
| |
| appendages of the Decapoda are developed in the Protozosea stage, but
| |
| completely aborted in the Zoasa stage, and subsequently redeveloped.
| |
| | |
| 2. In the case of the larva of Sergestes in the passage from the Acanthosoma (Mysis) stage to the Mastigopus stage the two hindermost thoracic
| |
| appendages become atrophied and redevelop again later.
| |
| | |
| Both of these cases clearly fit in very well with the view that there was an
| |
| actual period in the history of the Malacostraca in which the ancestors of
| |
| the present forms were without the appendages which are aborted and
| |
| redeveloped again in these larval forms. Claus' hypothesis affords no
| |
| explanation of these remarkable cases.
| |
| | |
| It is however always possible to maintain that the loss and reappearance
| |
| of the appendages in these cases may have no ancestral meaning ; and the
| |
| abortion of the first pair of maxillipeds and reduction of some of the other
| |
| appendages in the case of the Loricata is in favour of this explanation.
| |
| Similar examples of the abortion and reappearance of appendages, which
| |
| cannot be explained in the way attempted above, are afforded by the Mites
| |
| and also by the Insects, e.g. Bees.
| |
| | |
| On the other hand there is almost a conclusive indication that the loss
| |
| of the appendages in Sergestes has really the meaning assigned to it, in that
| |
| in the allied genius Leucifer the two appendages in question are actually
| |
| absent in the adult, so that the stage with these appendages absent is
| |
| permanently retained in an adult form. In the absence of the mandibular
| |
| palp in all the Zoaea forms, its actual atrophy in the Penaeus Zoasa, and its
| |
| | |
| | |
| | |
| 508 PHYLOGENY OF THE CRUSTACEA.
| |
| | |
| universal reappearance in adult Malacostraca, are cases which tell in favour
| |
| of the above explanation. The mandibular palp is permanently absent in
| |
| Phyllopods, which clearly shews that its absence in the Zoaea stage is due to
| |
| the retention of an ancestral peculiarity, and that its reappearance in the
| |
| adult forms was a late occurrence in the Malacostracan history.
| |
| | |
| The chief obvious difficulty of this view is the redevelopment of the
| |
| thoracic feet after their disappearance for a certain number of generations.
| |
| The possibility of such an occurrence appears to me however clearly demonstrated by the case of the mandibular palp, which has undoubtedly been
| |
| reacquired by the Malacostraca, and by the case of the two last thoracic
| |
| appendages of Sergestes just mentioned. The above difficulty may be
| |
| diminished if we suppose that the larvae of the Zoaea ancestors always
| |
| developed the appendages in question. Such appendages might first only
| |
| partially atrophy in a particular Zoaea form and then gradually come to
| |
| be functional again ; so that, as a form with functional thoracic limbs
| |
| came to be developed out of the Zoaea, we should find in the larval history
| |
| of this form that the limbs were developed in the pre-zoaeal larval stages,
| |
| partially atrophied in the Zoaea stage, and redeveloped in the adult. From
| |
| this condition it would not be difficult to pass to a further one in which the
| |
| development of the thoracic limbs became deferred till after the Zoaea stage.
| |
| | |
| The general arguments in favour of a Zoaea ancestor with partially or
| |
| completely aborted thoracic appendages having actually existed in the past
| |
| appear to me very powerful. In all the Malacostracan groups in which
| |
| the larva leaves the egg in an imperfect form a true Zoaea stage is found.
| |
| That the forms of these Zoaeas should differ considerably is only what might
| |
| be expected, considering that they lead a free existence and are liable to
| |
| be acted upon by natural selection, and it is probable that none of those
| |
| at present existing closely resemble the ancestral form. The spines from
| |
| their carapace, which vary so much, were probably originally developed,
| |
| as suggested by Fritz Miiller, as a means of defence. The simplicity of
| |
| the heart so different from that of Phyllopods in most forms of Zoaea
| |
| is a difficulty, but the reduction in the length of the heart may very
| |
| probably be a secondary modification ; the primitive condition being retained
| |
| in the Squilla Zoaea. In any case this difficulty is not greater on the
| |
| hypothesis of the Zoaea being an ancestral form, than on that of its being a
| |
| purely larval one.
| |
| | |
| The points of agreement in the number and character of the appendages,
| |
| form of the abdomen, etc. between the various types of Zoaea appear to me
| |
| too striking to be explained in the manner attempted by Claus. It seems
| |
| improbable that a peculiarity of form acquired by the larva of some ancestral
| |
| Malacostracan should have been retained so permanently in so many groups l
| |
| | |
| 1 A secondary larval form is less likely to be repeated in development than an
| |
| ancestral adult stage, because there is always a strong tendency for the former, which
| |
| is a secondarily intercalated link in the chain, to drop out by the occurrence of a
| |
| reversion to the original type of development.
| |
| | |
| | |
| | |
| CRUSTACEA. 509
| |
| | |
| | |
| | |
| more permanently indeed than undoubtedly ancestral forms like that of
| |
| Mysis and it would be still more remarkable that a Zoaea form should
| |
| have been two or more times independently developed.
| |
| | |
| There are perhaps not sufficient materials to reconstruct the characters of
| |
| the Zoaea ancestor, but it probably was provided with the anterior appendages up to the second pair of maxillipeds, and (?) with abdominal swimming feet. The heart may very likely have been many-chambered.
| |
| Whether gill pouches were present on the maxillipeds and abdominal feet
| |
| does not appear to me capable of being decided. The carapace and general
| |
| shape were probably the same as in existing Zoaeas. It must be left an open
| |
| question whether the six hindermost thoracic appendages were absent or
| |
| only very much reduced in size.
| |
| | |
| On the whole then it may be regarded as probable that the Malacostraca
| |
| are descended from Protophyllopod forms, in which, on the adoption of
| |
| swimming habits, six appendages of the middle region of the body were
| |
| reduced or aborted, and a Zoaea form acquired, and that subsequently the
| |
| lost appendages were redeveloped in the descendants of these forms, and
| |
| have finally become the most typical appendages of the group.
| |
| | |
| The relationship of the various Malacostracan groups is too difficult
| |
| a subject to be discussed here, but it seems to me most likely that in
| |
| addition to the groups with a Zoaea stage the Edriophthalmata and Cumaceae
| |
| are also post-zoaeal forms which have lost the Zoasa stage. Nebalia is
| |
| however very probably to be regarded as a prae-zoaeal form which has
| |
| survived to the present day ; and one might easily fancy that its eight thin
| |
| thoracic segments with their small Phyllopod-like feet might become nearly
| |
| aborted.
| |
| | |
| Copepoda. The Copepoda certainly appear to have diverged very
| |
| early from the main stem, as is shewn by their simple biramous feet and the
| |
| retention of the median eye as the sole organ of vision. It may be argued
| |
| that they have lost the eye by retrogressive changes, and in favour of this
| |
| view cases of the Pontellidae and of Argulus may be cited. It is however
| |
| more than doubtful whether the lateral eyes of the Pontellidae are related to
| |
| the compound Phyllopod eye, and the affinities of Argulus are still uncertain.
| |
| It would moreover be a great paradox if in a large group of Crustacea the
| |
| lateral eyes had been retained in a parasitic form only (Argulus), but lost in
| |
| all the free forms.
| |
| | |
| Cirripedia. The Cirripedia are believed by Claus to belong to the
| |
| same phylum as the Copepoda. This view does not appear to be completely
| |
| borne out by their larval history. The Nauplius differs very markedly from
| |
| that of the Copepoda, and this is still more true of the Cypris stage. The
| |
| Copepod-like appendages of this stage are chiefly relied upon to support the
| |
| above view, but this form of appendages was probably very primitive
| |
| and general, and the number (without taking into consideration the doubtful
| |
| case of Cryptophialus) does not correspond to that in Copepoda. On the
| |
| other hand the paired eyes and the bivalve shell form great difficulties in the
| |
| way of Claus' view. It is clear that the Cypris stage represents more or less
| |
| | |
| | |
| | |
| 510
| |
| | |
| | |
| | |
| PHYLOGENY OF THE CRUSTACEA.
| |
| | |
| | |
| | |
| closely an ancestral form of the Cirripedia, and that both the large bivalve
| |
| shell and the compound eyes were ancestral characters. These characters
| |
| would seem incompatible with Copepod affinities, but point to the independent derivation of the Cirripedia from some early bivalve Phyllopod form.
| |
| | |
| Ostracoda. The independent origin of the Ostracoda from the main
| |
| Crustacean stem seems probable. Claus points out that the Ostracoda
| |
| present by no means a simple organisation, and concludes that they were
| |
| not descended from a form with a more complex organisation and a larger
| |
| number of appendages. Some simplifications have however undoubtedly
| |
| taken place, as the loss of the heart, and of the compound eyes in many
| |
| forms. These simplifications are probably to be explained (as is done by
| |
| Claus) as adaptations due to the small size of body and its enclosure in a
| |
| thick bivalve shell. Although Claus is strongly opposed to the view that
| |
| | |
| | |
| | |
| I)
| |
| | |
| | |
| | |
| | |
| FIG. 737. FIGURES ILLUSTRATING THE DEVELOPMENT OF ASTACUS. (From
| |
| Parker ; after Reichenbach.)
| |
| | |
| A. Section through part of the ovum during segmentation, n. nuclei ; w.y. white
| |
| yolk ; y.p. yolk pyramids ; c. central yolk mass.
| |
| | |
| B and C. Longitudinal sections during the gastrula stage, a. archenteron ;
| |
| b. blastopore ; ms. mesoblast ; ec. epiblast ; en. hypoblast distinguished from epiblast
| |
| by shading.
| |
| | |
| I '. Highly magnified view of the anterior lip of blastopore to shew the origin of
| |
| the primary mesoblast from the wall of the archenteron. p.ms. primary mesoblast ;
| |
| ec. epiblast ; en. hypoblast.
| |
| | |
| I Two hypoblast cells to shew the amoeba-like absorption of yolk spheres.
| |
| y. yolk ; . nucleus ; /. pseudopodial process.
| |
| | |
| F. Hypoblast cells giving rise endogenously to the secondary mesoblast (s.nts.).
| |
| tt. nuclei.
| |
| | |
| | |
| | |
| CRUSTACEA. 5 1
| |
| | |
| | |
| | |
| the number of the appendages has been reduced, yet the very fact of the
| |
| (in some respects) complex organisation of this group might seem to indicate
| |
| that it cannot have diverged from the Phyllopod stem at so early a stage as
| |
| (on Claus' view of the Nauplius) would seem to be implied by the very small
| |
| number of appendages which is characteristic of it, and it therefore appears
| |
| most probable that the present number may be smaller than that of the
| |
| ancestral forms.
| |
| | |
| The formation of the germinal layers.
| |
| | |
| The formation of the germinal layers has been more fully
| |
| studied in various Malacostraca, more especially in the Decapoda,
| |
| than in other groups.
| |
| | |
| Decapoda. To Bobretzky (No. 472) is due the credit of
| |
| having been the pioneer in this line of investigation ; and his
| |
| researches have been followed up and enlarged by Haeckel,
| |
| Reichenbach (No. 488), and Mayer (No. 482). The segmentation
| |
| is centrolecithal and regular (fig. 237 A). At its close the
| |
| blastoderm is formed of a single uniform layer of lens-shaped
| |
| cells enclosing a central sphere of yolk, in which as a rule all
| |
| trace of the division into columns, present during the earlier
| |
| stages of segmentation, has disappeared ; though in Palaemon
| |
| the columns remain for a long period distinct. The cells of the
| |
| blastoderm are at first uniform, but in Astacus, Eupagurus,
| |
| and most Decapoda, soon become more columnar for a small
| |
| area, and form a circular patch. The whole patch either
| |
| becomes at once invaginated (Eupagurus, Palaemon, fig. 239 A)
| |
| or else the edge of it is invaginated as a roughly speaking
| |
| circular groove deeper anteriorly than posteriorly, within which
| |
| the remainder of the patch forms a kind of central plug, which
| |
| does not become invaginated till a somewhat later period
| |
| (Astacus, fig. 237 B and C). After the invagination of the
| |
| above patch the remainder of the blastoderm cells form the
| |
| epiblast.
| |
| | |
| The invaginated sack appears to be the archenteron and its
| |
| mouth the blastopore. The mouth finally becomes closed 1 , and
| |
| the sack itself then forms the mesenteron.
| |
| | |
| In Astacus the archenteron gradually grows forwards, its
| |
| opening is at first wide, but becomes continuously narrowed
| |
| | |
| 1 Bobretzky first stated that the invagination remained open, but subsequently
| |
| corrected himself. Zeit. /. Wiss. Zool., Bd. xxiv. p. 186.
| |
| | |
| | |
| | |
| 512
| |
| | |
| | |
| | |
| FORMATION OF THE LAYERS.
| |
| | |
| | |
| | |
| | |
| and is finally obliterated. Very shortly after this occurrence
| |
| there is formed, slightly in front of the point where the last trace
| |
| of the blastopore was observable, a fresh epiblastic invagination,
| |
| which gives rise to the proctodaeum, and the opening of which
| |
| remains as the definite anus. The proctodaeum (fig. 238 A, kg)
| |
| is very soon placed in communication with the mesenteron (mg).
| |
| The stomodaeum (fg) is formed during the same stage as the
| |
| proctodaeum. It gives rise to the oesophagus and stomach.
| |
| The hypoblast cells which form the wall of the archenteron
| |
| grow with remarkable rapidity at the expense of the yolk ; the
| |
| spherules of which they absorb and digest in an amceba-like
| |
| fashion by means of their pseudopodia. They become longer
| |
| and longer, and finally, after absorbing the whole yolk, acquire
| |
| a form almost exactly similar to
| |
| that of the yolk pyramids during segmentation (fig. 238 B).
| |
| They enclose the cavity of the
| |
| mesenteron, and their nuclei
| |
| and protoplasm are situated externally. The cells of the mesenteron close to its junction
| |
| with the proctodaeum differ
| |
| from those elsewhere in being
| |
| nearly flat.
| |
| | |
| In Palaemon (Bobretzky)
| |
| the primitive invagination (fig.
| |
| 239 A) has far smaller dimensions than in Astacus, and appears before the blastoderm
| |
| cells have separated from the
| |
| yolk pyramids. The cells which
| |
| are situated at the bottom of it
| |
| pass into the yolk, increase in
| |
| number, and absorb the whole
| |
| yolk, forming a solid mass of
| |
| hypoblast in which the outlines
| |
| of the individual cells would
| |
| seem at first not to be distinct.
| |
| | |
| | |
| | |
| | |
| FlG. 238. TWO LONGITUDINAL SECTIONS OF THE EMBRYO OF ASTACUS.
| |
| | |
| (From Parker ; after Bobretzky.)
| |
| | |
| A. Nauplius stage. B. Stage after
| |
| the hypoblast cells have absorbed the
| |
| food yolk. The ventral surface is turned
| |
| upwards, fg. stomodseum ; hg. proctodccum ; an. anus ; m. mouth ; mg. mesenteron ; abd. abdomen ; h. heart.
| |
| | |
| The blastopore in the mean
| |
| | |
| | |
| CRUSTACEA. 513
| |
| | |
| time becomes closed. Some of the nuclei now pass to the
| |
| periphery of the yolk mass ; the cells appertaining to them
| |
| gradually become distinct and assume a pyramidal form (fig.
| |
| 239 B, hy\ the inner ends of the cells losing themselves in a
| |
| central mass of yolk, in the interior of which nuclei are at first
| |
| present but soon disappear. The mesenteron thus becomes
| |
| constituted of a layer of pyramidal cells which merge into
| |
| a central mass of yolk. Some of the hypoblast cells adjoining
| |
| the junction of the proctodaeum and mesenteron become
| |
| flattened, and in the neighbourhood of these cells a lumen
| |
| | |
| | |
| | |
| | |
| FlG. 239. TWO STAGES IN THE DEVELOPMENT OF PAL^MON SEEN IN SECTION.
| |
| | |
| (After Bobretzky.)
| |
| | |
| A. Gastrula stage.
| |
| | |
| B. Longitudinal section through a late stage, hy. hypoblast ; sg. supra-resophageal ganglion ; vg. ventral nerve cord ; hd. proctodseum ; st. stomodseum.
| |
| | |
| first appears. The stomodaeum and proctodaeum are formed as
| |
| in Astacus. Fig. 239 B shews the relative positions of the
| |
| proctodaeum, stomodaeum, and mesenteron. Although the
| |
| process of formation of the hypoblast and mesenteron is
| |
| essentially the same in Astacus and Palaemon, yet the differences
| |
| between these two forms are very interesting, in that the yolk is
| |
| external to the mesenteron in Astacus, but enclosed within it in
| |
| Palaemon. This difference in the position of the yolk is rendered
| |
| possible by the fact that the invaginated hypoblast cells in
| |
| Palaemon do not, at first, form a continuous layer enclosing a
| |
| central cavity, while they do so in Astacus.
| |
| | |
| The mesoblast appears to be formed of cells budded off
| |
| from the anterior wall of the archenteron (Astacus, fig. 237 D),
| |
| B. II. 33
| |
| | |
| | |
| | |
| 514 FORMATION OF THE LAYERS.
| |
| | |
| or from its lateral walls generally (Palaemon). They make
| |
| their first appearance soon after the imagination of the hypoblast has commenced. The mesoblast cells are at first spherical,
| |
| and gradually spread, especially in an anterior direction, from
| |
| their point of origin.
| |
| | |
| According to Reichenbach there are formed in Astacus at the Nauplius
| |
| stage a number of peculiar cells which he speaks of as * secondary mesoblast
| |
| cells.' His account is not very clear or satisfactory, but it appears that they
| |
| originate (fig. 237 F) in the hypoblast cells by a kind of endogenous growth,
| |
| and though they have at first certain peculiar characters they soon become
| |
| indistinguishable from the remaining mesoblast cells.
| |
| | |
| Towards the end of the Nauplius period the secondary mesoblast cells
| |
| aggregate themselves into a rod close to the epiblast in the median ventral
| |
| line, and even bifurcate round the mouth and extend forwards to the
| |
| extremity of the procephalic lobes. This rod of cells very soon vanishes,
| |
| and the secondary mesoblast cells become indistinguishable from the
| |
| primary. Reichenbach believes, on not very clear evidence, that these cells
| |
| have to do with the formation of the blood.
| |
| | |
| General form of the body. The ventral thickening of epitlast
| |
| or ventral plate, continuous with the invaginated patch already
| |
| mentioned, forms the first indication of the embryo. It is at
| |
| first oval, but soon becomes elongated and extended anteriorly
| |
| into two lateral lobes the procephalic lobes. Its bilateral
| |
| symmetry is further indicated by a median longitudinal furrow.
| |
| The posterior end of the ventral plate next becomes raised into
| |
| a distinct lobe the abdomen which in Astacus at first lies in
| |
| front of the still open blastopore. This lobe rapidly grows in size,
| |
| and at its extremity is placed the narrow anal opening. It soon
| |
| forms a well-marked abdomen bent forwards over the region in
| |
| front (figs. 239 B, and 240 A and B). Its early development
| |
| as a distinct outgrowth causes it to be without yolk ; and so to
| |
| contrast very forcibly with the anterior thoracic and cephalic
| |
| regions of the body. In most cases this process corresponds to
| |
| the future abdomen, but in some cases (Loricata) it appears to
| |
| include part of the thorax. Before it has reached a considerable
| |
| development, three pairs of appendages spring from the region
| |
| of the head, viz. two pairs of antennae and the mandibles, and
| |
| inaugurate a so-called Nauplius stage (fig. 240 A). These three
| |
| appendages are formed nearly simultaneously, but the hindermost appears to become visible slightly before the two others
| |
| | |
| | |
| | |
| CRUSTACEA. 515
| |
| | |
| | |
| | |
| (Bobretzky). The mouth lies slightly behind the anterior pair
| |
| of antennae, but distinctly in front of the posterior pair. The
| |
| other appendages, the number of which at the time of hatching
| |
| varies greatly in the different Decapods (vide section on larval
| |
| development), sprout in succession from before backwards (fig.
| |
| 240 B). The food yolk in the head and thoracic region
| |
| gradually becomes reduced in quantity with the growth of the
| |
| embryo, and by the time of hatching the disparity in size between
| |
| the thorax and abdomen has ceased to exist.
| |
| | |
| Isopoda. The early embryonic phases of the Isopoda have
| |
| been studied by means of sections by Bobretzky (No. 498) and
| |
| Bullar (No. 499) and have been found to present considerable
| |
| | |
| | |
| | |
| | |
| | |
| FlG. 240. TWO STAGES IN THE DEVELOPMENT OF
| |
| | |
| A. Nauplius stage.
| |
| | |
| B. Stage with eight pairs of appendages, op. eyes ; at 1 , and at*, first and second
| |
| antennae; md. mandibles; mx l , mx 2 . first and second maxillae; mxp*. third maxillipeds ; Ib. upper lip.
| |
| | |
| variations. When laid the egg is enclosed in a chorion, but
| |
| shortly after the commencement of segmentation (Ed. van
| |
| Beneden and Bullar) a second membrane appears, which is
| |
| probably of the nature of a larval membrane.
| |
| | |
| In all the forms the segmentation is followed by the
| |
| formation of a blastoderm, completely enclosing the yolk, and
| |
| thickened along an area which will become the ventral surface of
| |
| the embryo. In this area the blastoderm is formed of at least
| |
| two layers of cells an external columnar epiblast, and an
| |
| internal layer of scattered cells which form the mesoblast and
| |
| probably in part also the hypoblast (Oniscus, Bobretzky ; Cymothoa, Bullar).
| |
| | |
| 332
| |
| | |
| | |
| | |
| 516 FORMATION OF THE LAYERS.
| |
| | |
| In Asellus aquaticus there is a centrolecithal segmentation,
| |
| ending in the formation of a blastoderm, which appears first
| |
| on the ventral surface and subsequently extends to the dorsal.
| |
| | |
| In Oniscus murarius, and Cymothoa the segmentation is
| |
| partial [for its peculiarities and relationship vide p. 120] and a
| |
| disc, formed of a single layer of cells, appears at a pole of the
| |
| egg which corresponds to the future ventral surface (Bobretzky).
| |
| This layer gradually grows round the yolk partly by division of
| |
| its cells, though a formation of fresh cells from the yolk may
| |
| also take place. Before it has extended far round the yolk, the
| |
| central part of it becomes two or more layers deep, and the cells
| |
| of the deeper layers rapidly increase in number, and are destined
| |
| to give rise to the mesoblast and probably also to part or the
| |
| whole of the hypoblast. In Cymothoa this layer does not at
| |
| first undergo any important change, but in Oniscus it becomes
| |
| very thick, and its innermost cells (Bobretzky) become imbedded
| |
| in the yolk, which they rapidly absorb; and increasing in
| |
| number first of all form a layer in the periphery of the yolk, and
| |
| finally fill up the whole of the interior of the yolk (fig. 241 A),
| |
| absorbing it in the process.
| |
| | |
| It appears possible that these cells do not, as Bobretzky believes, originate from the blastoderm, but from nuclei in the yolk which have escaped
| |
| his observation. This mode of origin would be similar to that by which yolk
| |
| cells originate in the eggs of the Insecta, etc. If Bobretzky's account is
| |
| correct we must look to Palaemon, as he himself suggests, to find an explanation of the passage of the hypoblast cells into the yolk. The thickening of
| |
| the primitive germinal disc would, according to this view, be equivalent to
| |
| the invagination of the archenteron in Astacus, Palaemon, etc.
| |
| | |
| Whatever may be the origin of the cells in the yolk they no
| |
| doubt correspond to the hypoblast of other types. In Cymothoa
| |
| nothing similar to them has been met with, but the hypoblast
| |
| has a somewhat different origin ; being apparently formed from
| |
| some of the indifferent cells below the epiblast, which collect as
| |
| a solid mass on the ventral surface, and then divide into two
| |
| masses which become hollow and give rise to the liver caeca.
| |
| Their fate, as well as that of the hypoblast in Oniscus, is dealt
| |
| with in connection with the alimentary tract. The completion
| |
| of the enclosure of the yolk by the blastoderm takes place on
| |
| the dorsal surface. In all the Isopods which have been carefully
| |
| | |
| | |
| | |
| CRUSTACEA. 517
| |
| | |
| | |
| | |
| studied, there appears before any other organ a provisional
| |
| structure formed from the epiblast and known as the dorsal
| |
| organ. An account of it is given in connection with the development of the organs. The general external changes undergone by the larva in its development are as follows. The
| |
| ventral thickened area of the blastoderm (ventral plate) shapes
| |
| itself and girths nearly the whole circumference of the ovum in
| |
| Oniscus (fig. 241 A) but is relatively much shorter in Cymothoa.
| |
| Anteriorly it dilates into the two procephalic lobes. In
| |
| Cymothoa it next becomes segmented; and the anterior segments are formed nearly simultaneously, and those of the
| |
| abdomen somewhat later. At the same time a median depres
| |
| | |
| | |
| | |
| FlG. 241. TWO LONGITUDINAL SECTIONS THROUGH THE EMBRYO OF ONISCUS
| |
| | |
| MURARIUS. (After Bobretzky.)
| |
| | |
| st. stomodaeum ; pr. proctodseum ; hy. hypoblast formed of large nucleated cells
| |
| imbedded in the yolk ; m. mesoblast ; vg. ventral nerve cord ; sg. supra- oesophageal
| |
| ganglion ; li. liver ; do. dorsal organ ; zp. rudiment of masticatory apparatus ; ol. upper
| |
| lip.
| |
| | |
| sion appears dividing the blastoderm longitudinally into two
| |
| halves. The appendages are formed later than their segments,
| |
| and the whole of them are formed nearly simultaneously, with
| |
| the exception of the last thoracic, which does not appear till
| |
| comparatively late after the hatching of the embryo. The late
| |
| development of the seventh thoracic segment and appendage is
| |
| a feature common to the majority of the Isopoda (Fritz Miiller).
| |
| In Oniscus the limbs are formed in nearly the same way as in
| |
| Cymothoa, but in Asellus they do not arise quite simultaneously.
| |
| First of all, the two antennae and mandibles (the future palp)
| |
| appear, inaugurating a stage often spoken of as the Nauplius
| |
| stage, which is supposed to correspond with the free Nauplius
| |
| | |
| | |
| | |
| 5l8 FORMATION OF THE LAYERS.
| |
| | |
| stage of Penaeus and Euphausia. At this stage a cuticle is shed
| |
| (Van Beneden) which remains as an envelope surrounding the
| |
| larva till the time of hatching. Similar cuticular envelopes are
| |
| formed in many Isopoda. Subsequently the appendages of the
| |
| thorax appear, and finally those of the abdomen. Later than
| |
| the appendages there arise behind the mouth two prominences
| |
| which resemble appendages, but give rise to a bilobed lower lip
| |
| (Dohrn).
| |
| | |
| In Asellus and Oniscus the ventral plate moulds itself to the
| |
| shape of the egg, and covers the greater part of the dorsal as
| |
| well as of the ventral side (fig. 241 A). As a result of this the
| |
| ventral surface of the embryo is throughout convex ; and in
| |
| Asellus a deep fold appears on the back of the embryo, so that
| |
| the embryo appears coiled up within the egg with its ventral
| |
| side outwards and its head and tail in contact. In Oniscus the
| |
| ventral surface is convex, but the dorsal surface is never bent in
| |
| as in Asellus. In Cymothoa the egg is very big and the
| |
| ventral plate does not extend nearly so far round to the dorsal
| |
| side as in Asellus, in consequence of which the ventral surface
| |
| is not nearly so convex as in other Isopoda. At the same time
| |
| the telson is early formed, and is bent forwards so as to lie
| |
| on the under side of the part of the blastoderm in front. In
| |
| having this ventral curvature of the telson Cymothoa forms
| |
| an exception amongst Isopods ; and in this respect is intermediate between the embryos of Asellus and those of the
| |
| Amphipoda.
| |
| | |
| Amphipoda. Amongst the Amphipoda the segmentation
| |
| is usually centrolecithal. In the case of Gammarus locusta
| |
| (Ed. van Beneden and Bessels, No. 503) it commences with
| |
| an unequal but total segmentation like that of the Frog (vide p.
| |
| 97), and the separation of a central yolk mass is a late occurrence ; and it is noticeable that the part of the egg with the
| |
| small segments eventually becomes the ventral surface. In the
| |
| fresh-water species of Gammarus (G. pulex and fluviatilis) the
| |
| segmentation is more like that of Insects ; the blastoderm cells
| |
| being formed nearly simultaneously over a large part of the
| |
| surface of the egg.
| |
| | |
| Both forms of segmentation give rise to a blastoderm covering the whole egg, which soon becomes thickened on the ventral
| |
| | |
| | |
| | |
| CRUSTACEA. 519
| |
| | |
| | |
| | |
| surface. There is formed, as in the Isopoda, a larval membrane
| |
| at about the time when the blastoderm is completed. Very
| |
| soon after this the egg loses its spherical shape, and becomes
| |
| produced into a pointed extremity the future abdomen which
| |
| is immediately bent over the ventral surface of the part in front.
| |
| The ventral curvature of the hinder part of the embryo at so
| |
| early an age stands in marked contrast to the usual condition of
| |
| Isopod embryos, and is only approached in this group, so far as
| |
| is known, in the case of Cymothoa.
| |
| | |
| At the formation of the first larval membrane the blastoderm
| |
| cells separate themselves from it, except at one part on the
| |
| dorsal surface. The patch of cells adherent at this part gives rise
| |
| to a dorsal organ, comparable with that in Oniscus, connecting
| |
| the embryo and its first larval skin. A perforation appears in it
| |
| at a later period.
| |
| | |
| The segments and limbs of the Amphipoda are all formed
| |
| before the larva leaves the egg.
| |
| | |
| Cladocera. The segmentation (Grobben, No. 455) takes place on the
| |
| normal centrolecithal type, but is somewhat unequal. Before the close of
| |
| the segmentation there may be seen at the apex of the vegetative pole one
| |
| cell marked off from the remainder by its granular aspect. It gives rise
| |
| to the generative organs. One of the cells adjoining it gives rise to the
| |
| hypoblast, and the other cells which surround it form the commencement
| |
| of the mesoblast. The remaining cells of the ovum form the epiblast. By
| |
| a later stage the hypoblast cell is divided into thirty-two cells and the genital
| |
| cell into four, while the mesoblast forms a circle of twelve cells round the
| |
| genital mass.
| |
| | |
| The hypoblast soon becomes involuted ; the blastopore probably closes,
| |
| and the hypoblast forms a solid cord of cells which eventually becomes the
| |
| mesenteron. The stomodaeum is said to be formed at the point of closure
| |
| of the blastopore. The mesoblast passes inwards and forms a mass adjoining the hypoblast, and somewhat later the genital mass also becomes
| |
| covered by the epiblast. The proctodseum appears to be formed later than
| |
| the stomodasum.
| |
| | |
| The embryo as first shewn by Dohrn passes through a Nauplius stage
| |
| in the brood-pouch, but is hatched, except in the case of the winter eggs of
| |
| Leptodora, in a form closely resembling the adult.
| |
| | |
| Copepoda. Amongst the free Copepoda the segmentation and
| |
| formation of the layers have recently been investigated by Hoek (No. 512).
| |
| He finds that there is, in both the fresh-water and marine forms studied
| |
| by him, a centrolecithal segmentation similar to that of Palaemon and
| |
| Pagurus (vide p. 112), which might from the surface be supposed to be
| |
| | |
| | |
| | |
| 520 FORMATION OF THE LAYERS.
| |
| | |
| complete and nearly regular. After the formation of the blastoderm an
| |
| invagination of some of its cells takes place and is completed in about a
| |
| quarter of an hour. The opening becomes closed. This invagination is
| |
| compared by Hoek to the invagination in Astacus, and is believed by him
| |
| to give rise to the mesenteron. Its point of closing corresponds with the
| |
| hind end of the embryo. On the ventral surface there appear two transverse furrows dividing the embryo into three segments, and a median
| |
| longitudinal furrow which does not extend to the front end of the foremost
| |
| segment. The three pairs of Nauplius appendages and upper lip become
| |
| subsequently formed as outgrowths from the sides of the ventral blastodermic thickening.
| |
| | |
| Amongst the parasitic Copepoda there are found two distinct types of
| |
| segmentation, analogous to those in the Isopoda. In the case of Condracanthus the segmentation is somewhat irregular, but on the type of Eupagurus, etc. (vide p. 112). In the other group (Anchorella, Clavella, Congericola, Caligus, Lerneopoda) the segmentation nearly resembles the ordinary
| |
| meroblastic type (vide p. 120), and is to be explained in the same manner as
| |
| in the cases of Oniscus and Cymothoa. The first blastodermic cells sometimes appear in a position corresponding with the head end of the embryo
| |
| (Anchorella), at other times at the hind end (Clavella), and sometimes in the
| |
| middle of the ventral surface. The dorsal surface of the yolk is always
| |
| the latest to be inclosed by the blastoderm cells. A larval cuticle similar
| |
| to that of the Isopoda is formed at the same time as the blastoderm. At
| |
| the sides of the ventral thickening of the blastoderm there grow out the
| |
| Nauplius appendages, of which only the first two appear in Anchorella.
| |
| In Anchorella and Lerneopoda the embryos are not hatched at the
| |
| Nauplius stage, but after the Nauplius appendages have been formed
| |
| a fresh cuticle the Nauplius cuticle is shed, and within it the embryo
| |
| develops till it reaches the so-called Cyclops stage (vide p. 490). The
| |
| embryo within the egg has its abdomen curved dorsalwards as amongst the
| |
| Isopoda.
| |
| | |
| Cinipedia. The segmentation of Balanus and Lepas commences by
| |
| the segregation of the constituents of the egg into a more protoplasmic
| |
| portion, and a portion formed mainly of food material. The former separates from the latter as a distinct segment, and then divides into two not
| |
| quite equal portions. The division of the protoplasmic part of the embryo
| |
| continues, and the resulting segments grow round the single yolk segment.
| |
| The point where they finally enclose it is situated on the ventral surface
| |
| (Lang) at about the position of the mouth (?).
| |
| | |
| After being enclosed by the protoplasmic cells the yolk divides, and gives
| |
| rise to a number of cells, which probably supply the material for the walls of
| |
| the mesenteron. The external layer of protoplasm forms the so-called
| |
| blastoderm, and soon (Arnold, Lang) becomes thickened on the dorsal
| |
| surface.
| |
| | |
| The embryo is next divided by two constrictions into three segments ;
| |
| and there are formed the three appendages corresponding to these, which are
| |
| | |
| | |
| | |
| CRUSTACEA. 52!
| |
| | |
| | |
| | |
| at first simple. The two posterior soon become biramous. The larva leaves
| |
| the egg before any further appendages become formed.
| |
| | |
| Comparative development of the organs.
| |
| | |
| Central nervous system. The ventral nerve cord of the
| |
| Crustacea develops as a thickening of the epiblast along the
| |
| median ventral line ; the differentiation of which commences in
| |
| front, and thence extends backwards. The ventral cord is at
| |
| first unsegmented. The supra-oesophageal ganglia originate as
| |
| thickenings of the epiblast of the procephalic lobes.
| |
| | |
| The details of the above processes are still in most cases very imperfectly known. The fullest account we have is that of Reichenbach (No. 488)
| |
| for Astacus. He finds that the supra- cesophageal ganglia and ventral cord
| |
| arise as a continuous formation, and not independently as would seem to be
| |
| the case in Chsetopoda. The supra-cesophageal ganglia are formed from the
| |
| procephalic lobes. The first trace of them is visible in the form of a pair of
| |
| pits, one on each side of the middle line. These pits become in the
| |
| Nauplius stage very deep, and their walls are then continued into two ridges
| |
| where the epiblast is several cells deep, which pass backwards one on each
| |
| side of the mouth. The walls of the pits are believed by Reichenbach to
| |
| give rise to the optic portions of the supra-cesophageal ganglia, and the
| |
| epiblastic ridges to the remainder of the ganglia and to the circum-cesophageal commissures. At a much later stage, when the ambulatory feet have
| |
| become formed, a median involution of epiblast in front of the mouth and
| |
| between the two epiblast ridges gives rise to a central part of the supracesophageal ganglia. Five elements are thus believed by Reichenbach to be
| |
| concerned in the formation of these ganglia, viz. two epiblast pits, two
| |
| epiblast ridges, and an involution of epiblast between the latter. It should
| |
| be noted however that the fate neither of the pair of pits, nor of the median
| |
| involution, appears to have been satisfactorily worked out. The two
| |
| epiblast ridges, which pass back from the supra-cesophageal ganglia on
| |
| each side of the mouth, are continued as a pair of thickenings of the epiblast
| |
| along the sides of a median ventral groove. This groove is deep in front
| |
| and shallows out posteriorly. The thickenings on the sides of this groove
| |
| no doubt give rise to the lateral halves of the ventral cord, and the cells of
| |
| the groove itself are believed by Reichenbach, but it appears to me without
| |
| sufficient evidence, to become invaginated also and to assist in forming the
| |
| ventral cord. When the ventral cord becomes separated from the epiblast
| |
| the two halves of it are united in the middle line, but it is markedly bilobed
| |
| in section.
| |
| | |
| In the Isopoda it would appear both from Bobretzky's and Bullar's
| |
| observations that the ventral nerve cord arises as an unpaired thickening of
| |
| the epiblast in which there is no trace of anything like a median involution.
| |
| After this thickening has become separated from the epiblast a slight
| |
| | |
| | |
| | |
| 522 DEVELOPMENT OF ORGANS.
| |
| | |
| median furrow indicates its constitution out of two lateral cords. The
| |
| supra-oesophageal ganglia are stated to be developed quite simply as a pair
| |
| of thickenings of the procephalic lobes, but whether they are from the
| |
| first continuous with the ventral cord does not appear to have been determined.
| |
| | |
| The later stages in the differentiation of the ventral cord are,
| |
| so far as is known, very similar throughout the Crustacea. The
| |
| ventral cord is, as has been stated, at first unsegmented (fig. 241
| |
| A, vg\ but soon becomes divided by a series of constrictions into
| |
| as many ganglia as there are pairs of appendages or segments
| |
| (fig. 241 B, vg).
| |
| | |
| There appears either on the ventral side (Oniscus) or in the
| |
| centre (Astacus, Palaemon) of the two halves of each segment or
| |
| ganglion a space filled with finely punctuated material, which is
| |
| the commencement of the commissural portion of the cords.
| |
| The commissural tissue soon becomes continuous through the
| |
| length of the ventral cord, and is also prolonged into the supracesophageal ganglia.
| |
| | |
| After the formation of the commissural tissue the remaining
| |
| cells of the cord form the true ganglion cells. A gradual
| |
| separation of the ganglia next takes place, and the cells become
| |
| confined to the ganglia, which are finally only connected by a
| |
| double band of commissural tissue. The commissural tissue not
| |
| only gives rise to the longitudinal cords connecting the successive
| |
| ganglia, but also to the transverse commissures which unite the
| |
| two halves of the individual ganglia.
| |
| | |
| The ganglia usually, if not always, appear at first to correspond in number with the segments, and the smaller number so
| |
| often present in the adult is due to the coalescence of originally
| |
| distinct ganglia.
| |
| | |
| Organs of special sense. Comparatively little is known on
| |
| this head. The compound eyes are developed from the coalescence of two structures, both however epiblastic, viz. (i) part of
| |
| the superficial epiblast of the procephalic lobes ; (2) part of the
| |
| supra-cesophageal ganglia. The former gives rise to the corneal
| |
| lenses, the crystalline cones, and the pigment surrounding
| |
| them ; the latter to the rhabdoms and the cells which encircle
| |
| them. Between these two parts a mesoblastic pigment is interposed.
| |
| | |
| | |
| | |
| CRUSTACEA.
| |
| | |
| | |
| | |
| 523
| |
| | |
| | |
| | |
| Of the development of the auditory and olfactory organs
| |
| almost nothing is known.
| |
| | |
| Dorsal organ. In a considerable number of the Malacostraca
| |
| and Branchiopoda a peculiar organ is developed from the epiblast
| |
| in the anterior dorsal region. This organ has been called the
| |
| dorsal organ. It appears to be of a glandular nature, and is
| |
| usually very large in the embryo or larva and disappears in the
| |
| adult ; but in some Branchiopoda it persists through life. In
| |
| most cases it is unpaired, but in some instances a paired organ
| |
| appears to take its place.
| |
| | |
| Various views as to its nature have been put forward. There
| |
| is but little doubt of its being glandular, and it is possible that it
| |
| is a provisional renal organ, though so far as I know concretions
| |
| have not yet been found in it.
| |
| | |
| Its development has been most fully studied in the Isopoda.
| |
| | |
| In Cymothoa (Bullar, No. 499) there appears on the dorsal surface, in the
| |
| region which afterwards becomes the first thoracic segment, an unpaired
| |
| linear thickening of the blastoderm. This soon becomes a circular patch,
| |
| the central part of which is invaginated so as to communicate
| |
| with the exterior by a narrow
| |
| opening only (fig. 242). It becomes at the same time attached
| |
| to the inner egg membrane. It
| |
| retains this condition till the close
| |
| of larval life.
| |
| | |
| In Oniscus (Dohrn, No. 500 ;
| |
| Bobretzky, No. 498) there appears
| |
| very early a dorsal patch of thickened cells. These cells become
| |
| attached at their edge to the
| |
| inner egg membrane and gradually separated from the embryo,
| |
| with which they finally only re- , FlG - W- DIAGRAMMATIC SECTION OF
| |
| . , ... CYMOTHOA SHEWING THE DORSAL ORGAN.
| |
| main in connection by a hollow (F rom Bullar.)
| |
| | |
| column of cells (fig. 241 A, do).
| |
| | |
| The original patch now gradually spreads over the inner egg membrane, and
| |
| forms a transverse saddle-shaped band of flattened cells which engirths the
| |
| embryo on all but the ventral side.
| |
| | |
| In the Amphipods the epiblast cells remain attached for a small area on
| |
| the dorsal surface to the first larval skin, when this is formed. This patch
| |
| of cells, often spoken of as a micropyle apparatus, forms a dorsal organ
| |
| equivalent to that in Oniscus. A perforation is formed in it at a later
| |
| | |
| | |
| | |
| | |
| | |
| 524
| |
| | |
| | |
| | |
| DEVELOPMENT OF ORGANS.
| |
| | |
| | |
| | |
| period. A perhaps homologous structure is found in the embryos of Euphausia, Cuma, etc.
| |
| | |
| In many Branchiopoda a dorsal organ is found. Its development has
| |
| been studied by Grobben in Moina. It
| |
| persists in the adult
| |
| in Branchipus, Limnadia, Estherea, etc.
| |
| | |
| In the Copepoda
| |
| a dorsal organ is
| |
| sometimes found in
| |
| the embryo ; Grobben at any rate believes that he has
| |
| detected an organ of
| |
| this nature in the
| |
| embryo of Cyclops
| |
| serrulatus.
| |
| | |
| A paired organ
| |
| which appears to be
| |
| | |
| | |
| | |
| | |
| FIG. 243. DIAGRAMMATIC SECTION OF AN EMBRYO
| |
| OF ASELLUS AQUATICUS TO SHEW THE PAIRED DORSAL
| |
| ORGAN. (From Bullar ; after E. van Beneden.)
| |
| | |
| | |
| | |
| of the same nature
| |
| has been found in
| |
| Asellus and Mysis.
| |
| | |
| In Asellus (Rathke (No. 501), Dohrn (No. 500), Van Beneden (No. 497))
| |
| this organ originates as two cellular masses at the sides of the body just
| |
| behind the region of the procephalic lobes. Each of them becomes trifoliate
| |
| and bends towards the ventral surface. In each of their lobes a cavity
| |
| arises and finally the three cavities unite, forming a trilobed cavity open to
| |
| the yolk. This organ eventually becomes so large that it breaks through the
| |
| egg membranes and projects at the sides of the embryo (fig. 243\ Though
| |
| formed before the appendages it does not attain its full development till
| |
| considerably after the latter have become well established.
| |
| | |
| In Mysis it appears during the Nauplius stage as a pair of cavities lined
| |
| by columnar cells, which atrophy very early.
| |
| | |
| Various attempts have been made to identify organs in other Arthropod
| |
| embryos with the dorsal organ of the Crustacea, but the only organ at all
| |
| similar which has so far been described is one found in the embryo of Linguatula (vide Chapter XIX.), but there is no reason to think that this organ is
| |
| really homologous with the dorsal organ of the Crustacea.
| |
| | |
| | |
| | |
| The mesoblast. The mesoblast in the types so far investigated arises from the same cells as the hypoblast, and appears
| |
| as a somewhat irregular layer between the epiblast and the
| |
| hypoblast. It gives rise to the same parts as in other forms, but
| |
| it is remarkable that it does not, in most Decapods and Isopods
| |
| | |
| | |
| | |
| CRUSTACEA. 525
| |
| | |
| | |
| | |
| (and so far we do not know about other forms), become divided
| |
| into somites, at any rate with the same distinctness that is usual
| |
| in Annelids and Arthropods. Not only so, but there is at first
| |
| no marked division into a somatic and splanchnic layer with an
| |
| intervening body cavity. Some of the cells become differentiated
| |
| into the muscles of the body wall and limbs ; and other cells,
| |
| usually in the form of a very thin layer, into the muscles of the
| |
| alimentary tract. In the tail of Palcsmon Bobretzky noticed
| |
| that the cells about to form the muscles of the body were
| |
| imperfectly divided into cubical masses corresponding with the
| |
| segments ; which however, in the absence of a central cavity,
| |
| differed from typical mesoblastic somites. In Mysis Metschnikoff states that the mesoblast becomes broken up into distinct
| |
| somites. Further investigations on this subject are required.
| |
| The body cavity has the form of irregular blood sinuses amongst
| |
| the internal organs.
| |
| | |
| Heart. The origin and development of the heart and vascular system
| |
| are but very imperfectly known.
| |
| | |
| In Phyllopods (Branchipus) Claus (No. 454) has shewn that the heart is
| |
| formed by the coalescence of the lateral parts of the mesoblast of the ventral
| |
| plates. The chambers are formed successively as the segments to which
| |
| they belong are established, and the anterior chambers are in full activity
| |
| while the posterior are not yet formed.
| |
| | |
| In Astacus and Palaemon, Bobretzky finds that at the stage before the
| |
| heart definitely appears there may be seen a solid mass of mesoblast cells
| |
| in the position which it eventually occupies 1 ; and considers it probable that
| |
| the heart originates from this mass. At the time when the heart can first
| |
| be made out and before it has begun to beat, it has the form of an oval sack
| |
| with delicate walls separated from the mesenteron by a layer of splanchnic
| |
| mesoblast. Its cavity is filled with a peculiar plasma which also fills up the
| |
| various cavities in the mesoblast. Around it a pericardial sack is soon
| |
| formed, and the walls of the heart become greatly thickened. Four bands
| |
| pass off from the heart, two dorsalwards which become fixed to the
| |
| integument, and two ventralwards. There is also a median band of cells
| |
| connecting the heart with the dorsal integument. The main arteries arise
| |
| as direct prolongations of the heart. Dohrn's observations on Asellus
| |
| greatly strengthen the view that the heart originates from a solid mesoblastic mass, in that he was able to observe the hollowing out of the mass in
| |
| | |
| 1 Reichenbach describes these cells, and states that there is a thickening of the
| |
| epiblast adjoining them. In one place he states that the heart arises from this thickening of epiblast, and in another that it arises from the mesoblast. An epiblastic origin
| |
| of the heart is extremely improbable.
| |
| | |
| | |
| | |
| 526 DEVELOPMENT OF ORGANS.
| |
| | |
| the living embryo (cf. the development of the heart in Spiders). Some of the
| |
| central cells (nuclei, Dohrn) become blood corpuscles. The formation of
| |
| these is not, according to Dohrn, confined to the heart, but takes place in
| |
| situ in all the parts of the body (antennae, appendages, etc.). The corpuscles
| |
| are formed as free nuclei and are primarily derived from the yolk, which at
| |
| first freely communicates with the cavities of the appendages.
| |
| | |
| Alimentary tract. In Astacus the formation of the mesenteron by
| |
| invagination, and the absorption of the yolk by the hypoblast cells, have
| |
| already been described. On the absorption of the yolk the mesenteron has
| |
| the form of a sack, the walls of which are formed of immensely long cells
| |
| the yolk pyramids at the base of which the nucleus is placed (fig. 238 B).
| |
| This sack gives rise both to the portion of the alimentary canal between the
| |
| abdomen and the stomach and to the liver. The epithelial wall of both of
| |
| these parts is formed by the outermost portions of the pyramids with the
| |
| nuclei and protoplasm becoming separated off from the yolk as a layer of
| |
| flat epithelial cells. The yolk then breaks up and forms a mass of nutritive
| |
| material filling up the cavity of the mesenteron.
| |
| | |
| The differentiation both of the liver and alimentary tract proper first
| |
| takes place on the ventral side, and commences close to the point where the
| |
| proctodasum ends, and extends forward from this point. A layer of epithelial
| |
| cells is thus formed on the ventral side of the mesenteron which very soon
| |
| becomes raised into a series of longitudinal folds, one of which in the
| |
| middle line is very conspicuous. The median fold eventually, by uniting
| |
| with a corresponding fold on the dorsal side, gives rise to the true mesenteron ; while the lateral folds form parallel hepatic cylinders, which in front
| |
| are not constricted off from the alimentary tract. The lateral parts of the
| |
| dorsal side of the mesenteron similarly give rise to hepatic cylinders. The
| |
| yolk pyramids of the anterior part of the mesenteron, which projects
| |
| forwards as a pair of diverticula on each side to the level of the stomach, are
| |
| not converted into hepatic cylinders till after the larva is hatched.
| |
| | |
| The proctodasum very early opens into the mesenteron, but the stomodaeum remains closed till the differentiation of the mid-gut is nearly
| |
| completed. The proctodaeum gives rise to the abdominal part of the intestine, and the stomodaeum to the oesophagus and stomach. The commencement of the masticatory apparatus in the latter appears very early as a
| |
| dorsal thickening of the epithelium.
| |
| | |
| The primitive mesenteron in Palaemon differentiates itself into the
| |
| permanent mid-gut and liver in a manner generally similar to that in
| |
| Astacus, though the process is considerably less complicated. A distinct
| |
| layer of cells separates itself from the outer part of the yolk pyramids,
| |
| and gives rise to the glandular lining both of the mid-gut and of the liver.
| |
| The differentiation of this layer commences behind, and the mid-gut very
| |
| soon communicates freely with the proctodasum. The lateral parts of
| |
| the primitive mesenteron become constricted into four wings, two directed
| |
| forwards and two backwards ; these, after the yolk in them has become
| |
| absorbed, constitute the liver. The median part simply becomes the me
| |
| | |
| | |
| CRUSTACEA. 527
| |
| | |
| | |
| | |
| senteron. The stomachic end of the stomodaeum lies in contact with the
| |
| mesenteron close to the point where it is continued into the hepatic
| |
| diverticula, and, though the partition-wall between the two becomes early
| |
| very thin, a free communication is not established till the yolk has been
| |
| completely absorbed.
| |
| | |
| The alimentary tract in the Isopoda is mainly if not entirely formed
| |
| from the proctodaeum and stomodaeum, both of which arise before any other
| |
| part of the alimentary system as epiblastic invaginations, and gradually
| |
| grow inwards (fig. 244). In Oniscus the liver is formed as two discs
| |
| at the surface of the yolk on each side of the anterior part of the body.
| |
| Their walls are composed of cubical cells derived from the yolk cells, the
| |
| | |
| | |
| | |
| | |
| pr
| |
| | |
| s r " a qcaggaw. rt -j_ .-. f .i~T' : . -^a^Mi^ . - .. >va^^^
| |
| | |
| Vff
| |
| | |
| FlG. 244. TWO LONGITUDINAL SECTIONS THROUGH THE EMBRYO OF ONISCUS
| |
| | |
| MURARIUS. (After Bobretzky.)
| |
| | |
| st. stomodaeum ; pr. proctodseum ; hy. hypoblast formed of large nucleated cells
| |
| imbedded in yolk ; m. mesoblast ; vg. ventral nerve cord ; jr^. supra- oesophageal ganglion ; li. liver; do. dorsal organ; zp. rudiment of masticatory apparatus.
| |
| | |
| origin of which was spoken of on p. 516. These two discs gradually take
| |
| the form of sacks (fig. 244 B, li.) freely open on their inner side to the
| |
| yolk. As these sacks continue to grow the stomodaeum and proctodaeum
| |
| do not remain passive. The stomodaeum, which gives rise to the oesophagus
| |
| and stomach of the adult, soon exhibits a posterior dilatation destined to
| |
| become the stomach, on the dorsal wall of which a well-marked prominence
| |
| the earliest trace of the future armature is soon formed (fig. 244 B,
| |
| xp}. The proctodaeum (pr) grows with much greater rapidity than the
| |
| stomodaeum, and its end adjoining the yolk becomes extremely thin or even
| |
| broken through. In the earliest stages it was surrounded by the yolk cells,
| |
| but in its later growth the yolk cells become gradually reduced in number
| |
| and appear to recede before it so much so that one is led to conclude
| |
| that the later growth of the proctodaeum takes place at the expense of the
| |
| yolk cells.
| |
| | |
| The liver sacks become filled with a granular material without a trace
| |
| of cells ; their posterior wall is continuous with the yolk cells, and their
| |
| anterior lies close behind the stomach. The proctodaeum continually
| |
| grows forwards till it approaches close to the stomodaeum, and the two
| |
| | |
| | |
| | |
| 528 DEVELOPMENT OF ORGANS.
| |
| | |
| liver sacks, now united into one at their base, become directly continuous
| |
| with the proctodaeum. By the stage when this junction is effected the yolk
| |
| cells have completely disappeared. It seems then that in Oniscus the yolk
| |
| cells (hypoblast) are mainly employed in giving rise to the walls of the
| |
| liver ; but that they probably also supply the material for the later growth
| |
| of the apparent proctodaeum. It becomes therefore necessary to conclude
| |
| that the latter, which might seem, together with the stomodasum, to form
| |
| the whole alimentary tract, does in reality correspond to the proctodaeum
| |
| and mesenteron together, though the digestive fluids are no doubt mainly
| |
| secreted not in the mesenteron but in the hepatic diverticula. The proctodaeum and stomodaeum at first meet each other without communicating, but
| |
| before long the partition between the two is broken through.
| |
| | |
| In Cymothoa (Bullar, No. 499) the proctodaeum and stomodaeum
| |
| develop in the same manner as in Oniscus, but the hypoblast has quite
| |
| a different form. The main mass of the yolk, which is much greater than
| |
| in Oniscus, is not contained in definite yolk cells, but the hypoblast is
| |
| represented by (i) two solid masses of cells, derived apparently from the
| |
| inner layer of blastoderm cells, which give rise to the liver ; and (2) by a
| |
| membrane enclosing the yolk in which nuclei are present.
| |
| | |
| The two hepatic masses lie on the surface of the yolk, and each of them
| |
| becomes divided into three short caecal tubes freely open to the yolk.
| |
| The stomodaeum soon reaches its full length, but the proctodaeum grows
| |
| forwards above the yolk till it meets the stomodaeum. By the time this
| |
| takes place the liver caeca have grown into three large tubes filled with
| |
| fluid, and provided with a muscular wall. They now lie above the yolk,
| |
| and no longer communicate directly with the cavity of the yolk sack,
| |
| but open together with the yolk sack into the point of junction of the
| |
| proctodaeum and stomodaeum. The yolk sack of Cymothoa no doubt
| |
| represents part of the mesenteron, but there is no evidence in favour of
| |
| any part of the apparent proctodaeum representing it also, though it is
| |
| quite possible that it may do so. The relations of the yolk sack and hepatic
| |
| diverticula in Cymothoa appear to hold good for Asellus and probably for
| |
| most Isopoda.
| |
| | |
| The differences between the Decapods and Isopods in the development
| |
| of the mesenteron are not inconsiderable, but they are probably to be
| |
| explained by the relatively larger amount of food yolk in the latter forms.
| |
| The solid yolk in the Isopods on this view represents the primitive mesenteron of Decapods after the yolk has been absorbed by the hypoblast cells.
| |
| Starting from this standpoint we find that in both groups the lateral parts of
| |
| the mesenteron become the liver. In Decapods the middle part becomes
| |
| directly converted into the mid-gut, the differentiation of it commencing
| |
| behind and proceeding forwards. In the Isopods, owing to the mesenteron
| |
| not having a distinct cavity, the differentiation of it, which proceeds forwards
| |
| as in Decapods, appears simply like a prolongation forwards of the proctoda?um, the cells for the prolongation being probably supplied from the yolk.
| |
| In Cymothoa the food yolk is so bulky that a special yolk sack is developed
| |
| | |
| | |
| | |
| CRUSTACEA.
| |
| | |
| | |
| | |
| 529
| |
| | |
| | |
| | |
| for its retention, which is not completely absorbed till some time after the
| |
| alimentary canal has the form of a continuous tube. The walls of this yolk
| |
| sack are morphologically a specially developed part of the mesenteron.
| |
| | |
| BIBLIOGRAPHY.
| |
| | |
| General Works.
| |
| | |
| (447) C. Spence Bate. " Report on the present state of our knowledge of the
| |
| Crustacea." Report of the British Association for 1878.
| |
| | |
| (448) C. Claus. Untersuchungen zur Erforschung der genealogischen Grundlage
| |
| des Crustaceen- Systems. Wien, 1876.
| |
| | |
| (449) A. Dohrn. "Geschichte des Krebsstammes. " Jenaische Zeitschrift,
| |
| Vol. VI. 1871.
| |
| | |
| (450) A. Gerstaecker. Bronris Thierreich, Bd. v. Arthropoda, 1866.
| |
| | |
| (451) Th. H. Huxley. The Anatomy of Invertebrated Animals. London,
| |
| 1877.
| |
| | |
| (452) Fritz Mliller. Fur Darwin, 1864. Translation, Facts for Darwin.
| |
| London, 1869.
| |
| | |
| Branchiopoda.
| |
| | |
| (453) Brauer. "Vorlaufige Mittheilung iiber die Entwicklung u. Lebensweise
| |
| des Lepidurus (Apus) productus." Sitz. der Ak. d. Wiss. Wien, Vol. LXIX., 1874.
| |
| | |
| (454) C. Claus. "Zur Kenntniss d. Baues u. d. Entwicklung von Branchipus
| |
| stagnalisu. Apus cancriformis." Abh. d. kb'nig. Gesell. der Wiss. Gb'ttingen, Vol. XVIII.
| |
| 1873.
| |
| | |
| (455) C. Grobben. "Zur Entwicklungsgeschichte d. Moina rectirostris."
| |
| Arbeit, a. d. zoologisch. Institute Wien, Vol. II., 1879.
| |
| | |
| (456) E. Grube. " Bemerkungen uber die Phyllopoden nebst einer Uebersicht
| |
| etc." Archivf. Naturgeschichte, Vol. xix., 1853.
| |
| | |
| (457) N. Joly. " Histoire d'un petit Crustace (Artemia salina, Leach} etc." Annales d. Sciences Natur., 2nd ser., Vol. xiii., 1840.
| |
| | |
| (458) N. Joly. " Recherches zoologiques anatomiques et physiologiques sur
| |
| 1'Isaura cycladoides ( = Estheria) nouveau genre, etc." Annales d. Sciences Nat., 2nd
| |
| ser., Vol. xvii., 1842.
| |
| | |
| (459) Lereboullet. " Observations sur la generation et le developpement de la
| |
| Ltmnadia de Hermann." Annales d. Sciences Natur., <$th ser., Vol. v., 1866.
| |
| | |
| (460) F. Leydig. " Ueber Artemia salina u. Branchipus stagnalis." Zeit. f.
| |
| wiss. ZooL, Vol. in., 1851.
| |
| | |
| (461) G. O. Sars. " Om en dimorph Udvikling samt Generationsvexel hos
| |
| Leptodora." Vidensk. Selskab. Forhand, 1873.
| |
| | |
| (462) G. Zaddach. De apodis cancreformis Schaeff. anatome et historia evolutionis. Dissertatio inanguralis zootomica. Bonnse, 1841.
| |
| | |
| Nebaliadce.
| |
| | |
| (463) C. Claus. " Ueber den Bau u. die systematische Stellung von Nebalia."
| |
| Zeit.f. wiss. Zool., Bd. xxn. 1872.
| |
| | |
| (464) E. Metschnikoff. Development of Nebalia (Russian), 1868.
| |
| | |
| B. II. 34
| |
| | |
| | |
| | |
| 530 BIBLIOGRAPHY.
| |
| | |
| | |
| | |
| Schizopoda.
| |
| | |
| (465) E. van Beneden, " Recherches sur 1'Embryogenie des Crustaces. n.
| |
| DeVeloppement des Mysis." Bullet, de rAcadtmie roy. de Belgique, second series,
| |
| Tom. xxvin. 1869.
| |
| | |
| (46G) C. Glaus. " Ueber einige Schizopoden u. niedere Malakostraken." Zett.
| |
| f. wiss. Zoologie, Bd. XII I., 1863.
| |
| | |
| (467) A. Dohrn. " Untersuchungen Ub. Bau u. Entwicklung d. Arthropoden."
| |
| Zeit.f. wiss. Zool.y Bd. XXL, 1871, .p. 375. Peneus zoaea (larva of Euphausia).
| |
| | |
| (468) E. Metschnikoff. " Ueber ein Larvenstadium von Euphausia." Zeit.
| |
| fiir wiss. Zool., Bd. xix., 1869.
| |
| | |
| (469) E. Metschnikoff. " Ueber den Naupliuszustand von Euphausia. " Zeit.
| |
| fiir wiss. Zool., Bd. XXI., 1871.
| |
| | |
| Decapoda.
| |
| | |
| (470) S pence Bate. "On the development of Decapod Crustacea." Phil.
| |
| Trans., 1858.
| |
| | |
| (471) Spence Bate. " On the development of Pagurus." Ann. and Mag. Nat.
| |
| History, Series 4, Vol. II., 1868.
| |
| | |
| (472) N. Bobretzky. Development of Astacus and Palamon. Kiew, 1873.
| |
| (Russian.)
| |
| | |
| (473) C. Glaus. "Zur Kenntniss d. Malakostrakenlarven. " Wiirzb. naturw.
| |
| Zeitschrift, 1861.
| |
| | |
| (474) R. Q. Couch. "On the Metamorphosis of the Decapod Crustaceans."
| |
| Report Cornwall Polyt. Society. 1848.
| |
| | |
| (475) Du Cane. "On the Metamorphosis of Crustacea." Ann. and Mag. of
| |
| Nat. History, 1839.
| |
| | |
| (476) Walter Faxon. " On the development of Palsemonetes vulgaris." Bull,
| |
| of the Mus. of Camp. Anat. Harvard, Cambridge, Mass., Vol. v., 1879.
| |
| | |
| (477) A. Dohrn. " Untersuchungen lib. Bau u. Entwicklung d. Arthropoden."
| |
| " Zur Entwicklungsgeschichte der Panzerkrebse. Scyllarus Palinurus." Zeit. f.
| |
| wiss. Zool., Bd. xx., 1870.
| |
| | |
| (478) A. Dohrn. "Untersuchungen lib. Bau u. Entwicklung d. Arthropoden.
| |
| Erster Beitrag z. Kenntniss d. Malacostraken u. ihrer Larven Amphion Reynaudi,
| |
| Lophogaster, Portunus, Porcellanus, Elaphocaris. " Zeit. f. wiss. Zool., Bd. xx.,
| |
| 1870.
| |
| | |
| (479) A. Dohrn. "Untersuchungen lib. Bau u. Entwicklung d. Arthropoden.
| |
| Zweiter Beitrag, etc." Zeit.f. wiss. Zool., Bd. xxi., 1871.
| |
| | |
| (480) N. Joly. " Sur la Caridina Desmarestii." Ann. Scien. Nat., Tom. xix.,
| |
| 1843.
| |
| | |
| (481) Lereboullet. " Recherches d . 1'embryologie comparee sur le developpement
| |
| du Brochet, de la Perche et de 1'Ecrevisse." Mem. Savans ktrang. Paris, Vol. xvn.,
| |
| 1862.
| |
| | |
| (482) P. Mayer. "Zur Entwicklungsgeschichte d. Dekapoden." Jenaische
| |
| Zeitschrift, Vol. XI., 1877.
| |
| | |
| (483) F r i t z M u 1 1 e r. " Die Verwandlung der Porcellana." Archivf. Natnrgeschichte, 1862.
| |
| | |
| | |
| | |
| CRUSTACEA. 531
| |
| | |
| | |
| | |
| (484) Fritz Muller. " Die Verwandlungen d. Garneelen," Archiv f. Naturgesch., Tom. xxix.
| |
| | |
| (485) Fritz Muller. " Ueber die Naupliusbrut d. Garneelen." Zeit f. wiss.
| |
| Zool., Bd. xxx., 1878.
| |
| | |
| (486) T. J. Parker. "An account of Reichenbach's researches on the early
| |
| development of the Fresh-water Crayfish." Quart. J. of M. Science, Vol. xvin.,
| |
| 1878.
| |
| | |
| (487) H. Rathke. Ueber die Bildung u. Entivicklung d. Flusskrebses. Leipzig, 1829.
| |
| | |
| (488) H. Reichenbach. " Die Embryoanlage u. erste Entwicklung d. Flusskrebses." Zeit.f. wiss. Zool., Vol. xxix., 1877.
| |
| | |
| (489) F. Richters. " Ein Beitrag zur Entwicklungsgeschichte d. Loricaten."
| |
| Zeit.f. wiss. Zool., Bd. xxiil., 1873.
| |
| | |
| (490) G. O. Sars. " Om Hummers posiembryonale Udvikling. " Vidensk Selsk.
| |
| Fork. Christiania, 1874.
| |
| | |
| (491) Sidney J. Smith. " The early stages of the American Lobster. " Trans,
| |
| of the Connecticut Acad. of Arts and Sciences, Vol. n., Part 2, 1873.
| |
| | |
| (492) R. v. Willemoes Suhm. " Preliminary note on the development of some
| |
| pelagic Decapoda." Proc. of Royal Society, 1876.
| |
| | |
| Stomatopoda.
| |
| | |
| (493) W. K. Brooks. " On the larval stages of Squilla empusa." Chesapeake
| |
| Zoological Laboratory, Scientific results of the Session of 1878. Baltimore, 1879
| |
| (494) C. Claus. "Die Metamorphose der Squilliden." Abhand. der kbnigl.
| |
| Gesell. der Wiss. zu Gbttingen, 1871.
| |
| | |
| (495) Fr. Muller. " Bruchstuck a. der Entwicklungsgeschichte d. Maulfusser I.
| |
| und II." Archiv f. Naturgeschichte, Vol. xxvin., 1862, and Vol. xxix., 1863.
| |
| | |
| Cumacea.
| |
| | |
| (496) A. Dohrn. " Ueber den Bau u. Entwicklung d. Cumaceen." Jenaische
| |
| Zeitschrift, Vol. v., 1870.
| |
| | |
| Isopoda.
| |
| | |
| (497) Ed. van Beneden. " Recherches sur 1'Embryogenie des Crustaces. I.
| |
| Asellus aquaticus." Bull, de FAcad. roy Belgique, 2me serie, Tom. xxvni., No. 7,
| |
| 1869.
| |
| | |
| (498) N. Bobretzky. " Zur Embryologie des Oniscus murarius." Zeit. fur
| |
| wiss. Zool., Bd. xxiv., 1874.
| |
| | |
| (499) J. F. Bullar. "On the development of the parasitic Isopoda." Phil.
| |
| Trans., Part II., 1878.
| |
| | |
| (500) A. Dohrn. " Die embryonale Entwicklung des Asellus aquaticus." Zeit.
| |
| f. wiss. Zool., Vol. xvn., 1867.
| |
| | |
| (501) H. Rathke. Untersuchungen iiber die Bildung tmd Entwicklung der
| |
| Wasser-Assel. Leipzig, 1832.
| |
| | |
| (502) H. Rathke. Zur Morphologic. Reisebemerkungen aus Taurien. Riga u.
| |
| Leipzig, 1837. (Bopyrus, Idothea, Ligia, lanira.)
| |
| | |
| 342
| |
| | |
| | |
| | |
| 532 BIBLIOGRAPHY.
| |
| | |
| | |
| | |
| A mphipoda.
| |
| | |
| (503) Ed. van Beneden and E. Bessels. "M&noire sur la formation du
| |
| blastoderme chez les Amphipodes, les Lerneens et les Cope"podes." Classe des Sciences
| |
| deTAcad. roy. de Belgique, Vol. xxxiv., 1868.
| |
| | |
| (504) De la Valletta St George. " Studien iiber die Entwicklung der Amphipoden." Abhand. d. naturfor. Gesell. zu Halle, Bd. v., 1860.
| |
| | |
| Copepoda.
| |
| | |
| (505) E. van Beneden and E. Bessels. " Me*moire sur la formation du blastoderme chez les Amphipodes, les Lerndens et Copepodes." Classe des Sciences de
| |
| FAcad. roy. de Belgique, Vol. xxxiv., 1868.
| |
| | |
| (506) E. van Beneden. " Recherches sur 1'Embryogenie des Crustaces iv. Anchorella, Lerneopoda, Branchiella, Hessia." Bull, de FAcad. roy. de Belgique, 2me
| |
| serie, T. xxix., 1870.
| |
| | |
| (507) C. Claus. Zur Anatomie u. Entwicklungsgeschichte d. Copepoden.
| |
| | |
| (508) C. Claus. " Untersuchungen Uber die Organisation u. Verwandschaft d.
| |
| Copepoden." Wiirzburger naturwiss. Zeitschrift, Bd. III., 1862.
| |
| | |
| (509) C. Claus. " Ueber den Bau u. d. Entwicklung von Achtheres percarum."
| |
| Zeit.f. wiss. Zool., Bd. XL, 1862.
| |
| | |
| (510) C. Claus. Die freilebenden Copepoden mit besonderer Berucksichtigung der
| |
| Fauna Deutschlands, des Nordsee u. des Mittelmeeres. Leipzig, 1863.
| |
| | |
| (511) C. Claus. " Ueber d. Entwicklung, Organisation u. systematische Stellung
| |
| d. Argulidse." Zeit.f. wiss. Zool., Bd. xxv., 1875.
| |
| | |
| (512) P. P. C. Hoek. " Zur Entwicklungsgeschichte d. Entomostracen." Niederldndisches Archiv, Vol. IV., 1877.
| |
| | |
| (513) N o r d m a n n. Mikrographische Beitrdge zur Naturgeschichte der ivirbellosen
| |
| l^hiere. Zweites Heft. 1832.
| |
| | |
| (514) Salensky. " Sphseronella Leuckartii." Archivf. Naturgeschichte, 1868.
| |
| | |
| (515) F. Vejdovsky. "Untersuchungen Ub. d. Anat. u. Metamorph. v. Tracheliastes polycolpus." Zeit.f. wiss. Zool., Vol. xxix., 1877.
| |
| | |
| Cirripedia.
| |
| | |
| (516) C. Spence Bate. "On the development of the Cirripedia." Annals
| |
| and Mag. of Natur. History. Second Series, Vin., 1851.
| |
| | |
| (517) E. van Beneden. " DeVeloppement des Sacculines." Bull, de I" Acad.
| |
| roy. de Belg., 1870.
| |
| | |
| (518) C. Claus. Die Cypris-dhnliche Larve der Cifripedien. Marburg, 1869.
| |
| | |
| (519) Ch. Darwin. A monograph of the sub-class Cirripedia, i Vols., Ray
| |
| Society, 18514.
| |
| | |
| (520) A. Dohrn. ' Untersuchungen iiber Bau u. Entwicklung d. Arthropoden
| |
| ix. Eine neue Naupliusform (Archizoea gigas)." Zeit. f. wiss. Zool., Bd. xx.,
| |
| 1870.
| |
| | |
| (521) P. P. C. Hoek. "Zur Entwicklungsgeschichte der Entomostraken i.
| |
| Kinbryologie von Balanus." Niederldndisches Archiv fur Zoologie, Vol. III., 1876 7.
| |
| | |
| (522) R. Kossmann. "Suctoria u. Lepadidoc." Arbeiten a. d. zool.-zoot. Instituted. Univer. Wiirz., Vol. I., 1873.
| |
| | |
| | |
| | |
| CRUSTACEA. 533
| |
| | |
| | |
| | |
| (523) Aug. Krohn. " Beobachtungen iiber die Entwicklung der Cirripedien."
| |
| Wiegmanris Archiv fur Naturgesch., xxvi., 1860.
| |
| | |
| (524) E. Metschnikoff. Sitzungsberichte d. Versammlung deutscher Naturforscher zu Hannover, 1865. (Balanus balanoides.)
| |
| | |
| (525) Fritz Muller. "Die Rhizocephalen." Archiv f. Naturgeschichte,
| |
| 1862-3.
| |
| | |
| (526) F. C. Noll. " Kochlorine hamata, ein bohrendes Cirriped." Zeit.f. wiss.
| |
| Zool., Bd. xxv., 1875.
| |
| | |
| (527) A. Pagenstecher. " Beitrage zur Anatomic und Entwicklungsgeschichte
| |
| von Lepas pectinata." Zeit.f. wiss. ZooL, Vol. xni., 1863.
| |
| | |
| (528) J. V. Thompson. Zoological Researches and Illustrations, Vol. I., Part I.
| |
| Memoir IV. On the Cirripedes or Barnacles. 8vo. Cork, 1830.
| |
| | |
| (529) J. V. Thompson. " Discovery of the Metamorphosis in the second type
| |
| of the Cirripedes, viz. the Lepades completing the natural history of these singular
| |
| animals, and confirming their affinity with the Crustacea." Phil. Trans. 1835. Part
| |
| n.
| |
| | |
| (530) R. von Willemoes Suhm. "On the development of Lepas fascicularis."
| |
| Phil. Trans., Vol. 166, 1876.
| |
| | |
| Ostracoda.
| |
| | |
| (531) C. Glaus. " Zur naheren Kenntniss der Jugendformen von Cypris ovum."
| |
| Zeit.f. wiss. ZooL, Bd. xv., 1865.
| |
| | |
| (532) C. Glaus. "Beitrage zur Kenntniss d. Ostracoden. Entwicklungsgeschichte von Cypris ovum." Schriften d. Gesell. zur Befdrderung d. gesamm. Naturwiss. zu Marburg, Vol. IX., 1868.
| |
| | |
| | |
| | |
| CHAPTER XIX.
| |
| | |
| | |
| | |
| PCECILOPODA, PYCNOGONIDA, TARDIGRADA, AND LINGUATULIDA; AND COMPARATIVE SUMMARY OF
| |
| ARTHROPODAN DEVELOPMENT.
| |
| | |
| THE groups dealt with in the present Chapter undoubtedly
| |
| belong to the Arthropoda. They are not closely related, and in
| |
| the case of each group it is still uncertain with which of the
| |
| main phyla they should be united. It is possible that they may
| |
| all be offshoots from the Arachnidan phylum.
| |
| | |
| PCECILOPODA.
| |
| | |
| The development of Limulus has been studied by Dohrn (No. 533) and
| |
| Packard (No. 534). The ova are laid in the sand near the spring-tide
| |
| marks. They are enveloped in a thick chorion formed of several layers ;
| |
| and (during the later stages of development at any rate) there is a membrane within the chorion which exhibits clear indications of cell outlines 1 .
| |
| | |
| There is a centrolecithal segmentation, which ends in the formation of
| |
| a blastoderm enclosing a central yolk mass. A ventral plate is then
| |
| formed, which is thicker in the region where the abdomen is eventually
| |
| developed. Six segments soon become faintly indicated in the cephalothoracic region, the ends of which grow out into prominent appendages
| |
| (fig. 245 A) ; of these there are six pairs, which increase in size from before
| |
| backwards. A stomodaeum (m) is by this time established and is placed well
| |
| in front of the foremost pair of appendages'*-.
| |
| | |
| In the course of the next few days the two first appendages of the
| |
| abdominal region become formed (vide fig. 245 C shewing those abdominal
| |
| appendages at a later stage), and have a very different shape and direction
| |
| to those of the cephalothorax. The appendages of the latter become
| |
| | |
| 1 The nature of the inner membrane is obscure. It is believed by Packard to be
| |
| moulted after the formation of the limbs, and to be equivalent to the amnion of Insects,
| |
| while by Dohrn it is regarded as a product of the follicle cells.
| |
| | |
| 2 Dohrn finds at first only five appendages, but thinks that the sixth (the anterior
| |
| one) may have been present but invisible.
| |
| | |
| | |
| | |
| PCECILOPODA.
| |
| | |
| | |
| | |
| 535
| |
| | |
| | |
| | |
| flexed in the middle in such a way that their ends become directed towards
| |
| the median line (fig. 245 B). The body of the embryo (fig. 245 B) is
| |
| now distinctly divided into two regions the cephalothoracic in front, and
| |
| the abdominal behind, both divided into segments.
| |
| | |
| | |
| | |
| | |
| FIG. 245. THREE STAGES IN THE DEVELOPMENT OF LIMULUS POLYPHEMUS.
| |
| (Somewhat modified from Packard.)
| |
| | |
| A. Embryo in which the thoracic limbs and mouth have become developed on
| |
| the ventral plate. The outer line represents what Packard believes to be the amnion.
| |
| | |
| B. Later embryo from the ventral surface.
| |
| | |
| C. Later embryo, just before the splitting of the chorion from the side. The full
| |
| number of segments of the abdomen, and three abdominal appendages, have become
| |
| established ; m. mouth ; I IX. appendages.
| |
| | |
| Round the edge of the ventral plate there is a distinct ridge the
| |
| rudiment of the cephalothoracic shield.
| |
| | |
| With the further growth of the embryo the chorion becomes split
| |
| and cast off, the embryo being left enclosed within the inner membrane.
| |
| The embryo has a decided ventral flexure, and the abdominal region
| |
| grows greatly and forms a kind of cap at the hinder end, while its
| |
| vaulted dorsal side becomes divided into segments (fig. 245 C). Of these
| |
| there are according to Dohrn seven, but according to Packard nine, of
| |
| which the last forms the rudiment of the caudal spine.
| |
| | |
| In the thoracic region the nervous system is by this stage formed as
| |
| a ganglionated cord (Dohrn), with no resemblance to the peculiar cesophageal ring of the adult. The mouth is stated by Dohrn to lie between the
| |
| second pair of limbs, so that, if the descriptions we have are correct, it must
| |
| have by this stage changed its position with reference to the appendages.
| |
| Between the thorax and abdomen two papillae have arisen which form the
| |
| | |
| | |
| | |
| 536
| |
| | |
| | |
| | |
| PCEC1LOPODA.
| |
| | |
| | |
| | |
| so-called lower lip of the adult, but from their position and late development
| |
| they can hardly be regarded as segmental appendages. In the course of
| |
| further changes all the parts become more distinct, while the membrane in
| |
| which the larva is placed becomes enormously distended (fig. 246 A). The
| |
| rudiments of the compound eyes are formed on the third (Packard) or fourth
| |
| (Dohrn) segment of the cephalothorax, and the simple eyes near the median
| |
| line in front. The rudiments of the inner process of the chelae of the cephalothoracic appendages arise as buds. The abdominal appendages become
| |
| more plate-like, and the rudiments of a third pair appear behind the two
| |
| already present. The heart appears on the dorsal surface.
| |
| | |
| An ecdysis now takes place, and in the stage following the limbs have
| |
| approached far more closely to their adult state (fig. 246 A). The
| |
| cephalothoracic appendages become fully jointed ; the two anterior abdominal appendages (vn.) have approached, and begin to resemble the oper
| |
| | |
| | |
| ce.
| |
| | |
| | |
| | |
| | |
| | |
| VIII
| |
| | |
| | |
| | |
| FlO. 246. TWO STAGES IN THE DEVELOPMENT OF LlMULUS POLYPHEMUS.
| |
| | |
| (After Dohrn.)
| |
| | |
| A. An advanced embryo enveloped in the distended inner membrane shortly
| |
| before hatching ; from the ventral side.
| |
| | |
| B. A later embryo at the Trilobite stage, from the dorsal side.
| |
| I., vii., VIII. First, seventh, and eight appendages.
| |
| | |
| cs. caudal spine ; se. simple eye ; ce. compound eye.
| |
| | |
| culum of the adult, and on the second pair is formed a small inner ramus.
| |
| The segmentation of the now vaulted cephalothorax becomes less obvious,
| |
| though still indicated by the arrangement of the yolk masses which form
| |
| the future hepatic diverticula.
| |
| | |
| Shortly after this stage the embryo is hatched, and at about the time of
| |
| hatching acquires a form (fig. 246 B) in which it bears, as pointed out by
| |
| Dohrn and Packard, the most striking resemblance to a Trilobite.
| |
| | |
| Viewed from the dorsal surface (fig. 246 B) it is divided into two
| |
| distinct regions, the cephalothoracic in front and the abdominal behind.
| |
| The cephalothoracic has become much flatter and wider, has lost all trace
| |
| of its previous segmentation, and has become distinctly trilobed. The
| |
| | |
| | |
| | |
| PCECILOPODA. 537
| |
| | |
| | |
| | |
| central lobe forms a well-marked keel, and at the line of insertion of the
| |
| rim-like edge of the lateral lobes are placed the two pairs of eyes (se and
| |
| ce). The abdominal region is also distinctly trilobed and divided into nine
| |
| segments ; the last, which is merely formed of a median process, being the
| |
| rudiment of the caudal spine. The edges of the second to the seventh are
| |
| armed with a spine. The changes in the appendages are not very considerable. The anterior pair nearly meet in the middle line in front or
| |
| the mouth ; and the latter structure is completely covered by an upper
| |
| lip. Each abdominal appendage of the second pair is provided with four
| |
| gill-lamellas, attached close to its base.
| |
| | |
| Three weeks after hatching an ecdysis takes place, and the larva passes
| |
| from a trilobite into a limuloid form. The segmentation of the abdomen
| |
| has become much less obvious, and this part of the embryo closely resembles its permanent form. The caudal spine is longer, but is still relatively
| |
| short. A fourth pair of abdominal appendages is established, and the first
| |
| pair have partially coalesced, while the second and third pairs have become
| |
| jointed, their outer ramus containing four and their inner three joints.
| |
| Additional gill-lamellae attached to the two basal joints of the second and
| |
| third abdominal appendages have appeared.
| |
| | |
| The further changes are not of great importance. They are effected in
| |
| a series of successive moults. The young larvae swim actively at the
| |
| surface.
| |
| | |
| Our, in many respects, imperfect knowledge of the development of
| |
| Limulus is not sufficient to shew whether it is more closely related to the
| |
| Crustacea or to the Arachnida, or is an independent phylum.
| |
| | |
| The somewhat Crustacean character of biramous abdominal feet, etc.
| |
| is not to be denied, but at the same time the characters of the embryo
| |
| appear to me to be decidedly more arachnidan than crustacean. The
| |
| embryo, when the appendages are first formed, has a decidedly arachnidan facies. It will be remembered that when the limbs are first formed
| |
| they are all post-oral. They resemble in this respect the limbs of the
| |
| Arachnida, and it seems to be probable that the anterior pair is equivalent
| |
| to the cheliceras of Arachnida, which, as shewn in a previous section, are
| |
| really post-oral appendages in no way homologous with antennae 1 .
| |
| | |
| The six thoracic appendages may thus be compared with the six
| |
| Arachnidan appendages; which they resemble in their relation to the
| |
| mouth, their basal cutting blades, etc.
| |
| | |
| The existence of abdominal appendages behind the six cephalothoracic
| |
| does not militate against the Arachnidan affinities of Limulus, because in
| |
| the Arachnida rudimentary abdominal appendages are always present in
| |
| the embryo. The character of the abdominal appendages is probably
| |
| | |
| 1 Dohrn believes that he has succeeded in shewing that the first pair of appendages
| |
| of Limulus is innervated in the embryo from the supra-cesophageal ganglia. His
| |
| observations do not appear to me conclusive, and, arguing from what we know of the
| |
| development of the Arachnida, the innervation of these appendages in the adult can be
| |
| of no morphological importance.
| |
| | |
| | |
| | |
| 538 PYCNOGONIDA.
| |
| | |
| | |
| | |
| secondarily adapted to an aquatic respiration, since it is likely (for the
| |
| reasons already mentioned in connection with the Tracheata) that if Limulus
| |
| has any affinities with the stock of the Tracheata it is descended from airbreathing forms, and has acquired its aquatic mode of respiration. The
| |
| anastomosis of the two halves of the generative glands is an Arachnidan
| |
| character, and the position of the generative openings in Limulus is more
| |
| like that in the Scorpion than in Crustacea.
| |
| | |
| A fuller study of the development would be very likely to throw
| |
| further light on the affinities of Limulus, and if Packard's view about the
| |
| nature of the inner egg membrane were to be confirmed, strong evidence
| |
| would thereby be produced in favour of the Arachnidan affinities.
| |
| | |
| (533) A. Dohrn. "Untersuch. Ub. Bau u. Entwick. d. Arthropoden (Limulus
| |
| polyphemus)." Jenaische Zeitschrift, Vol. vi., 1871.
| |
| | |
| (534) A. S. Packard. "The development of Limulus polyphemus." Mem.
| |
| Boston Soc. Nat. History, Vol. II., 1872.
| |
| | |
| PYCNOGONIDA.
| |
| | |
| The embryos, during the first phases of their development, are always
| |
| carried by the male in sacks which are attached to a pair of appendages
| |
| (the third) specially formed for this purpose. The segmentation of the
| |
| ovum is complete, and there is in most forms developed within the eggshell a larva with three pairs of two-jointed appendages, and a rostrum
| |
| placed between the front pair.
| |
| | |
| It will be convenient to take Achelia kevis, studied by Dohrn (No. 536),
| |
| as type.
| |
| | |
| The larva of Achelia when hatched is provided with the typical three
| |
| pairs of appendages. The foremost of them is chelate, and the two following pairs are each provided with a claw. Of the three pairs of larvalappendages Dohrn states that he has satisfied himself that the anterior is
| |
| innervated by the supra-cesophageal ganglion, and the two posterior by
| |
| separate nerves coming from two imperfectly united ventral ganglia. The
| |
| larva is provided with a median eye formed of two coalesced pigment
| |
| spots, and with a simple stomach.
| |
| | |
| The gradual conversion of the larva into the adult takes place by the
| |
| elongation of the posterior end of the body into a papilla, and the formation there, at a later period, of the anus ; while at the two sides of the
| |
| anal papilla rudiments of a fresh pair of appendages the first pair of ambulatory limbs of the adult make their appearance. The three remaining
| |
| pairs of limbs become formed successively as lateral outgrowths, and their
| |
| development is accomplished in a number of successive ecdyses. As they
| |
| are formed caeca from the stomach become prolonged into them. For each
| |
| of them there appears a special ganglion. While the above changes are
| |
| taking place the three pairs of larval appendages undergo considerable
| |
| reduction. The anterior pair singly becomes smaller, the second loses
| |
| its claw, and the third becomes reduced to a mere stump. In the adult the
| |
| | |
| | |
| | |
| PENTASTOMIDA. 539
| |
| | |
| | |
| | |
| second pair of appendages becomes enlarged again and forms the so-called
| |
| palpi, while the third pair develops in the male into the egg-carrying appendages, but is aborted in the female. The first pair form appendages lying
| |
| parallel to the rostrum, which are sometimes called pedipalpi and sometimes antennae.
| |
| | |
| The anal papilla is a rudimentary abdomen, and, as Dohrn has shewn,
| |
| contains rudiments of two pairs of ganglia.
| |
| | |
| The larvae of Phoxichilidium are parasitic in various Hydrozoa (Hydractinia, etc.). After hatching they crawl into the Hydractinia stock. They
| |
| are at first provided with the three normal pairs of larval appendages. The
| |
| two hinder of these are soon thrown off, and the posterior part of the trunk,
| |
| with the four ambulatory appendages belonging to it, becomes gradually
| |
| developed in a series of moults. The legs, with the exception of the hindermost pair, are fully formed at the first ecdysis after the larva has become
| |
| free. In the genus Pallene the metamorphosis is abbreviated, and the'
| |
| young are hatched with the full complement of appendages.
| |
| | |
| The position of the Pycnogonida is not as yet satisfactorily settled.
| |
| The six-legged larva has none of the characteristic features of the Nauplius,
| |
| except the possession of the same number of appendages.
| |
| | |
| The number of appendages (7) of the Pycnogonida does not coincide
| |
| with that of the Arachnida. On the other hand, the presence of chelate
| |
| appendages innervated in the adult by the supra-cesophageal ganglia rather
| |
| points to a common phylum for the Pycnogonida and Arachnida ; though as
| |
| shewn above (p. 455) all the appendages in the embryo of true Arachnida
| |
| are innervated by post-oral ganglia. The innervation of these appendages
| |
| in . the larvae of Pycnogonida requires further investigation. Against
| |
| such a relationship the extra pair of appendages in the Pycnogonida is
| |
| no argument, since the embryos of most Arachnida are provided with four
| |
| such extra pairs. The two groups must no doubt have diverged very
| |
| early.
| |
| | |
| BIBLIOGRAPHY.
| |
| | |
| (535) G. Cavanna. " Studie e ricerche sui Picnogonidi." Pubblicazioni del R.
| |
| Institute di Studi stiperiori in Firenze, 1877.
| |
| | |
| (536) An. Dohrn. " Ueber Entwickhuig u. Baud. Pycnogoniden." Jenaische
| |
| Zeitschrift, Vol. v. 1870, and " Neue Untersuchungen lib. Pycnogoniden." Mitthdl.
| |
| a. d. zoologischen Station zu Neafel, Bd. I. 1878.
| |
| | |
| (537) G. Hodge. " Observations on a species of Pycnogon, etc." Annal. and
| |
| Mag. of Nat. Hist. Vol. ix. 1862.
| |
| | |
| (538) C. Semper. " Ueber Pycnogoniden u. ihre in Hydroiden schmarotzenden
| |
| Larvenformen." Arbeiten a. d. zool.-zoot. Instit. Wiirzburg, Vol. I. 1874.
| |
| | |
| PENTASTOMIDA.
| |
| | |
| The development and metamorphosis of Pentastomum taenoides have
| |
| been thoroughly worked out by Leuckart (No. 540) and will serve as type
| |
| for the group.
| |
| | |
| | |
| | |
| 540 PENTASTOMIDA.
| |
| | |
| | |
| | |
| In the sexual state it inhabits the nasal cavities of the dog. The early
| |
| embryonic development takes place as the ovum gradually passes down the
| |
| uterus. The segmentation appears to be complete ; and gives rise to an
| |
| oval mass in which the separate cells can hardly be distinguished. This
| |
| gradually differentiates itself into a characteristic embryo, divided into a tail
| |
| and trunk. The tail is applied to the ventral surface of the trunk, and on
| |
| the latter two pairs of stump-like unsegmented appendages arise, each
| |
| provided with a pair of claws. At the anterior extremity of the body is
| |
| formed the mouth, with a ventral spine and lateral hook, which are perhaps
| |
| degenerated jaws. The spine functions as a boring apparatus, and an
| |
| apparatus with a similar function is formed at the end of the tail. A larval
| |
| cuticle now appears, which soon becomes detached from the embryo, except
| |
| on the dorsal surface, where it remains firmly united to a peculiar papilla.
| |
| This papilla becomes eventually divided into two parts, one of which remains
| |
| attached to the cuticle, while the part connected with the embryo forms a
| |
| raised cross placed in a cup- shaped groove. The whole structure has been
| |
| compared, on insufficient grounds, to the dorsal organ of the Crustacea.
| |
| | |
| The eggs, containing the embryo in the condition above described, are
| |
| eventually carried out with the nasal slime, and, if transported thence into
| |
| the alimentary cavity of a rabbit or hare, the embryos become hatched by
| |
| the action of the gastric juice. From the alimentary tract of their new host
| |
| they make their way into the lungs or liver. They here become enveloped
| |
| in a cyst, in the interior of which they undergo a very remarkable metamorphosis. They are, however, so minute and delicate that Leuckart was
| |
| unable to elucidate their structure till eight weeks after they had been
| |
| swallowed. At this period they are irregularly-shaped organisms, with a
| |
| most distant resemblance to the earlier embryos. They are without their
| |
| previous appendages, but the alimentary tract is now distinctly differentiated.
| |
| The remains of two cuticles in the cyst seem to shew that the above changes
| |
| are effected in two ecdyses.
| |
| | |
| In the course of a series of ecdyses the various organs of the larval form
| |
| known as Pentastomum denticulatum continue to become differentiated.
| |
| After the first (= third) ecdysis the cesophageal nerve-ring and sexually
| |
| undifferentiated generative organs are developed. At the fourth (=sixth)
| |
| ecdysis the two pairs of hooks of the adult are formed in pockets which
| |
| appeared at a somewhat earlier stage ; and the body acquires an annulated
| |
| character. At a somewhat earlier period rudiments of the external generative organs indicate the sex of the larva.
| |
| | |
| After a number of further ecdyses, which are completed in about six
| |
| months after the introduction of the embryos into the intermediate host, the
| |
| larva attains its full development, and acquires a form in which it has long
| |
| been known as Pentastomum denticulatum. It now leaves its cyst and
| |
| begins to move about. It is in a state fit to be introduced into its final host ;
| |
| but if it be not so introduced it may become encysted afresh.
| |
| | |
| If the part of a rabbit or hare infected by a Pentastomum denticulatum
| |
| be eaten by a dog or wolf, the parasite passes into the nasal cavity of the
| |
| | |
| | |
| | |
| TARDIGRADA. 541
| |
| | |
| | |
| | |
| latter, and after further changes of cuticle becomes a fully-developed sexual
| |
| Pentastomum taenioides, which does not differ to any very marked extent
| |
| from P. denticulatum.
| |
| | |
| In their general characters the larval migrations of Pentastomum are
| |
| similar to those of the Cestodes.
| |
| | |
| The internal anatomy of the adult Pentastomum, as well as the
| |
| characters of the larva with two pairs of clawed appendages, are perhaps
| |
| sufficient to warrant us in placing it with the Arthropoda, though it would
| |
| be difficult to shew that it ought not to be placed with such a form as
| |
| Myzostomum (vide p. 369). There do not appear to be any sufficient
| |
| grounds to justify its being placed with the Mites amongst the Arachnida.
| |
| If indeed the rings of the body of the Pentastomida are to be taken as
| |
| implying a true segmentation, it is clear that the Pentastomida cannot be
| |
| associated with the Mites.
| |
| | |
| BIBLIOGRAPHY.
| |
| | |
| (539) P. J. van Beneden. " Recherches s. 1'organisation et le developpement d.
| |
| Linguatules." Ann. d. Sden. Nat., 3 Ser., Vol. XI.
| |
| | |
| (540) R. Leuckart. " Bau u. Entwicklungsgeschichte d. Pentastomen." Leipzig
| |
| and Heidelberg. 1860.
| |
| | |
| TARDIGRADA.
| |
| | |
| Very little is known with reference to the development of the Tardigrada.
| |
| A complete and regular segmentation (von Siebold, Kaufmann, No. 541) is
| |
| followed by the appearance of a groove on the ventral side indicating a
| |
| ventral flexure. At about the time of the appearance of the groove the cells
| |
| become divided into an epiblastic investing layer and a central hypoblastic
| |
| mass.
| |
| | |
| The armature of the pharynx is formed very early at the anterior
| |
| extremity, and the limbs arise in succession from before backwards.
| |
| | |
| The above imperfect details throw no light on the systematic position of
| |
| this group.
| |
| | |
| Tardigrada.
| |
| | |
| (541) J. Kaufmann. " Ueber die Entwicklung u. systematische Stellung d.
| |
| Tardigraden." Zeit.f. wiss, ZooL, Bd. HI. 1851.
| |
| | |
| Summary of Arthropodan Development.
| |
| The numerous characters common to the whole of the
| |
| Arthropoda led naturalists to unite them in a common phylum,
| |
| but the later researches on the genealogy of the Tracheata and
| |
| Crustacea tend to throw doubts on this conclusion, while there
| |
| is not as yet sufficient evidence to assign with certainty a
| |
| definite position in either of these classes to the smaller groups
| |
| described in the present chapter. There seems to be but little
| |
| | |
| | |
| | |
| 542 SUMMARY.
| |
| | |
| | |
| | |
| doubt that the Tracheata are descended from a terrestrial Annelidan type related to Peripatus. The affinities of Peripatus to
| |
| the Tracheata are, as pointed out in a previous chapter (p. 386),
| |
| very clear, while at the same time it is not possible to regard
| |
| Peripatus simply as a degraded Tracheate, owing to the fact
| |
| that it is provided with such distinctly Annelidan organs as
| |
| nephridia, and that its geographical distribution shews it to be a
| |
| very ancient form.
| |
| | |
| The Crustacea on the other hand are clearly descended from
| |
| a Phyllopod-like ancestor, which can be in no way related to
| |
| Peripatus.
| |
| | |
| The somewhat unexpected conclusion that the Arthropoda
| |
| have a double phylum is on the whole borne out by the anatomy
| |
| of the two groups. Without attempting to prove this in detail,
| |
| it may be pointed out that the Crustacean appendages are
| |
| typically biramous, while those of the Tracheata are never at
| |
| any stage of development biramous 1 ; and the similarity between
| |
| the appendages of some of the higher Crustacea and those of
| |
| many Tracheata is an adaptive one, and could in no case be
| |
| used as an argument for the affinity of the two groups.
| |
| | |
| The similarity of many organs is to be explained by both
| |
| groups being descendants of Annelidan ancestors. The similarity of the compound eye in the two groups cannot however
| |
| be explained in this way, and is one of the greatest difficulties
| |
| of the above view. It is moreover remarkable that the eye of
| |
| Peripatus 2 is formed on a different type to either the single or
| |
| compound eyes of most Arthropoda.
| |
| | |
| The conclusion that the Crustacea and Tracheata belong to
| |
| two distinct phyla is confirmed by a consideration of their
| |
| development. They have no doubt in common a centrolecithal
| |
| segmentation, but, as already insisted on, the segmentation is
| |
| no safe guide to the affinities.
| |
| | |
| In the Tracheata the archenteron is never, so far as we
| |
| know, formed by an invagination 3 , while in Crustacea the
| |
| | |
| 1 The biflagellate antennae of Pauropus amongst the Myriapocls can hardly be
| |
| considered as constituting an exception to this rule.
| |
| | |
| 3 I hope to shew this in a paper I am preparing on the anatomy of Peripatus.
| |
| | |
| 8 Stecker's description of an invagination in the Chilognatha cannot be accepted
| |
| without further confirmation ; -vide p. 388.
| |
| | |
| | |
| | |
| SUMMARY. 543
| |
| | |
| | |
| | |
| evidence is in favour of such an invagination being the usual,
| |
| and, without doubt, the primitive, mode of origin.
| |
| | |
| The mesoblast in the Tracheata is formed in connection with
| |
| a median thickening of the ventral plate. The unpaired plate
| |
| of mesoblast so formed becomes divided into two bands, one on
| |
| each side of the middle line.
| |
| | |
| In both Spiders and Myriopods, and probably Insects, the
| |
| two plates of mesoblast are subsequently divided into somites,
| |
| the lumen of which is continued into the limbs.
| |
| | |
| In Crustacea the mesoblast usually originates from the walls
| |
| of the invagination, which gives rise to the mesenteron.
| |
| | |
| It does not become divided into two distinct bands, but
| |
| forms a layer of scattered cells between the epiblast and hypoblast, and does not usually break up into somites ; and though
| |
| somites are stated in some cases to be found they do not
| |
| resemble those in the Tracheata.
| |
| | |
| The proctodaeum is usually formed in Crustacea before and
| |
| rarely later 1 than the stomodaeum. The reverse is true for the
| |
| Tracheata. In Crustacea the proctodseum and stomodaeum,
| |
| especially the former, are very long, and usually give rise to the
| |
| greater part of the alimentary tract, while the mesenteron is
| |
| usually short.
| |
| | |
| In the Tracheata the mesenteron is always considerable, and
| |
| the proctodaeum is always short. The derivation of the Malpighian bodies from the proctodaeum is common to most
| |
| Tracheata. Such diverticula of the proctodaeum are not found
| |
| in Crustacea.
| |
| | |
| 1 This is stated to be the case in Moina (Grobben).
| |
| | |
| | |
| | |
| CHAPTER XX.
| |
| | |
| | |
| | |
| ECHINODERMATA 1 .
| |
| | |
| THE development of the Echinodermata naturally falls into
| |
| two sections:
| |
| | |
| (i) The development of the germinal layers and of the
| |
| systems of organs; (2) the development of the larval appendages
| |
| and the metamorphosis.
| |
| | |
| The Development of the Germinal Layers and of tJie Systems
| |
| | |
| of Organs.
| |
| | |
| The development of the systems of organs presents no very
| |
| important variations within the limits of the group.
| |
| | |
| Holothuroidea. The Holothurians have been most fully
| |
| studied (Selenka, No. 563), and may be conveniently taken as
| |
| type.
| |
| | |
| The segmentation is nearly regular, though towards its close,
| |
| and in some instances still earlier, a difference becomes apparent
| |
| between the upper and the lower poles.
| |
| | |
| At the close of segmentation (fig. 247 A) the egg has a
| |
| nearly spherical form, and is constituted of a single layer of
| |
| columnar cells enclosing a small segmentation cavity. The
| |
| lower pole is slightly thickened, and the egg rotates by means of
| |
| fine cilia.
| |
| | |
| An invagination now makes its appearance at the lower
| |
| pole (fig. 247 B), and simultaneously there become budded off
| |
| from tJie cells undergoing the invagination amoeboid cells, which
| |
| | |
| 1 The following classification of the Echinodermata is employed in this chapter.
| |
| | |
| I. Holothuroidea. IV. Echinoidea.
| |
| | |
| II. Asteroidea. V. Crinoidea.
| |
| | |
| III. Ophiuroidea.
| |
| | |
| | |
| | |
| ECHINODERMATA. 545
| |
| | |
| | |
| | |
| eventually form the muscular system and the connective tissue.
| |
| These cells very probably have a bilaterally symmetrical origin.
| |
| This stage represents the gastrula stage which is common to all
| |
| Echinoderms. The invaginated sack is the archenteron. As it
| |
| grows larger one side of the embryo becomes flattened, and the
| |
| other more convex. On the flattened side a fresh invagination
| |
| | |
| | |
| | |
| | |
| FIG. 247. TWO STAGES IN THE DEVELOPMENT OF HOLOTHURIA TUBULOSA
| |
| | |
| VIEWED IN OPTICAL SECTION. (After Selenka.)
| |
| A. Blastosphere stage at the close of segmentation. B. Gastrula stage.
| |
| mr. micropyle ; //. chorion; s.c. segmentation cavity; bl. blastoderm; ep. epiblast;
| |
| hy. hypoblast; ms. amoeboid cells derived from hypoblast ; a.e. archenteron.
| |
| | |
| arises, the opening of which forms the permanent mouth, the
| |
| opening of the first invagination remaining as the permanent
| |
| anus (fig. 248 A).
| |
| | |
| These changes give us the means of attaching definite names
| |
| to the various parts of the embryo. It deserves to be noted in
| |
| the first place that the embryo has assumed a distinctly bilateral
| |
| form. There is present a more or less concave surface extending from the mouth to near the anus, which will be spoken
| |
| of as the ventral surface. The anus is situated at the posterior
| |
| extremity. The convex surface opposite the ventral surface
| |
| forms the dorsal surface, which terminates anteriorly in a
| |
| rounded prse-oral prominence.
| |
| | |
| It will be noticed in fig. 248 A that in addition to the
| |
| primitive anal invagination there is present a vesicle (?/.).
| |
| This vesicle is directly formed by a constriction of the primitive
| |
| B. II. 35
| |
| | |
| | |
| | |
| 54 6
| |
| | |
| | |
| | |
| HOLOTHUROIDEA.
| |
| | |
| | |
| | |
| archenteron (fig. 249 Vpv.), and is called by Selenka the vasoperitoneal vesicle. It gives origin to the epithelioid lining of
| |
| the body cavity and water-vascular system of the adult 1 . In the
| |
| parts now developed we have the rudiments of all the adult organs.
| |
| The mouth and anal involutions (after the separation of the
| |
| vaso-peritoneal vesicle) meet and unite, a constriction indicating
| |
| their point of junction (fig. 248 B). Eventually the former gives
| |
| | |
| | |
| | |
| | |
| FIG. 248. THREE STAGES IN THE DEVELOPMENT OF HOLOTHURIA TUBULOSA
| |
| | |
| VIEWED FROM THE SIDE IN OPTICAL SECTION. (After Selenka.)
| |
| tn. mouth; oe. oesophagus; st. stomach; i. intestine; a. anus; I.e. longitudinal
| |
| ciliated band; v.p. vaso-peritoneal vesicle; p.v. peritoneal vesicle; p.r. right peritoneal vesicle ; //. left peritoneal vesicle ; w.v. water- vascular vesicle ; p. dorsal pore
| |
| of water- vascular system ; ms. muscle cells.
| |
| | |
| rise to the mouth and cesophagus, and the latter to the remainder of the alimentary canal 2 .
| |
| | |
| The vaso-peritoneal vesicle undergoes a series of remarkable
| |
| changes. After its separation from the archenteron it takes
| |
| up a position on the left side of this, elongates in an anteroposterior direction, and from about its middle sends a narrow
| |
| diverticulum towards the dorsal surface of the body, where an
| |
| | |
| 1 The origin of the vaso-peritoneal vesicle is not quite the same in all the species.
| |
| In Holothuria tubulosa it is separated from the csecal end of the archenteron; the
| |
| remainder of which then grows towards the oral invagination. In Cucumaria the
| |
| archenteron forks (fig. 249) ; and one fork forms the vaso-peritoneal vesicle, and the
| |
| other the major part of the mesenteron.
| |
| | |
| 2 There appears to be some uncertainty as to how much of the larval cesophagus is
| |
| derived from the stomodaeal invagination.
| |
| | |
| | |
| | |
| ECHINODERMATA.
| |
| | |
| | |
| | |
| 547
| |
| | |
| | |
| | |
| opening to the exterior becomes formed (fig. 248 B, /.). The
| |
| diverticulum becomes the madreporic canal, and the opening
| |
| the dorsal pore.
| |
| | |
| The vaso-peritoneal vesicle next divides into two, an anterior vesicle (fig. 248 B, w.v.), from which is derived the
| |
| epithelium of the water-vascular system, and a posterior (fig.
| |
| 248 B, /.?;.), which gives rise to the epithelioid lining of the body
| |
| cavity. The anterior vesicle (fig. 248 C, w.v.) becomes fivelobed, takes a horseshoe-shaped form, and grows round the
| |
| oesophagus (fig. 256, w.v.r). The five lobes form the rudiments
| |
| of the water-vascular prolongations into the tentacles. The
| |
| remaining parts of the water-vascular system are also developed
| |
| as outgrowths of the original vesicle. Five of these, alternating
| |
| with the original diverticula, form the five ambulacral canals,
| |
| from which diverticula are produced into the ambulacral feet ; a
| |
| sixth gives rise to the Polian vesicle. The remaining parts of
| |
| the original vesicle form the water-vascular ring.
| |
| | |
| We must suppose that eventually the madreporic canal loses
| |
| its connection with the exterior so as to hang loosely in the
| |
| interior, though the steps of this process do not appear to
| |
| have been made out.
| |
| | |
| The original hinder peritoneal vesicle grows rapidly,
| |
| and divides into two (fig. 248 C,
| |
| pi. and pr.}, which encircle the
| |
| two sides of the alimentary
| |
| canal, and meet above and
| |
| below it. The outer wall of
| |
| each of them attaches itself to
| |
| the skin, and the inner one to
| |
| the alimentary canal and watervascular system ; in both cases
| |
| the walls remain separated
| |
| from the adjacent parts by a
| |
| layer of the amoeboid cells
| |
| already spoken of. The cavity
| |
| of the peritoneal vesicles becomes the permanent body
| |
| cavity. Where the walls of
| |
| | |
| | |
| | |
| | |
| -ME
| |
| | |
| | |
| | |
| FIG. 249. LONGITUDINAL SECTION
| |
| | |
| THROUGH AN EMBRYO OF CUCUMARIA
| |
| DOLIOLUM AT THE END OF THE FOURTH
| |
| DAY.
| |
| | |
| Vpv. vaso-peritoneal vesicle; ME.
| |
| mesenteron; Blp., Ptd. blastopore, proctodaeum.
| |
| | |
| 352
| |
| | |
| | |
| | |
| 548 HOLOTHUROIDEA.
| |
| | |
| | |
| | |
| the two vesicles meet on the dorsal side, a mesentery, suspending the alimentary canal and dividing the body cavity longitudinally, is often formed. In other parts the partition walls
| |
| between the two sacks appear to be absorbed.
| |
| | |
| The amoeboid cells, which were derived from the invaginated
| |
| cells, arrange themselves as a layer round all the organs (fig.
| |
| 249). Some of them remain amoeboid, attach themselves to the
| |
| skin, and form part of the cutis; and in these cells the calcareous spicula of the larva and adult are formed. Others
| |
| form the musculature of the larval alimentary tract, while the
| |
| remainder give rise to the musculature and connective tissue of
| |
| the adult.
| |
| | |
| The development of the vascular system is not known, but the discovery
| |
| of Kowalevsky, confirmed by Selenka, that from the walls of the watervascular system corpuscles are developed, identical with those in the bloodvessels, indicates that it probably develops in connection with the watervascular system. The observations of Hoffmann and Perrier on the communication of the two systems in the Echinoidea point to the same conclusion.
| |
| Though nothing very definite is known with reference to the development of
| |
| the nervous system, Metschnikoff suggests that it develops in connection
| |
| with the thickened bands of epiblast which are formed by a metamorphosis
| |
| of the ciliated bands of the embryo, and accompany the five radial tubes
| |
| (vide p. 555). In any case its condition in the adult leaves no doubt of its
| |
| being a derivative of the epiblast.
| |
| | |
| From the above description the following general conclusions
| |
| may be drawn :
| |
| | |
| (1) The blastosphere stage is followed by a gastrula stage.
| |
| | |
| (2) The gastrula opening forms the permanent anus, and the
| |
| mouth is formed by a fresh invagination.
| |
| | |
| (3) The mesoblast arises entirely from the invaginated cells,
| |
| but in two ways :
| |
| | |
| (a) As scattered amoeboid cells, which give origin to the
| |
| muscles and connective tissue (including the cutis) of the body
| |
| wall and alimentary tract.
| |
| | |
| (&) As a portion separated off from the archenteron,
| |
| which gives rise both to the epithelioid lining of the body cavity,
| |
| and of the water-vascular system.
| |
| | |
| (4) The oesophagus is derived from an invagination of the
| |
| epiblast, and the remainder of the alimentary canal from the
| |
| archenteron.
| |
| | |
| | |
| | |
| ECHINODERMATA. 549
| |
| | |
| | |
| | |
| (5) The embryonic systems of organs pass directly into those
| |
| of the adult.
| |
| | |
| The development of Synapta diverges, as might be expected, to a very
| |
| small extent from that of Holothuria.
| |
| | |
| Asteroidea. In Asterias the early stages of development conform to
| |
| our type. There arise, however, two bilaterally symmetrical vaso-peritoneal
| |
| diverticula from the archenteron. These diverticula give rise both to the
| |
| lining of the body cavity and water-vascular system. With reference to
| |
| the exact changes they undergo there is, however, some difference of opinion.
| |
| Agassiz (543) maintains that both vesicles are concerned in the formation of
| |
| the water-vascular system, while Metschnikoff (560) holds that the watervascular system is entirely derived from the anterior part of the larger left
| |
| vesicle, while the right and remainder of the left vesicle form the body
| |
| cavity. MetschnikofFs statements appear to be the most probable. The
| |
| anterior part of the left vesicle, after separating from the posterior, grows
| |
| into a five-lobed rosette (fig. 260, /), and a madreporic canal (h] with a dorsal
| |
| pore opening to the exterior. The rosette appears not to grow round the
| |
| oesophagus, as in the cases hitherto described. But the latter is stated to
| |
| disappear, and a new oesophagus to be formed, which pierces the rosette,
| |
| and places the old mouth in communication with the stomach. Except
| |
| where the anus is absent in the adult, the larval anus probably persists.
| |
| | |
| Ophiuroidea. The early development of the Ophiuroidea is not so
| |
| fully known as that of other types. Most species have a free-swimming
| |
| larva, but some (Amphiura) are viviparous.
| |
| | |
| The early stages of the free-swimming larvae have not been described,
| |
| but I have myself observed in the case of Ophiothrix fragilis that the
| |
| segmentation is uniform, and is followed by the normal invagination. The
| |
| opening of this no doubt remains as the larval anus, and there are probably
| |
| two outgrowths from this to form the vaso-peritoneal vesicles. Each of these
| |
| divides into two parts, an anterior lying close to the oesophagus, and a
| |
| posterior close to the stomach. The anterior on the right side aborts ; that
| |
| on the left side becomes the water-vascular vesicle, early opens to the
| |
| exterior, and eventually grows round the oesophagus, which, as in Holothurians, becomes the oesophagus of the adult. The posterior vesicles give rise
| |
| to the lining of the body cavity, but are stated by Metschnikoff to be at first
| |
| solid, and only subsequently to acquire a cavity the permanent body cavity.
| |
| The anus naturally disappears, since it is absent in the adult. In the
| |
| viviparous type the first stages are imperfectly known, but it appears that
| |
| the blastopore vanishes before the appearance of the mouth. The development of the ^vaso-peritoneal bodies takes place as in the free-swimming
| |
| larvae.
| |
| | |
| Echinoidea. In the Echinoidea (Agassiz, No. 542, Selenka, No. 564)
| |
| there is a regular segmentation and the normal invagination (fig. 250 A).
| |
| The amoeboid mesoblast cells arise as two laterally placed masses, and give
| |
| rise to the usual parts. The archenteron grows forward and bends towards
| |
| | |
| | |
| | |
| 550
| |
| | |
| | |
| | |
| CRINOIDEA.
| |
| | |
| | |
| | |
| the ventral side (fig. 250 B). It becomes (fig. 250 C) divided into three
| |
| chambers, of which the two hindermost (d and c) form the stomach and
| |
| intestine ; while the anterior forms the oesophagus, and gives rise to the
| |
| | |
| | |
| | |
| | |
| FIG. 250. THREE SIDE VIEWS OF EARLY STAGES IN THE DEVELOPMENT OF
| |
| | |
| STRONGYLOCENTRUS. (From Agassiz.)
| |
| | |
| a, anus (blastopore) ; d. stomach ; o. oesophagus ; c . rectum ; w. vaso-peritoneal
| |
| vesicle ; v. ciliated ridge ; r. calcareous rod.
| |
| | |
| vaso-peritoneal vesicles. These latter appear as a pair of outgrowths
| |
| (fig. 251), but become constricted off as a single two-horned vesicle, which
| |
| subsequently divides into two. The left of these
| |
| is eventually divided, as in Asteroids, into a
| |
| peritoneal and water-vascular sack, while the
| |
| right forms the right peritoneal sack. An oral
| |
| invagination on the flattened ventral side meets
| |
| the mesenteron after its separation from the
| |
| vaso-peritoneal vesicle. The larval anus persists, as also does the larval mouth, but owing
| |
| to the manner in which the water-vascular
| |
| rosette is established the larval oesophagus appears to be absorbed, and to be replaced by a
| |
| fresh oesophagus.
| |
| | |
| Crinoidea. Antedon, the only Crinoid
| |
| so far studied (Gotte, No. 549), presents some
| |
| not inconsiderable variations from the usual
| |
| Echinoderm type. The blastopore is placed on
| |
| the somewhat flattened side of the oval blastosphere, and not, as is usual, at the hinder end.
| |
| | |
| The blastopore completely closes, and is not converted into the permanent anus. The archenteron gives rise to the epithelioid lining of both body
| |
| cavity and water-vascular system. These parts do not, however, appear as
| |
| a single or paired outgrowth from the archenteron, but as three distinct
| |
| outgrowths which are not formed contemporaneously. Two of them are first
| |
| | |
| | |
| | |
| | |
| FIG. -251. DORSO-VENTRAL VIEW OF AN EARLY
| |
| LARVA OF STRONGYLOCENTRUS. (From Agassiz.)
| |
| | |
| a. anus ; d. stomach ; o.
| |
| oesophagus ; w. vaso-peritoneal vesicle; r. calcareous
| |
| rod.
| |
| | |
| | |
| | |
| ECHINODERMATA.
| |
| | |
| | |
| | |
| 551
| |
| | |
| | |
| | |
| formed and become the future body cavity; but their lumens remain distinct.
| |
| Jngmally appearing as lateral outgrowths, the right one assumes a dorsal
| |
| position and sends a prolongation into the stalk (fig. 252 rp'\ and
| |
| the left one assumes first a ventral, and then an oral position (fur
| |
| 252 lp\
| |
| | |
| The third outgrowth of the archenteron gives rise to the water-vascular
| |
| vesicle. It first grows round the region of the future oesophagus and so
| |
| forms the water-vascular ring.
| |
| The wall of the ring then
| |
| grows towards the body wall
| |
| so as to divide the oral (left)
| |
| peritoneal vesicle into two
| |
| distinct vesicles, an anterior
| |
| and a posterior, shewn in fig.
| |
| 253, lp' and lp. Before this
| |
| division is completed, the
| |
| water-vascular ring is produced in front into five pro
| |
| | |
| | |
| | |
| FIG. 252. LONGITUDINAL SECTION THROUGH
| |
| AN ANTEDON LARVA. (From Carpenter: after
| |
| Gotte.)
| |
| | |
| | |
| | |
| al. mesenteron ; -wv. water- vascular ring ;
| |
| lp. left (oral) peritoneal vesicle; rp. right peritoneal vesicle ; rp'. continuation of right vesicle
| |
| into the stalk ; st. stalk.
| |
| | |
| | |
| | |
| cessesthe future tentacles
| |
| (fig. 252, wv) which project
| |
| into the cavity of the oral
| |
| vesicle (lp\ After the oral
| |
| peritoneal space has become
| |
| completely divided into two parts, the anterior dilates (fig. 253, //) greatly,
| |
| and forms a large vestibule at the anterior end of the body. This vestibule
| |
| (lp'} next acquires a communication with the mesenteron, shewn in fig. 253
| |
| at m. The anterior wall of this vestibule is finally broken through. By this
| |
| rupture the mesenteron is placed in communication with the exterior by the
| |
| opening at m, while at the same time the tentacles of the water-vascular ring
| |
| (/) project freely to the exterior. Such is Gotte's account of the prge-oral
| |
| body space, but, as he himself points out, it involves our believing that the
| |
| lining of the diverticulum derived from the primitive alimentary vesicle
| |
| becomes part of the external skin. This occurrence is so remarkable, that
| |
| more evidence appears to me requisite before accepting it.
| |
| | |
| The formation of the anus occurs late. Its position appears to be the
| |
| same as that of the blastopore, and is indicated by a papilla of the mesenteron attaching itself to the skin on the ventral side (fig. 253, an). It eventually becomes placed in an interradial space within the oral disc of the adult.
| |
| The water-vascular ring has no direct communication with the exterior, but
| |
| the place of the madreporic canal of other types appears to be taken in
| |
| the larva by a single tube leading from the exterior into the body cavity, the
| |
| external opening of which is placed on one of the oral plates (vide p. 571) in
| |
| the next interradial space to the right of the anus, and a corresponding
| |
| diverticulum of the water-vascular ring opening into the body cavity. The
| |
| line of junction between the left and right peritoneal vesicles forms in the
| |
| larva a ring-like mesentery dividing the oral from the aboral part of the body
| |
| | |
| | |
| | |
| 552
| |
| | |
| | |
| | |
| CRINOIDEA.
| |
| | |
| | |
| | |
| cavity. In the adult 1 the oral section of the larval body cavity becomes the
| |
| ventral part of the circumvisceral division of the body cavity, and the
| |
| subtentacular canals of the arms and disc ; while the aboral section becomes
| |
| the dorsal part of the circumvisceral division of the body cavity, the cceliac
| |
| canals of the arms, and the cavity of the centro-dorsal piece. The primitive
| |
| | |
| | |
| | |
| ,+wr
| |
| | |
| | |
| | |
| | |
| FIG. 253. LONGITUDINAL SECTION THROUGH THE CALYX OF AN ADVANCED
| |
| PENTRACRINOID ANTEDON LARVA WITH CLOSED VESTIBULE.
| |
| | |
| (From Carpenter ; after Gotte.)
| |
| | |
| ae. epithelium of oral vestibule; ;//. mouth; al. mesenteron; an. rudiment of
| |
| permanent anus; lp. posterior part of left (oral) peritoneal sack; lp' '. anterior part of
| |
| left (oral) peritoneal sack; wr. water-vascular ring; /. tentacle; mt. mesentery;
| |
| rp. right peritoneal sack; rp '. continuation of right peritoneal sack into the stalk;
| |
| r. roof of tentacular vestibule.
| |
| | |
| distinction between the sections of the larval body cavity becomes to a large
| |
| extent obliterated, while the axial and intervisceral sections of the bodycavity of the adult are late developments.
| |
| | |
| The more important points in the development indicated in
| |
| the preceding pages are as follows :
| |
| | |
| (i) The blastosphere is usually elongated in the direction
| |
| of the axis of invagination, but in Comatula it is elongated
| |
| transversely to this axis.
| |
| | |
| 1 Vide P. H. Carpenter, "On the genus Actinometra." Linnean Trans., and
| |
| Series, Zoology, Vol. n., Part I., 1879.
| |
| | |
| | |
| | |
| ECHINODERMATA. 553
| |
| | |
| | |
| | |
| (2) The blastopore usually becomes the permanent anus,
| |
| but it closes at the end of larval life (there being no anus in the
| |
| adult) in Ophiuroids and some Asteroids, while in Comatula it
| |
| closes very early, and a fresh anus is formed at the point where
| |
| the blastopore was placed.
| |
| | |
| (3) The larval mouth always becomes the mouth of the
| |
| adult.
| |
| | |
| (4) The archenteron always gives rise to outgrowths which
| |
| form the peritoneal membrane and water-vascular systems. In
| |
| Comatula there are three such outgrowths, two paired, which
| |
| form the peritoneal vesicles, and one unpaired, which forms the
| |
| water-vascular vesicle. In Asteroids and Ophiuroids there are
| |
| two outgrowths. In Ophiuroids both of these are divided into a
| |
| peritoneal and a water-vascular vesicle, but the right watervascular vesicle atrophies. In Asteroids only one water-vascular
| |
| vesicle is formed, which is derived from the left peritoneal vesicle.
| |
| In Echinoids and Holothuroids there is a single vaso-peritoneal
| |
| vesicle.
| |
| | |
| (5) The water- vascular vesicle grows round the larval
| |
| oesophagus in Holothuroids, Ophiuroids, and Comatula ; in
| |
| these cases the larval oesophagus is carried on into the adult.
| |
| In other forms the water-vascular vesicle forms a ring which
| |
| does not enclose the cesophagus (Asteroids and Echinoids);
| |
| in such cases a new oesophagus is formed, which perforates this
| |
| ring.
| |
| | |
| Development of the larval appendages and metamorphosis.
| |
| | |
| Holothuroidea. The young larva of Synapta, to which J.
| |
| Muller gave the name Auricularia (fig. 255), is in many respects
| |
| the simplest form of Echinoderm larva. With a few exceptions
| |
| the Auricularia type of larva is common to the Holothuria.
| |
| | |
| It is (fig. 254 A and fig. 255) bilaterally symmetrical, presenting a flattened ventral surface, and a convex dorsal one.
| |
| The anus (an) is situated nearly at the hinder pole, and the
| |
| mouth (m) about the middle of the ventral surface. In front
| |
| of the mouth is a considerable process, the prae-oral lobe.
| |
| Between the mouth and anus is a space, more or less concave
| |
| according to the age of the embryo, interrupted by a ciliated
| |
| | |
| | |
| | |
| 554
| |
| | |
| | |
| | |
| AURICULARIA.
| |
| | |
| | |
| | |
| A similar ciliated ridge is
| |
| A E
| |
| | |
| | |
| | |
| ridge a little in front of the anus,
| |
| present on the ventral surface
| |
| of the prae-oral lobe immediately in front of the mouth.
| |
| The anal and oral ridges are
| |
| connected by two lateral ciliated bands, the whole forming
| |
| a continuous band, which,
| |
| since the mouth lies in the
| |
| centre of it (fig. 255), may be
| |
| regarded as a ring completely
| |
| surrounding the body behind
| |
| the mouth, or more naturally
| |
| as a longitudinal ring.
| |
| | |
| The bilateral Auricularia
| |
| is developed from a slightly
| |
| elongated gastrula with an uniform covering of cilia. The
| |
| gastrula becomes flattened on the oral side. At the same time
| |
| the cilia become specially developed on the oral and anal ridges,
| |
| and then on the remainder of the ciliated ring, while they are
| |
| | |
| | |
| | |
| | |
| FIG. 254. A. THE LARVA OF A HOLOTHUROID. B. THE LARVA OF AN ASTEROID.
| |
| | |
| ;//. mouth; st. stomach; a. anus; l.c>
| |
| primitive longitudinal ciliated band; pr.c.
| |
| prae-oral ciliated band.
| |
| | |
| | |
| | |
| | |
| FIG. 155. DIAGRAMMATIC FIGURES REPRESENTING THE EVOLUTION OF AN
| |
| AURICULARIA FROM THE SIMPLEST ECHINODERM LARVAL FORM. (Copied from
| |
| MUller.)
| |
| | |
| The black line represents the ciliated ridge. The shaded part is the oral side of
| |
| the ring, the clear part the aboral side.
| |
| | |
| /;;. mouth; an. anus.
| |
| | |
| simultaneously obliterated elsewhere ; and so a complete Auricularia is developed. The water-vascular ring in the fully-developed
| |
| larva has already considerably advanced in the growth round the
| |
| oesophagus (fig. 256 w.v.r).
| |
| | |
| Most Holothurian larvae, in their transformation from the
| |
| bilateral Auricularia form to the radial form of the adult, pass
| |
| through a stage in which the cilia form a number of transverse
| |
| | |
| | |
| | |
| ECHINODERMATA.
| |
| | |
| | |
| | |
| 555
| |
| | |
| | |
| | |
| | |
| -2>.v
| |
| | |
| | |
| | |
| rings, usually five in number, surrounding the body. The
| |
| stages in this metamorphosis are shewn in figs. 256, 257, and
| |
| 258.
| |
| | |
| The primitive ciliated band,
| |
| at a certain stage of the metamorphosis, breaks up into a
| |
| number of separate portions
| |
| (fig. 256), the whole of which are
| |
| placed on the ventral surface.
| |
| Four of these (fig. 257 A and B)
| |
| arrange themselves in the form
| |
| of an angular ring round the
| |
| mouth, which at this period projects considerably. The remaining portions of the primitive
| |
| band change their direction from
| |
| a longitudinal one to a transverse (fig. 257 B), and eventually
| |
| grow into complete rings (fig.
| |
| 2570). Of these there are five.
| |
| The middle one (257 B) is the
| |
| first to develop, and is formed
| |
| from the dorsal parts of the
| |
| primitive ring. The two hinder
| |
| rings develop next, and last of
| |
| all the two anterior ones, one of
| |
| which appears to be in front of the mouth (fig. 257 C).
| |
| | |
| The later development of the mouth, and of the ciliated ridge
| |
| surrounding it, is involved in some obscurity. It appears from
| |
| Metschnikoff (No. 560) that an invagination of the oesophagus
| |
| takes place, carrying with it the ciliated ridge around the mouth.
| |
| This ridge becomes eventually converted into the covering for
| |
| the five tentacular outgrowths of the water- vascular ring (fig.
| |
| 258), and possibly also forms the nervous system.
| |
| | |
| The opening of the cesophageal invagination is at first behind
| |
| the foremost ciliated ring, but eventually comes to lie in front of
| |
| it, and assumes a nearly terminal though slightly ventral position
| |
| (fig. 258). No account has been given of the process by which
| |
| this takes place, but the mouth is stated by Metschnikoff (though
| |
| | |
| | |
| | |
| FIG. 256. FULL-GROWN LARVA OF
| |
| SYNAPTA. (After Metschnikoff.)
| |
| | |
| m. mouth ; st. stomach ; a. anus ;
| |
| p.v. left division of perivisceral cavity,
| |
| which is still connected with the watervascular system ; w.v.r. water-vascular
| |
| ring which has not yet completely encircled the oesophagus; I.e. longitudinal
| |
| part of ciliated band ; pr.c. prae-oral part
| |
| of ciliated band.
| |
| | |
| | |
| | |
| 556
| |
| | |
| | |
| | |
| BIPINNARIA.
| |
| | |
| | |
| | |
| Miiller differs from him on this point) to remain open throughout. The further changes in the metamorphosis are not considerable. The ciliated bands disappear, and a calcareous ring
| |
| of ten pieces, five ambulacral and five interambulacral, is formed
| |
| round the oesophagus. A provisional calcareous skeleton is also
| |
| developed.
| |
| | |
| All the embryonic systems of organs pass in this case
| |
| directly into those of the adult.
| |
| | |
| The metamorphosis of most Holothuroidea is similar to that just
| |
| described. In Cucumaria (Selenka) there is however no Auricularia stage,
| |
| and the uniformly ciliated stage is succeeded by one with five transverse
| |
| | |
| | |
| | |
| | |
| FIG. 257. THREE STAGES IN THE DEVELOPMENT OF SYNAPTA. A and B
| |
| are viewed from the ventral surface, and C from the side. (After Metschnikoff.)
| |
| | |
| m. mouth; oe. oesophagus; pv. walls of the perivisceral cavity; wv. longitudinal
| |
| vessel of the water- vascular system; p. dorsal pore of water-vascular system;
| |
| cr. ciliated ring formed round the mouth from parts of the primitive ciliated
| |
| band.
| |
| | |
| bands of cilia, and a prae-oral and an anal ciliated cap. The mouth is at
| |
| first situated ventrally behind the prse-oral cap of cilia, but the prae-oral
| |
| cap becomes gradually absorbed, and the mouth assumes a terminal
| |
| position.
| |
| | |
| In Psolinus (Kowalevsky) there is no embryonic ciliated stage, and the
| |
| adult condition is attained without even a metamorphosis. There appear to
| |
| | |
| | |
| | |
| ECHINODERMATA.
| |
| | |
| | |
| | |
| 557
| |
| | |
| | |
| | |
| | |
| be five plates surrounding the
| |
| mouth, which are developed before
| |
| any other part of the skeleton, and
| |
| are regarded by P. H. Carpenter
| |
| (No. 548) as equivalent to the five
| |
| oral plates of the Crinoidea. The
| |
| larval condition with ciliated bands
| |
| is often spoken of as the pupa stage,
| |
| and during it the larvae of Holothurians proper use their embryonic
| |
| tube feet to creep about.
| |
| | |
| Asteroidea. The commonest and most thoroughly
| |
| investigated form of Asteroid
| |
| larva is a free swimming form
| |
| known as Bipinnaria.
| |
| | |
| This form in passing from
| |
| the spherical to the bilateral
| |
| condition passes through at
| |
| first almost identical changes
| |
| to the Auricularian larva.
| |
| The cilia become at an early
| |
| period confined to an oral
| |
| and anal ridge.
| |
| | |
| The anal ridge gradually extends dorsalwards, and finally
| |
| forms a complete longitudinal post-oral ring (fig. 259 A) ; the
| |
| oral ridge also extends dorsalwards, and forms a closed prae-oral
| |
| ring (fig. 259 A), the space within which is left unshaded in my
| |
| figure.
| |
| | |
| The presence of two rings instead of one distinguishes the
| |
| Bipinnaria from the Auricularia. The two larvae are shewn side
| |
| by side in fig. 254, and it is obvious that the two bands of the
| |
| Bipinnaria are (as pointed out by Gegenbaur) equivalent to the
| |
| single band of the Auricularia divided into two. Ontologically,
| |
| however, the two bands of Bipinnaria do not appear to arise
| |
| from the division of a single band.
| |
| | |
| As the Bipinnaria grows older, a series of arms grows out
| |
| along lines of the two ciliated bands (fig. 259 C), and, in many
| |
| cases, three special arms are formed, not connected with the
| |
| ciliated bands, and covered with warts. These latter arms are
| |
| | |
| | |
| | |
| FlG. 258. A LATE STAGE IN THE DEVELOPMENT OF SYNAPTA. (After Metschnikoff.)
| |
| | |
| The figure shews the vestibular cavity
| |
| with retracted tentacles ; the ciliated bands ;
| |
| the water-vascular system, etc.
| |
| | |
| p. dorsal pore of water-vascular system ;
| |
| pv. walls of perivisceral cavity; ms. amoeboid cells.
| |
| | |
| | |
| | |
| 558
| |
| | |
| | |
| | |
| BIPINNARIA.
| |
| | |
| | |
| | |
| known as brachiolar arms, and the larvae provided with them
| |
| as Brachiolaria (fig. 259 D).
| |
| | |
| As a rule the following arms can be distinguished (fig. 259 C and D), on
| |
| the hinder ring (Agassiz' nomenclature) a median anal pair, a dorsal anal
| |
| pair, and a ventral anal pair, a dorsal oral pair, and an unpaired anterior
| |
| dorsal arm ; on the prae-oral ring a ventral oral pair, and sometimes (Miiller)
| |
| an unpaired anterior ventral arm.
| |
| | |
| The three brachiolar arms arise as processes from the base of the
| |
| unpaired dorsal arm, and the two ventral oral arms. The extent of the
| |
| development of the arms varies with the species.
| |
| | |
| | |
| | |
| | |
| | |
| FIG. 259. DIAGRAMMATIC REPRESENTATION OF VARIOUS FORMS OF ASTEROID
| |
| LARWE. A, B, C, BIPINNARIA; D, BRACHIOLARIA. (Copied from Muller.)
| |
| The black lines represent the ciliated bands ; and the shading the space between
| |
| the prae-oral and the post-oral bands.
| |
| | |
| m. mouth; an. anus.
| |
| | |
| The changes by which the Bipinnaria or Brachiolaria becomes
| |
| converted into the adult starfish are very much more complicated
| |
| than those which take place in Holothurians. For an accurate
| |
| knowledge of them we are largely indebted to Alex. Agassiz
| |
| (No. 543). The development of the starfish takes place entirely
| |
| at the posterior end of the larva close to the stomach.
| |
| | |
| On the right and dorsal side of the stomach, and externally
| |
| to the rig/it peritoneal space, are formed five radially situated
| |
| calcareous rods arranged in the form of a somewhat irregular
| |
| pentagon. The surface on which they are deposited has a
| |
| spiral form, and constitutes together with its calcareous rods, the
| |
| | |
| | |
| | |
| ECHINODERMATA. 559
| |
| | |
| | |
| | |
| abactinal or dorsal surface of the future starfish. Close to its
| |
| dorsal, i.e. embryonic dorsal, edge lies the dorsal pore of the
| |
| water-vascular system (madreporic canal), and close to its ventral
| |
| edge the anus. On the left and ventral side of the stomach is
| |
| placed the water-vascular rosette, the development of which was
| |
| described on p. 549. It is situated on the actinal or ventral surface
| |
| of the future starfish, and is related to the left peritoneal vesicle.
| |
| | |
| Metschnikoff (No. 560) and Agassiz (No. 543) differ slightly as to the
| |
| constitution of the water- vascular rosette. The former describes and figures
| |
| it as a completely closed rosette, the latter states that ' it does not form a
| |
| completely closed curve but is always open, forming a sort of twisted
| |
| crescent-shaped arc.'
| |
| | |
| The water-vascular rosette is provided with five lobes, corresponding to which are folds in the larval skin, and each lobe
| |
| corresponds to one of the calcareous plates developed on the
| |
| abactinal disc. The plane of the actinal surface at first meets
| |
| that of the abactinal at an acute or nearly right angle. The two
| |
| surfaces are separated by the whole width of the stomach. The
| |
| general appearance of the larva from the ventral surface after
| |
| the development of the water-vascular rosette (i) and abactinal
| |
| disc (A) is shewn in fig. 260.
| |
| | |
| As development proceeds the abactinal surface becomes a
| |
| firm and definite disc, owing to the growth of the original
| |
| calcareous spicules into more or less definite plates, and to the
| |
| development of five fresh plates nearer the centre of the disc and
| |
| interradial in position. Still later a central calcareous plate
| |
| appears on the abactinal surface, which is thus formed of a
| |
| central plate, surrounded by a ring of five interradial plates, and
| |
| then again by a ring of five radial plates. The abactinal disc
| |
| now also grows out into five short processes, separated by five
| |
| shallow notches. These processes are the rudiments of the five
| |
| arms, and each of them corresponds to one of the lobes of the
| |
| water-vascular rosette. A calcareous deposit is formed round
| |
| the opening of the water-vascular canal, which becomes the
| |
| madreporic tubercle 1 . At about this stage the absorption of the
| |
| larval appendages takes place. The whole anterior part of the
| |
| | |
| 1 The exact position of the madreporic tubercle in relation to the abactinal plates
| |
| does not seem to have been made out. It might have been anticipated that it would
| |
| be placed in one of the primary interradial plates, but this does not seem to be the
| |
| case. The position of the anus is also obscure.
| |
| | |
| | |
| | |
| 5 6o
| |
| | |
| | |
| | |
| BIPINNARIA.
| |
| | |
| | |
| | |
| | |
| larva with the great prae-oral lobe has hitherto remained
| |
| unchanged, but now it contracts and undergoes absorption, and
| |
| becomes completely withdrawn into the disc of the future starfish.
| |
| The larval mouth is transported into
| |
| the centre of the actinal disc. In the
| |
| larvae observed by Agassiz and Metschnikoff nothing was cast off, but the
| |
| whole absorbed.
| |
| | |
| According to M tiller and Koren and
| |
| Danielssen this is not the case in the larva
| |
| observed by them, but part of the larva is
| |
| thrown off, and lives for some time independently.
| |
| | |
| After the absorption of the larval
| |
| appendages the actinal and abactinal
| |
| surfaces of the young starfish approach
| |
| each other, owing to the flattening of
| |
| the stomach ; at the same time they
| |
| lose their spiral form, and become flat
| |
| discs, which fit each other. Each of
| |
| the lobes of the rosette of the watervascular system becomes one of the
| |
| radial water-vascular canals. It first
| |
| becomes five-lobed, each lobe forming
| |
| a rudimentary tube foot, and on each ^ d ctinal disc of youn Aste '
| |
| side of the middle lobe two fresh ones
| |
| | |
| next spring out, and so on in succession. The terminal median
| |
| lobe forms the tentacle at the end of the arm, and the eye is
| |
| developed at its base. The growth of the water-vascular canals
| |
| keeps pace with that of the arms, and the tube feet become
| |
| supported at their base by an ingrowth of calcareous matter.
| |
| The whole of the calcareous skeleton of the larva passes directly
| |
| into that of the adult, and spines are very soon formed on the
| |
| plates of the abactinal surface. The original radial plates,
| |
| together with the spines which they have, are gradually pushed
| |
| outwards with the growth of the arms by the continual addition
| |
| of fresh rows of spines between the terminal plate and the plate
| |
| next to it. It thus comes about that the original radial plates
| |
| persist at the end of the arms, in connection with the unpaired
| |
| | |
| | |
| | |
| FIG. 260. BIPINNARIA
| |
| LARVA OF AN ASTEROID. (From
| |
| Gegenbaur ; after Miiller.)
| |
| | |
| b. mouth ; a. anus ; h. madreporic canal ; t. ambulacral
| |
| rosette ; c . stomach ; d. g. e.
| |
| etc. arms of Bipinnaria ; A.
| |
| | |
| | |
| | |
| ECHINODERMATA. 561
| |
| | |
| | |
| | |
| tentacles which form the apex of the radial water-vascular
| |
| tubes.
| |
| | |
| It has already been mentioned that according to Metschnikoff (No. 560)
| |
| a new oesophagus is formed which perforates the water-vascular ring, and
| |
| connects the original stomach with the original mouth. Agassiz (No. 543)
| |
| maintains that the water-vascular ring grows round the primitive oesophagus.
| |
| He says " During the shrinking of the larva the long oesophagus becomes
| |
| " shortened and contracted, bringing the opening of the mouth of the larva
| |
| " to the level of the opening of the oesophagus, which eventually becomes
| |
| "the true mouth of the starfish." The primitive anus is believed by
| |
| Metschnikoff to disappear, but by Agassiz to remain. This discrepancy
| |
| very possibly depends upon these investigators having worked at different
| |
| species.
| |
| | |
| There is no doubt that the whole of the larval organs, with
| |
| the possible exception of the oesophagus, and anus (where absent
| |
| in the adult), pass directly into the corresponding organs of the
| |
| starfish and that the prae-oral part of the body and arms of the
| |
| larva are absorbed and not cast off.
| |
| | |
| In addition to the Bipinnarian type of Asteroid larva a series of other
| |
| forms has been described by Miiller (No. 561), Sars, Keren, and Danielssen
| |
| (No. 554) and other investigators, which are however very imperfectly
| |
| known. The best-known form is one first of all discovered by Sars in
| |
| Echinaster Sarsii, and the more or less similar larvae subsequently investigated by Agassiz, Busch, Miiller, Wyville Thomson, etc. of another species
| |
| of Echinaster and of Asteracanthion. These larvae on leaving the egg have
| |
| an oval form, and are uniformly covered by cilia. Four processes (or in
| |
| Agassiz' type one process) grow out from the body ; by these the larvae fix
| |
| themselves. In the case of Echinaster the larvae are fixed in the ventral
| |
| concavity of the disc of the mother, between the five arms, where a temporary brood-pouch is established. The main part of the body is converted
| |
| directly into the disc of the young starfish, while the four processes come to
| |
| spring from the ventral surface, and are attached to the water- vascular ring.
| |
| Eventually they atrophy completely. Of the internal structure but little is
| |
| known ; till the permanent mouth is formed, after the development of the
| |
| young starfish is pretty well advanced, the stomach has no communication
| |
| with the exterior.
| |
| | |
| A second abnormal type of development is presented by the embryo of
| |
| Pteraster miliaris, as described by Koren and Danielssen 1 . The larvae to
| |
| the number of eight to twenty develop in a peculiar pouch on the dorsal
| |
| surface of the body. The early stages are not known, but in the later ones
| |
| the whole body assumes a pentagonal appearance with a mouth at one edge
| |
| | |
| 1 The following statements are taken from the abstract in Bronn's Thierreichs.
| |
| B. II. 36
| |
| | |
| | |
| | |
| 5 62
| |
| | |
| | |
| | |
| OPHIUROID PLUTEUS.
| |
| | |
| | |
| | |
| of the disc. At a later stage the anus is formed on the dorsal side of an arm
| |
| opposite the mouth. The stomach is surrounded by a water-vascular ring,
| |
| from which the madreporic canal passes to the dorsal surface, but does not
| |
| open. At a later stage the embryonic mouth and anus vanish, to be replaced
| |
| by a permanent mouth and anus in the normal positions.
| |
| | |
| A third, and in some respects very curious, form is a worm like larva of
| |
| Miiller, which is without bands of cilia. The dorsal surface of the youngest
| |
| larva is divided by transverse constrictions into five segments. On the
| |
| under side of the first of these is a five-lobed disc, each lobe being provided
| |
| with a pair of tube feet.
| |
| | |
| At a later period only three segments are visible on the dorsal surface,
| |
| but the ventral surface has assumed a pentagonal aspect. The later stages
| |
| are not known.
| |
| | |
| Ophiuroidea. The full-grown larva of the Ophiuroids is
| |
| known as a Pluteus. It commences with the usual more or less
| |
| spherical form ; from this it passes to a form closely resembling
| |
| | |
| | |
| | |
| | |
| FIG. 261. DIAGRAMMATIC FIGURES SHEWING THE EVOLUTION OK AN OPHIUROID PLUTEUS FROM A SIMPLE ECHINODERM LARVA. (Copied from Miiller.) The
| |
| calcareous skeleton is not represented.
| |
| | |
| ///. mouth; an. anus; d. anterior arms; d'. lateral arms; e'. posterior arms; tf.
| |
| anterolateral arms.
| |
| | |
| that of Auricularia with a rounded dorsal surface, and a flattened
| |
| ventral one. Soon however it becomes distinguished by the
| |
| growth of a post-anal lobe and the absence of a prae-oral lobe
| |
| (fig. 261 B). The post-anal lobe forms the somewhat rounded
| |
| apex of the body. In front of the mouth, and between the
| |
| mouth and anus, arise the anal and oral ciliated ridges, which
| |
| soon become continued into a single longitudinal ciliated ring.
| |
| At the same time the body becomes prolonged into a series of
| |
| | |
| | |
| | |
| ECHINODERMATA.
| |
| | |
| | |
| | |
| 563
| |
| | |
| | |
| | |
| processes along the ciliated band, which is continued to the
| |
| extremity of each. The primitive ciliated ring never becomes
| |
| broken up into two or more rings. A ciliated crown is usually
| |
| developed at the extremity of the post-anal lobe. The arms are
| |
| arranged in the form of a ring surrounding the mouth, and are
| |
| all directed forwards.
| |
| | |
| The first arms to appear are two lateral ones, which usually remain the
| |
| most conspicuous (fig. 261 B and C, cf\ Next arises a pair on the sides of
| |
| the mouth, which may be called the mouth or anterior arms (C, d}. A pair
| |
| ventral to and behind the lateral arms is then formed, constituting the
| |
| posterior arms (D, e'\ and finally a pair between the lateral arms and the
| |
| anterior, constituting the anterolateral arms (D,^).
| |
| | |
| The concave area between the arms forms the greater part of
| |
| the ventral surface of the body. Even before the appearance of
| |
| any of the arms, and before the formation of the mouth, two
| |
| calcareous rods are formed, which meet behind at the apex of
| |
| the post-anal lobe, and are continued as a central support into
| |
| each of the arms as they are successively formed. These rods
| |
| are shewn at their full development in fig. 262. The important
| |
| points which distinguish a Pluteus
| |
| larva from the Auricularia or
| |
| Bipinnaria are the following :
| |
| | |
| (i) The presence of the postanal lobe at the hind end of the
| |
| body. (2) The slight development of a prae-oral lobe. (3) The
| |
| provisional calcareous skeleton in
| |
| the larval arms.
| |
| | |
| Great variations are presented
| |
| in the development of the arms
| |
| and provisional skeleton. The
| |
| presence of lateral arms is however
| |
| a distinctive characteristic of the
| |
| Ophiuroid Pluteus. The other
| |
| arms may be quite absent, but
| |
| the lateral arms never.
| |
| | |
| The formation of the permanent Ophiuroid takes place in
| |
| much the same way as in the Asteroidea.
| |
| | |
| 36-2
| |
| | |
| | |
| | |
| | |
| FIG. 262.
| |
| OPHIUROID.
| |
| after Miiller.)
| |
| | |
| | |
| | |
| PLUTEUS LARVA OF AN
| |
| (From Gegenbaur ;
| |
| | |
| | |
| | |
| A. rudiment of young Ophiuroid ;
| |
| (?. lateral arms; d. anterior arms;
| |
| e . posterior arms.
| |
| | |
| | |
| | |
| 564
| |
| | |
| | |
| | |
| OPHIUROID PLUTEUS.
| |
| | |
| | |
| | |
| There is formed (fig. 262) on the right and dorsal side of stomach the
| |
| abactinal disc supported by calcareous plates, at first only five in number
| |
| and radial in position 1 . The disc is at first not symmetrical, but becomes so
| |
| at the time of the resorption of the larval arms. It grows out into five
| |
| processes the five future rays. The original five radial plates remain as the
| |
| terminal segments of the adult rays, and new plates are always added
| |
| between the ultimate and penultimate plate (Mu'ller), though it is probable
| |
| that in the later stages fresh plates are added in the disc.
| |
| | |
| The ventral surface of the permanent Ophiuroid is formed by the concave
| |
| surface between the mouth and anus. Between this and the stomach is
| |
| | |
| | |
| | |
| | |
| FIG. 263. DIAGRAMMATIC FIGURES SHEWING THE EVOLUTION OF ECHINOID
| |
| PLUTEI. (Copied from Miiller.) The calcareous skeleton is not represented. E.
| |
| Pluteus of Spatangus.
| |
| | |
| m. mouth; an. anus; d. anterior arms; d' . point where lateral arms arise in the
| |
| Ophiuroid Pluteus; e. anterointernal arms; e. posterior arms; g'. anterolateral arms;
| |
| g. anteroexternal arms.
| |
| | |
| situated the water-vascular ring. It is at first not closed, but is horseshoeshaped, with five blind appendages (fig. 262). It eventually grows round
| |
| the cesophagus, which, together with the larval mouth, is retained in the
| |
| adult. The five blind appendages become themselves lobed in the same
| |
| way as in Asterias, and grow out along the five arms of the disc and become
| |
| the radial canals and tentacles. All these parts of the water-vascular system
| |
| are of course covered by skin, and probably also surrounded by mesoblast
| |
| cells, in which at a later period the calcareous plates which lie ventral to the
| |
| radial canal are formed. The larval anus disappears. As long as the larval
| |
| appendages are not absorbed the ventral and dorsal discs of the permanent
| |
| Ophiuroid fit as little as in the case of the Brachiolaria, but at a certain
| |
| period the appendages are absorbed. The calcareous rods of the larval arms
| |
| | |
| 1 Whether interradial plates are developed as in Asterias is not clear. They seem
| |
| to be found in Ophiopholis bellis, Agassiz, but have not been recognised in other
| |
| forms (vide Carpenter, No. 548, p. 369).
| |
| | |
| | |
| | |
| ECHINODERMATA. 565
| |
| | |
| | |
| | |
| break up, the arms and anal lobe become absorbed, and the dorsal and
| |
| ventral discs, with the intervening stomach and other organs, are alone left.
| |
| After this the discs fit together, and there is thus formed a complete young
| |
| Ophiuroid.
| |
| | |
| The whole of the internal organs of the larva (except the anus), including
| |
| the mouth, cesophagus, the body cavity, etc. are carried on directly into the
| |
| adult.
| |
| | |
| The larval skeleton is, as above stated, absorbed.
| |
| | |
| The viviparous larva of Amphiura squamata does not differ very greatly
| |
| from the larvae with very imperfect arms. It does not develop a distinct
| |
| ciliated band, and the provisional skeleton is very imperfect. The absence
| |
| of these parts, as well as of the anus, mentioned on p. 549, may probably be
| |
| correlated with the viviparous habits of the larva. With reference to the
| |
| passage of this larva into the adult there is practically nothing to add to
| |
| what has just been stated. When the development of the adult is fairly
| |
| advanced the part of the body with the provisional skeleton forms an
| |
| elongated rod-like process attached to the developing disc. It becomes
| |
| eventually absorbed.
| |
| | |
| Echinoidea. The Echinus larva (fig. 263} has a Pluteus
| |
| form like that of the Ophiuroids, and in most points, such as the
| |
| | |
| | |
| | |
| | |
| | |
| FIG. 264. Two LARV/E OF STRONGYLOCENTRUS. (From Agassiz.)
| |
| m. mouth; a. anus; o. cesophagus; d. stomach; c. intestine; '. and v. ciliated
| |
| ridges; iv. water- vascular tube; r. calcareous rods.
| |
| | |
| presence of the anal lobe, the ciliated band, the provisional
| |
| skeleton, etc., develops in the same manner. The chief difference
| |
| between the two Pluteus forms concerns the development of the
| |
| lateral arms. These, which form the most prominent arms in
| |
| the Ophiuroid Pluteus, are entirely absent in the Echinoid
| |
| | |
| | |
| | |
| 5 66
| |
| | |
| | |
| | |
| ECHINOID PLUTEUS.
| |
| | |
| | |
| | |
| Pluteus, which accordingly has, as a rule, a much narrower form
| |
| than the Ophiuroid Pluteus.
| |
| | |
| A pair of ciliated epaulettes on each side of and behind the
| |
| ciliated ring is very characteristic of some Echinoid larvae.
| |
| They are originally developed from the ciliated ring (fig. 266 A
| |
| | |
| | |
| | |
| | |
| | |
| FIG. 265. LATERAL AND VENTRAL VIEW OF A LARVA OF STRONGYLOCENTRUS.
| |
| | |
| (From Agassiz.) General references as in fig. 264.
| |
| | |
| b. dorsal opening of madreporic canal; e '. posterior arms ; e'". anterior arms;
| |
| f lV . anterointernal arms.
| |
| | |
| and B, z>"). The presence of three processes from the anal lobe
| |
| supported by calcareous rods is characteristic of the Spatangoid
| |
| Pluteus (fig. 263 E).
| |
| | |
| The first two pairs of arms to develop, employing the same names as in
| |
| Ophiuroids, are the anterior attached to the oral process (fig. 263 C, d] and
| |
| the posterior pair (*?') A pair of anterolateral arms next becomes developed
| |
| (j^). A fourth pair (not represented in Ophiuroids) appears on the inner
| |
| side of the anterior pair forming an anterointernal pair (e}, and in the
| |
| Spatangoid Pluteus a fifth pair may be added on the external side of the
| |
| anterior pair forming an anteroexternal pair (g).
| |
| | |
| Each of the first-formed paired calcareous rods is composed of three
| |
| processes, two of which extend into the anterior and posterior arms ; and the
| |
| third and strongest passes into the anal lobe, and there meets its fellow
| |
| (fig. 265). A transverse bar in front of the arms joins the rods of the two
| |
| sides meeting them at the point where the three processes diverge. The
| |
| process in the anterolateral arm (fig. 266 B) is at first independent of this
| |
| system of rods, but eventually unites with it. Although our knowledge of
| |
| | |
| | |
| | |
| ECHINODERMATA. 567
| |
| | |
| | |
| | |
| the Pluteus types in the different groups is not sufficient to generalise with
| |
| great confidence, a few points seem to have been fairly determined 1 . The
| |
| Plutei of Strongylocentrus (figs. 266 and 267) and Echinus have eight arms
| |
| and four ciliated epaulettes. The only Cidaris-like form, the Pluteus of
| |
| which is known, is Arbacia : it presents certain peculiarities. The anal lobe
| |
| develops a pair of posterior (auricular) appendages, and the ciliated ring,
| |
| besides growing out into the normal eight appendages, has a pair of short
| |
| blunt anterior and posterior lobes. An extra pair of non-ciliated accessory
| |
| mouth arms appears also to be developed. Ciliated epaulettes are not
| |
| present. So far as is known the Clypeastroid larva is chiefly characterized
| |
| by the round form of the anal lobe. The calcareous rods are latticed. In the
| |
| Pluteus of Spatangoids there are (fig. 263) five pairs of arms around the
| |
| mouth pointing forwards, and three arms developed from the anal lobe
| |
| pointing backwards. One of these is unpaired, and starts from the apex of
| |
| the anal lobe. All the arms have calcareous rods which, in the case of the
| |
| posterior pair, the anterolateral pair, and the unpaired arm of the anal lobe,
| |
| are latticed. Ciliated epaulettes are not developed.
| |
| | |
| Viviparous larvae of Echinoids have been described by Agassiz 2 .
| |
| | |
| The development of the permanent Echinus has been chiefly worked out
| |
| by Agassiz and Metschnikoff.
| |
| | |
| In the Pluteus of Echinus lividus the first indication of the adult arises,
| |
| when three pairs of arms are already developed, as an invagination of the
| |
| skin on the left side, between the posterior and anterolateral arms, the
| |
| bottom of which is placed close to the water-vascular vesicle (fig. 266 B, u/\
| |
| The base of this invagination becomes very thick, and forms the ventral disc
| |
| of the future Echinus. The parts connecting this disc with the external
| |
| skin become however thin, and, on the narrowing of the external aperture of
| |
| invagination and the growth of the thickened disc, constitute a covering for
| |
| the disc, called by Metschnikoff the amnion. The water- vascular vesicle
| |
| adjoining this disc grows out into five processes, forming as many tube feet,
| |
| which cause the surface of the involuted disc to be produced into the same
| |
| number of processes. The external opening of the invagination of the disc
| |
| never closes, and after the development of the tube feet begins to widen
| |
| again, and the amnion to atrophy. Through the opening of the invagination
| |
| the tube feet now project. The dorsal and right surface of the Pluteus,
| |
| which extends so as to embrace the opening of the madreporic canal and
| |
| the anus, forms the abactinal or dorsal surface of the future Echinus
| |
| (fig. 267, a). This disc fits on to the actinal invaginated surface which arises
| |
| on the left side of the Pluteus. On the right surface of the larva (dorsal of
| |
| permanent Echinus) two pedicellariae appear, and at a later period spines
| |
| are formed, which are at first arranged in a ring-like form round the edge of
| |
| the primitively flat test. While these changes are taking place, and the two
| |
| surfaces of the future Echinus are gradually moulding themselves so as to
| |
| | |
| 1 Vide especially Muller, Agassiz, and Metschnikoff.
| |
| | |
| 2 For viviparous Echini vide Agassiz, Proc. Amer. Acad. 1876.
| |
| | |
| | |
| | |
| 5 68
| |
| | |
| | |
| | |
| ECHINOID PLUTEUS.
| |
| | |
| | |
| | |
| form what is obviously a young Echinus, the arms of the Pluteus with their
| |
| contained skeleton have been gradually undergoing atrophy. They become
| |
| irregular in form, their contained skeleton breaks up into small pieces, and
| |
| they are gradually absorbed.
| |
| | |
| The water-vascular ring is from the first complete, so that, as in
| |
| Asterias, it is perforated through the centre by a new oesophagus. According
| |
| | |
| | |
| | |
| | |
| | |
| FIG. 266. SIDE AND DORSAL VIEW OF A LARVA OF STRONGYLOCENTRUS.
| |
| | |
| (From Agassiz.) General reference letters as in figs. 264 and 265.
| |
| e" . anterolateral arms; v" '. ciliated epaulettes; ?&'. invagination to form the disc
| |
| of Echinus.
| |
| | |
| to Agassiz the first five tentacles or tube feet grow into the radial canals,
| |
| and form the odd terminal tentacles exactly as in Asterias 1 . Spatangus
| |
| only differs in development from Echinus in the fact that the opening of the
| |
| invagination to form the ventral disc becomes completely closed, and that
| |
| the tube feet have eventually to force their way through the larval epidermis
| |
| of the amnion, which is ruptured in the process and eventually thrown
| |
| off.
| |
| | |
| Crinoidea. The larva of Antedon, while still within the
| |
| egg-shell, assumes an oval form and uniform ciliation. Before it
| |
| | |
| 1 Gotte (No. 549) supported by Muller's and Krohn's older, and in some points
| |
| extremely erroneous observations, has enunciated the view that the radial canals in
| |
| Echinoids and Holothuroids have a different nature from those in Asteroids and
| |
| Ophiuroids.
| |
| | |
| | |
| | |
| ECHINODERMATA.
| |
| | |
| | |
| | |
| 569
| |
| | |
| | |
| | |
| becomes hatched the uniform layer of cilia is replaced by four
| |
| transverse bands of cilia, and a tuft of cilia at the posterior
| |
| extremity. In this condition it escapes from the egg-shell
| |
| | |
| | |
| | |
| | |
| FIG. 267. FULL-GROWN LARVA OF STRONGYLOCENTRUS. (From Agassiz.)
| |
| The figure shews the largely-developed abactinal disc of the young Echinus
| |
| enclosing the larval stomach. Reference letters as in previous figs.
| |
| | |
| (fig. 268 A), and becomes bilateral, owing to a flattening of the
| |
| ventral surface. On the flattened surface appears a ciliated
| |
| | |
| | |
| | |
| 570
| |
| | |
| | |
| | |
| CRINOID LARVA.
| |
| | |
| | |
| | |
| depression corresponding in position with the now closed blastopore (vide p. 550). The third ciliated band bends forward
| |
| to pass in front of this (fig. 269). Behind the last ciliated band
| |
| there is present a small depression of unknown function, also
| |
| | |
| | |
| | |
| | |
| FIG. 768. THREB STAGES IN THE DEVELOPMENT OF ANTEDON (COMATULA.)
| |
| | |
| (From Lubbock; after Thomson.)
| |
| | |
| A. larva just hatched; B. larva with rudiment of the calcareous plates; C. Pentacrinoid larva.
| |
| | |
| | |
| | |
| ECHINODERMATA.
| |
| | |
| | |
| | |
| 571
| |
| | |
| | |
| | |
| situated on the ventral surface. The posterior extremity of the
| |
| embryo elongates to form the rudiment of the future stem, and
| |
| a fresh depression, marking the position of the future mouth,
| |
| makes its appearance on the anterior and ventral part.
| |
| | |
| While the ciliated bands are still at their full development,
| |
| the calcareous skeleton of the future calyx makes its appearance
| |
| in the form of two rows, each of five plates, formed of a network
| |
| of spicula (figs. 268 B and 269). The plates of the anterior ring
| |
| are known as the orals, those of the posterior as the basals.
| |
| The former surround the left, i.e. anterior
| |
| peritoneal sack ; the latter the right, i.e.
| |
| posterior peritoneal sack. The two rows
| |
| of plates are at first not quite transverse,
| |
| but form two oblique circles, the dorsal
| |
| end being in advance of the ventral.
| |
| The rows soon become transverse, while
| |
| the originally somewhat ventral oral
| |
| surface is carried into the centre of the
| |
| area enclosed by the oral plates.
| |
| | |
| By the change in position of the
| |
| original ventral surface relatively to the
| |
| axis of the body, the bilateral symmetry
| |
| of the larva passes into a radial symmetry. While the first skeletal elements
| |
| of the calyx are being formed, the
| |
| skeleton of the stem is also established.
| |
| The terminal plate is first of all established, then the joints, eight at first, of
| |
| the stem. The centro-dorsal plate is
| |
| stated by Thomson to be formed as the
| |
| uppermost joint of the stem 1 . The larva, after the completion
| |
| of the above changes, is shewn in fig. 268 B, and somewhat more
| |
| diagrammatically in fig. 269.
| |
| | |
| After the above elements of the skeleton have become established the ciliated bands undergo atrophy, and shortly after
| |
| 1 Gotte (No. 549) on the other hand holds that the centro-dorsal plate is developed
| |
| by the coalescence of a series of at first independent rods, which originate simultaneously with, and close to, the lower edges of the basals, and that it is therefore
| |
| similar in its origin to the basals.
| |
| | |
| | |
| | |
| | |
| FIG. 269. LARVA OF
| |
| ANTEDON WITH RUDIMENTS
| |
| OF CALCAREOUS SKELETON.
| |
| (From Carpenter; after
| |
| Thomson.)
| |
| | |
| i. Terminal plate at the
| |
| end of the stem ; 3. basals ;
| |
| or. orals ; bl. position of blastopore.
| |
| | |
| | |
| | |
| 572
| |
| | |
| | |
| | |
| CRINOID LARVA.
| |
| | |
| | |
| | |
| wards the larva becomes attached by the terminal plate of its
| |
| stem. It then passes into the Pentacrinoid stage! The larva in
| |
| this stage is shewn in fig. 268 C and fig. 270. New joints are
| |
| added at the upper end of the stem next the calyx, and a new
| |
| element the radials makes its appearance as a ring of five
| |
| small plates, placed in the space between the basals and orals,
| |
| and in the intervals alternating with them
| |
| (fig. 270, 4). The roof of the oral vestibule (vide fig. 253 and p. 551) has in
| |
| the meantime become ruptured ; and
| |
| the external opening of the mouth thus
| |
| becomes established. Surrounding the
| |
| mouth are five petal-like lobes, each of
| |
| them supported by an oral plate (fig.
| |
| 268 C). In the intervals between them
| |
| five branched and highly contractile tentacles, which were previously enclosed
| |
| within the vestibule, now sprout out :
| |
| they mark the position of the future
| |
| radial canals, and are outgrowths of the
| |
| water-vascular ring. At the base of each
| |
| of them a pair of additional tentacles is
| |
| soon formed. Each primary tentacle corresponds to one of the radials. These
| |
| latter are therefore, as their name implies,
| |
| radial in position; while the basals and
| |
| orals are interradial. In addition to the
| |
| contractile radial tentacles ten non-contractile tentacles, also diverticula of the
| |
| water- vascular ring, are soon formed, two
| |
| for each interradius.
| |
| | |
| In the course of the further development the equatorial space between the FlG - 2 7<>. YOUNG PEN
| |
| . TACRINOID LARVA OF AN
| |
| | |
| orals and the basals enlarges, and gives TEDON. (From Carpenter ;
| |
| rise to a wide oral disc, the sides of which after w >' ville Thom s"-)
| |
| | |
| - , , . ... . i. terminal plate of stem;
| |
| | |
| are formed by the radials resting on the c d. centro-donal plate; 3 .
| |
| basals; while in the centre of it are bftsals J 4- radials; or. orals.
| |
| placed the five orals, each with its special lobe.
| |
| | |
| The anus, which is formed on the ventral side in the position
| |
| | |
| | |
| | |
| | |
| ECHINODERMATA. 573
| |
| | |
| | |
| | |
| of the blastopore (p. 551), becomes surrounded by an anal plate,
| |
| which is interradial in position, and lies on the surface of the
| |
| oral disc between the orals and radials. On the oral plate in
| |
| the next interradius is placed the opening of a single funnel
| |
| leading into the body cavity, which Ludwig regards as equivalent to the opening of the madreporic canal (vide p. 55 1) 1 .
| |
| | |
| From the edge of the vestibule the arms grow out, carrying
| |
| with them the tentacular prolongation of the water-vascular ring.
| |
| Two additional rows of radials are soon added.
| |
| | |
| The stalked Pentacrinoid larva becomes converted, on the
| |
| absorption of the stalk, into the adult Antedon. The stalk is
| |
| functionally replaced by a number of short cirri springing from
| |
| the centro-dorsal plate. The five basals coalesce into a single
| |
| plate, known as the rosette, and the five orals disappear, though
| |
| the lobes on which they were placed persist. In some stalked
| |
| forms, e.g. Rhizocrinus Hyocrinus, the orals are permanently
| |
| retained. The arms bifurcate at the end of the third radial, and
| |
| the first radial becomes in Antedon rosacea (though not in all
| |
| species of Antedon) concealed from the surface by the growth of
| |
| the centro-dorsal plate. An immense number of funnels, leading
| |
| into the body cavity, are formed in addition to the single one
| |
| present in the young larva. These are regarded by Ludwig as
| |
| equivalent to so many openings of the madreporic canal ; and
| |
| there are developed, in correspondence with them, diverticula of
| |
| the water-vascular ring.
| |
| | |
| Comparison of Echinoderm Larvce and General Conclusions.
| |
| | |
| In any comparison of the various types of Echinoderm larvae
| |
| it is necessary to distinguish between the free-swimming forms,
| |
| and the viviparous or fixed forms. A very superficial examination suffices to shew that the free-swimming forms agree very
| |
| much more closely amongst themselves than the viviparous
| |
| | |
| 1 I have made no attempt to discuss the homologies of the plates of the larval
| |
| Echinodermata because the criteria for such a discussion are still in dispute. The
| |
| suggestive memoirs of P. H. Carpenter (No. 548) on this subject may be consulted by
| |
| the reader. Carpenter attempts to found his homologies on the relation of the plates
| |
| to the primitive peritoneal vesicles, and I am inclined to believe that this method of
| |
| dealing with these homologies is the right one. Ludwig (No. 559) by regarding the
| |
| opening of the madreporic canal as a fixed point has arrived at very different results.
| |
| | |
| | |
| | |
| 574
| |
| | |
| | |
| | |
| COMPARISON OF ECHINODERM LARV.-E.
| |
| | |
| | |
| | |
| forms. We are therefore justified in concluding that in the
| |
| viviparous forms the development is abbreviated and modified.
| |
| | |
| All the free forms are nearly alike in their earliest stage after
| |
| the formation of the archenteron. The surface between the
| |
| anus and the future mouth becomes flattened, and (except in
| |
| Antedon, Cucumaria, Psolinus, etc. which practically have an
| |
| abbreviated development like that of the viviparous forms) a
| |
| ridge of cilia becomes established in front of the mouth, and a
| |
| second ridge between the mouth and the anus. This larval
| |
| form, which is shewn in fig. 264 A, is the type from which the
| |
| various forms of Echinoderm larvae start.
| |
| | |
| In all cases, except in Bipinnaria, the two ciliated ridges
| |
| soon become united, and constitute a single longitudinal postoral ciliated ring.
| |
| | |
| The larvae in their further growth undergo various changes,
| |
| and in the later stages they may be divided into two groups :
| |
| | |
| (1) The Pluteus larva of Echinoids and Ophiuroids.
| |
| | |
| (2) The Auricularia (Holothuroids) and Bipinnaria (Asteroids) type.
| |
| | |
| The first group is characterized by the growth of a number
| |
| of arms more or less surrounding the mouth, and supported
| |
| by calcareous rods. The ciliated band retains its primitive
| |
| condition as a simple longitudinal band throughout larval life.
| |
| There is a very small prae-oral lobe, while an anal lobe is very
| |
| largely developed.
| |
| | |
| The Auricularia and Bi- A. B
| |
| | |
| pinnaria resemble each other
| |
| in shape, in the development
| |
| of a large prae-oral lobe, and
| |
| in the absence of provisional
| |
| calcareous rods ; but differ in
| |
| the fact that the ciliated band
| |
| is single in Auricularia (fig
| |
| 271 A), and is double in Bipinnaria (fig. 271 B).
| |
| | |
| TheBipinnarialarvashews
| |
| | |
| THUROID. B. THE LARVA OF AN ASTEa great tendency to develop RIAS.
| |
| | |
| soft arms; while in the Auri- . ' mouth; st. stomach; a. anus; I.e.
| |
| , . ,_, , *_ 1-1- primitive longitudinal ciliated band; pr.c.
| |
| | |
| cularia the longitudinal ciliat- p r3 e-oral ciliated band.
| |
| | |
| | |
| | |
| | |
| FlG
| |
| | |
| | |
| THE LARVA OF A
| |
| | |
| | |
| | |
| ECHINODERMATA. 575
| |
| | |
| | |
| | |
| ed band breaks up into a number of transverse ciliated bands.
| |
| This condition is in .some instances reached directly, and such
| |
| larvae undoubtedly approximate to the larvae of Antedon, in
| |
| which the uniformly ciliated condition is succeeded by one with
| |
| four transverse bands, of which one is prae-oral.
| |
| | |
| All or nearly all Echinoderm larvae are bilaterally symmetrical,
| |
| and since all Echinodermata eventually attain a radial symmetry, a change necessarily takes place from the bilateral to the
| |
| radial type.
| |
| | |
| In the case of the Holothurians and Antedon, and generally
| |
| in the viviparous types, this change is more or less completely
| |
| effected in the embryonic condition ; but in the Bipinnaria and
| |
| Pluteus types a radial symmetry does not become apparent till
| |
| after the absorption of the larval appendages. It is a remarkable fact, which seems to hold for the Asteroids, Ophiuroids, Echinoids, and Crinoids, that the dorsal side of the larva is
| |
| not directly converted into the dorsal disc of the adult; but
| |
| the dorsal and right side becomes the adult dorsal or abactinal
| |
| surface, while the ventral and left becomes the actinal or ventral
| |
| surface.
| |
| | |
| It is interesting to note with reference to the larvae of the
| |
| Echinodermata that the various existing types of larvae must
| |
| have been formed after the differentiation of the existing groups
| |
| of the Echinodermata ; otherwise it would be necessary to adopt
| |
| the impossible position that the different groups of Echinodermata were severally descended from the different types of larvae.
| |
| The various special appendages, etc. of the different larvae have
| |
| therefore a purely secondary significance; and their atrophy
| |
| at the time of the passage of the larva into the adult, which
| |
| is nothing else but a complicated metamorphosis, is easily explained.
| |
| | |
| Originally, no doubt, the transition from the larva to the
| |
| adult was very simple, as it is at present in most Holothurians ;
| |
| but as the larvae developed various provisional appendages, it
| |
| became necessary that these should be absorbed in the passage
| |
| to the adult state.
| |
| | |
| It would obviously be advantageous that their absorption
| |
| should be as rapid as possible, since the larva in a state of
| |
| transition to the adult would be in a very disadvantageous
| |
| | |
| | |
| | |
| 576 COMPARISON OF ECHINODERM
| |
| | |
| position. The rapid metamorphosis, which we find in Asteroids,
| |
| Ophiuroids, and Echinoids in the passage from the larval to the
| |
| adult state, has no doubt arisen for this reason.
| |
| | |
| In spite of the varying provisional appendages possessed by
| |
| Echinoderm larvae it is possible, as stated above (p. 574), to
| |
| recognise a type of larva, of which all the existing Echinoderm
| |
| larval forms are modifications. This type does not appear to
| |
| me to be closely related to that of the larvae of any group
| |
| described in the preceding pages. It has no doubt certain
| |
| resemblances to the trochosphere larva of Chaetopoda, Mollusca,
| |
| etc., but the differences between the two types are more striking
| |
| than the resemblances. It firstly differs from the trochosphere
| |
| larva in the character of the ciliation. Both larvae start from the
| |
| uniformly ciliated condition, but while the prae-oral ring is almost
| |
| invariable, and a peri-anal ring very common in the trochosphere;
| |
| in the Echinoderm larva such rings are rarely found ; and even
| |
| when present, i.e. the prae-oral ring of Bipinnaria and the terminal
| |
| though hardly peri-anal patch of Antedon, do not resemble
| |
| closely the more or less similar structures of the trochosphere.
| |
| The two ciliated ridges (fig. 264 A) common to all the Echinoderm larvae, and subsequently continued into a longitudinal ring,
| |
| have not yet been found in any trochosphere. The transverse
| |
| ciliated rings of the Holothurian and Crinoid larvae are of no
| |
| importance in the comparison between the trochosphere larvae
| |
| and the larvae of Echinodermata, since such rings are frequently
| |
| secondarily developed. Cf. Pneumodermon and Dentalium amongst Mollusca.
| |
| | |
| In the character of the prae-oral lobe the two types again
| |
| differ. Though the prae-oral lobe is often found in Echinoderm
| |
| larvae it is never the seat of an important (supra-oesophageal)
| |
| ganglion and organs of special sense, as it invariably is in the
| |
| trochosphere.
| |
| | |
| Nothing like the vaso-peritoneal vesicles of the Echinoderm
| |
| larvae has been found in the trochosphere ; nor have the characteristic trochosphere excretory organs been found in the Echinoderm larvae.
| |
| | |
| The larva which most nearly approaches those of the Echinodermata is the larva of Balanoglossus described in the next
| |
| chapter.
| |
| | |
| | |
| | |
| ECHINODERMATA. 577
| |
| | |
| | |
| | |
| BIBLIOGRAPHY.
| |
| | |
| (542) Alex. Agassiz. Revision of the Echini. Cambridge, U.S. 1872 74.
| |
| | |
| (543) Alex. Agassiz. " North American Starfishes." Memoirs of the Museum
| |
| of Comparative Anatomy and Zoology at Harvard College, Vol. v., No. i. 1877
| |
| (originally published in 1864).
| |
| | |
| (544) J. Barrois. " Embryogenie de 1'Asteriscus verruculatus " Journal dc
| |
| VAnat. et Phys. 1879.
| |
| | |
| (545) A. Baur. Beitrdge zur Naturgeschichte d. Synapta digitata. Dresden,
| |
| 1864.
| |
| | |
| (546) H. G. Bronn. Klassen u. Ordnungen etc. Strahlenthiere, Vol. II. 1860.
| |
| | |
| (547) W. B. Carpenter. "Researches on the structure, physiology and development of Antedon." Phil. Trans. CLVI. 1866, and Proceedings of the Roy. Soc.,
| |
| No. 166. 1876.
| |
| | |
| (548) P. H. Carpenter. " On the oral and apical systems of the Echinoderms."
| |
| Quart. J. of Micr. Science, Vol. xvm. and xix. 18789.
| |
| | |
| (549) A. Gotte. " Vergleichende Entwicklungsgeschichte d. Comatula mediterranea." Arch, fur micr. Anat., Vol. xn. 1876.
| |
| | |
| (550) R. Greeff. "Ueber die Entwicklung des Asteracanthion rubens vom Ei
| |
| bis zur Bipinnaria u. Brachiolaria." Schriften d. Gesellschaft zur Beforderung d. gesammten Natunvissenschaften zu Marburg, Bd. xn. 1876.
| |
| | |
| (551) R. Greeff. "Ueber den Bau u. die Entwicklung d. Echinodermen." Sitz.
| |
| d. Gesell. z. Beforderung d. gesam. Naturwiss. zu Marburg, No. 4. 1879.
| |
| | |
| (552) T. H. Huxley. "Report upon the researches of Miiller into the anat.
| |
| anddevel. of the Echinoderms." Ann. and Mag. of Nat. Hist., 2nd Ser., Vol. vin.
| |
| 1851.
| |
| | |
| (553) Koren and Danielssen. "Observations sur la Bipinnaria asterigera.
| |
| Ann. Scien. Nat., Ser. in., Vol. vii. 1847.
| |
| | |
| (554) Koren and Danielssen. "Observations on the development of the Starfishes." Ann. and Mag. of Nat. Hist., Vol. XX. 1857.
| |
| | |
| (555) A. Kowalevsky. " Entwicklungsgeschichte d. Holothurien. " Mhn.Ac.
| |
| Petersbourg, Ser. VII., Tom. XL, No. 6.
| |
| | |
| (556) A. Krohn. "Beobacht. a. d. Entwick. d. Holothurien u. Seeigel."
| |
| Miillers Archiv, 1851.
| |
| | |
| (557) A. Krohn. "Ueb. d. Entwick. d. Seesterne u. Holothurien." Miillcr's
| |
| Archiv, 1853.
| |
| | |
| (558) A. Krohn. "Beobacht. lib. Echinodermenlarven." Mailer's Archiv,
| |
| 1854.
| |
| | |
| (559) H. Ludwig. "Ueb. d. primar. Steinkanal d. Crinoideen, nebst vergl.
| |
| anat. Bemerk. lib. d. Echinodermen." Zeit.f. wiss. ZooL, Vol. xxxiv. 1880.
| |
| | |
| (560) E. Metschnikoff. "Studien iib. d. Entwick. d. Echinodermen u.
| |
| Nemertinen." Mem. Ac. Petersboiirg, Series vii., Tom. xiv., No. 8. 1869.
| |
| | |
| (561) 1 Joh. Miiller. "Ueb. d. Larven u. d. Metamorphosed. Echinodermen."
| |
| Abhandlungen d. Berlin. Akad. (Five Memoirs), 1848, 49, 50, 52 (two Memoirs).
| |
| | |
| (562) Joh. Mtiller. "Allgemeiner Plan d. Entwicklung d. Echinodermen."
| |
| Abhandl. d. Berlin. Akad., 1853.
| |
| | |
| 1 The dates in this reference are the dates of publication.
| |
| B. II. 37
| |
| | |
| | |
| | |
| 578 BIBLIOGRAPHY.
| |
| | |
| | |
| | |
| (563) E. Selenka. "Zur Entwicklung d. Holothurien." Zeit. f. wiss. Zool.,
| |
| Bd. xxvii. 1876.
| |
| | |
| (564) E. Selenka. "Keimblatter u. Organanlage bei Echiniden." Zeit.f.-wiss.
| |
| Zool., Vol. xxxin. 1879.
| |
| | |
| (565) Sir Wyville Thomson. "On the Embryology of the Echinodermata."
| |
| Natural History Review, 1 864.
| |
| | |
| (566) Sir Wyville Thomson. "On the Embryogeny of Antedon rosaceus."
| |
| Phil. Trans. 1865.
| |
| | |
| | |
| | |
| CHAPTER XXI.
| |
| | |
| | |
| | |
| ENTEROPNEUSTA.
| |
| | |
| | |
| | |
| THE larva of Balanoglossus is known as Tornaria. The prselarval development is not known, and the youngest stage (fig.
| |
| 272) so far described (Gotte, No. 569) has
| |
| many remarkable points of resemblance to
| |
| a young Bipinnaria.
| |
| | |
| A mouth (m\ situated on the ventral
| |
| surface, leads into an alimentary canal with
| |
| a terminal anus (an). A prae-oral lobe is
| |
| well developed, as in Bipinnaria, but there
| |
| is no post-anal lobe. The bands of cilia
| |
| have the same general form as in Bipinnaria. There is a prae-oral band, and a
| |
| longitudinal post-oral band ; and the two
| |
| bands nearly meet at the apex of the praeoral lobe (fig. 273). A contractile band
| |
| | |
| | |
| | |
| | |
| an
| |
| | |
| FIG. 272. EARLY
| |
| STAGE IN THE DEVELOPMENT OF TORNARIA.
| |
| (After Gotte.)
| |
| | |
| W. so-called watervascular vesicle developing as an outgrowth
| |
| of the mesenteron; m.
| |
| | |
| passes from the oesophagus to the apex of mouth; an. anus,
| |
| the prae-oral lobe, and a diverticulum (fig. 272, W) from the
| |
| alimentary tract, directed towards the dorsal surface, is present.
| |
| Contractile cells are scattered in the space between the body
| |
| wall and the gut.
| |
| | |
| In the following stage (fig. 274 A) a conspicuous transverse
| |
| post-oral band of a single row of long cilia is formed, and the
| |
| original bands become more sinuous. The alimentary diverticulum of the last stage becomes an independent vesicle opening
| |
| by a pore on the dorsal surface (fig. 274 A, w). The contractile
| |
| cord is now inserted on this vesicle. Where this cord joins the
| |
| apex of the prae-oral lobe between the two anterior bands of
| |
| cilia a thickening of the epiblast (? a ganglion) has become
| |
| | |
| 372
| |
| | |
| | |
| | |
| 580
| |
| | |
| | |
| | |
| ENTEROPNEUSTA.
| |
| | |
| | |
| | |
| | |
| C.C.
| |
| | |
| | |
| | |
| an.
| |
| | |
| FIG. 273. YOUNG TORNARIA.
| |
| | |
| (After Miiller.)
| |
| | |
| m. mouth ; an. anus ; w. watervascular vesicle ; oc. eye-spots ; c.c.
| |
| contractile cord.
| |
| | |
| | |
| | |
| established, and on it are placed
| |
| two eye-spots (fig. 273 oc, and
| |
| fig. 274 A). A deep bay is
| |
| formed on the ventral surface of
| |
| the larva.
| |
| | |
| As the larva grows older the
| |
| original bands of cilia become
| |
| more sinuous, and a second
| |
| transverse band with small cilia
| |
| is formed (in the Mediterranean
| |
| larva) between the previous
| |
| transverse band and the anus.
| |
| The water-vascular vesicle is
| |
| prolonged into two spurs, one
| |
| on each side of the stomach.
| |
| A pulsating vesicle or heart is
| |
| also formed (fig. 274 B, ht), and arises, according to Spcngel
| |
| (No. 572), as a thickening of the epidermis.
| |
| It subsequently becomes enveloped in a
| |
| pericardium, and is
| |
| placed in a depression
| |
| in the water-vascular
| |
| vesicle. Two pairs of
| |
| diverticula, one behind
| |
| the other, grow out
| |
| (Agassiz, No. 568) from
| |
| the gastric region of
| |
| the alimentary canal.
| |
| The two parts of each
| |
| pair form flattened
| |
| compartments, which
| |
| together give rise to a
| |
| complete investment of
| |
| the adjoining parts of
| |
| the alimentary tract.
| |
| The two parts of each
| |
| coalesce, and thus form
| |
| | |
| | |
| | |
| | |
| FlG. 274. TWO STAGKS IN THK 1 >KY KI.< >I'M KN I
| |
| | |
| OF TORNARIA. (After Metschnikoff.)
| |
| | |
| The black lines represent the ciliated hands.
| |
| m. mouth; an. anus; br. branchial cleft; ///.
| |
| | |
| heart ; c. Ixxly cavity between splanchnic and
| |
| | |
| somatic mesoblast layers; 7.-'. watcr-vascvilar vesicle:
| |
| | |
| v. circular blood-vessel.
| |
| | |
| | |
| | |
| ENTEROPNEUSTA.
| |
| | |
| | |
| | |
| 5 8l
| |
| | |
| | |
| | |
| a double-walled cylinder round the alimentary tract, but their
| |
| cavities remain separated by a dorsal and ventral septum.
| |
| | |
| Eventually (Spengel) the cavity of the anterior cylinder
| |
| forms the section of the body cavity in the collar of the adult,
| |
| and that of the posterior (fig. 274 B, c) the remainder of the
| |
| body cavity. The septa, separating the two halves of each,
| |
| remain as dorsal and ventral mesenteries.
| |
| | |
| The conversion of Tornaria (fig. 274 A) into Balanoglossus
| |
| (fig. 274 B) is effected in a few hours, and consists mainly in
| |
| certain changes in configuration, and in the disappearance of
| |
| the longitudinal ciliated band.
| |
| | |
| The body of the young Balanoglossus (fig. 274 B) is divided
| |
| into three regions (i) the proboscidian region, (2) the collar,
| |
| (3) the trunk proper. The proboscidian region is formed by the
| |
| elongation of the prae-oral lobe into an oval body with the eyespots at its extremity, and provided with strong longitudinal
| |
| muscles. The heart (hi) and water-vascular vesicle lie near its
| |
| base, but the contractile cord connected with the latter is no longer
| |
| present. The mouth is placed on
| |
| the ventral side at the base of the
| |
| prae-oral lobe, and immediately behind it is the collar. The remainder
| |
| of the body is more or less conical,
| |
| and is still girt with the larval
| |
| transverse ciliated band, which lies
| |
| in the middle of the gastric region
| |
| in the Mediterranean species, but
| |
| in the cesophageal region in the
| |
| American one.
| |
| | |
| The whole of the body, including
| |
| the proboscis, becomes richly ciliated.
| |
| | |
| One of the most important cha- S us WITH FOUR BRANCHIAL
| |
| racters of the adult Balanoglossus CLEFTS * (After Alex. Agossiz.)
| |
| | |
| r . m. mouth ; an. anus ; br. bran
| |
| consists in the presence of respira- chial cleft . hL heart ; IV. watertory structures comparable with the vascular vesicle,
| |
| vertebrate gill slits. The earliest traces of these structures
| |
| are distinctly formed while the larva is still in the Tornaria
| |
| | |
| | |
| | |
| | |
| FIG. 275. LATE STAGE IN THE
| |
| DEVELOPMENT OF BALANOGLOS
| |
| | |
| | |
| 582 I'N I'KUOl'NKUSTA.
| |
| | |
| | |
| | |
| condition, as one pair of pouches from the oesophagus in the
| |
| Mediterranean species, and four pairs in the American one
| |
| (fig. 275, br).
| |
| | |
| In the Mediterranean Tornaria the two pouches meet the
| |
| skin dorsally, and in the young Balanoglossus (fig. 274 B, br)
| |
| acquire an external opening on the dorsal side. In the American
| |
| species the first four pouches are without external openings
| |
| till additional pouches have been formed. Fresh gill pouches
| |
| continue to be formed both in the American and probably
| |
| the Mediterranean species, but the conversion of the simple
| |
| pouches into the complicated gill structure of the adult
| |
| has only been studied by Agassiz (No. 568) in the American
| |
| species. It would seem in the first place that the structure of
| |
| the adult gill slits is much less complicated in the American than
| |
| in the Mediterranean species. The simple pouches of the young
| |
| become fairly numerous. They are at first circular ; they then
| |
| become elliptical, and the dorsal wall of each slit becomes folded ;
| |
| subsequently fresh folds are formed which greatly increase the
| |
| complexity of the gills. The external openings are not acquired
| |
| till comparatively late.
| |
| | |
| Our knowledge of the development of the internal organs, mainly
| |
| derived from Agassiz, is still imperfect. The vascular system appears early
| |
| in the form of a dorsal and a ventral vessel, both pointed, and apparently
| |
| ending blindly at their two extremities. The two spurs of the water-vascular
| |
| vesicle, which in the Tornaria stage rested upon the stomach, now grow
| |
| round the oesophagus, and form an anterior vascular ring, which Agassiz
| |
| describes as becoming connected with the heart, though it still communicates
| |
| with the exterior by the dorsal pore and seems to become connected with the
| |
| remainder of the vascular system. According to Spengel (No. 572) the
| |
| dorsal vessel becomes connected with the heart, which remains through life
| |
| in the proboscis : the cavity of the water-vascular vesicle forms the cavity of
| |
| the proboscis in the adult, and its pore remains as a dorsal (not, as usually
| |
| stated, ventral) pore leading to the exterior.
| |
| | |
| The eye-spots disappear.
| |
| | |
| Tornaria is a very interesting larval form, since it is intermediate in structure between the larva of an Echinoderm and
| |
| trochosphere type common to the Mollusca, Chxtopoda, etc.
| |
| The shape of the body especially the form of the ventral
| |
| depression, the character of the longitudinal ciliated band, the
| |
| structure and derivation of the water-vascular vesicle, and the
| |
| | |
| | |
| | |
| ENTEROPNEUSTA. 583
| |
| | |
| | |
| | |
| formation of the walls of the body cavity as gastric diverticula,
| |
| are all characters which point to a connection with Echinodcrm
| |
| larvae.
| |
| | |
| On the other hand the eye-spots at the end of the prae-oral
| |
| lobe 1 , the contractile band passing from the oesophagus to the
| |
| eye-spots (fig. 273), the two posterior bands of cilia, and the
| |
| terminal anus are all trochosphere characters.
| |
| | |
| The persistence of the prae-oral lobe as the proboscis is
| |
| interesting, as tending to shew that Balanoglossus is the surviving representative of a primitive group.
| |
| | |
| *
| |
| | |
| BIBLIOGRAPHY.
| |
| | |
| (567) A. Agassiz. "Tornaria." Ann. Lyceum Nat. Hist.\u\. New York,
| |
| 1866.
| |
| | |
| (568) A. Agassiz. "The History of Balanoglossus and Tornaria." Mem.
| |
| Amer. Acad. of Arts and Stien., Vol. IX. 1873.
| |
| | |
| (569) A. Gotte. " Entwicklangsgeschichte d. Comatula Mediterranea." Archiv
| |
| fur mikr. Anat., Bd. xii., 1876, p. 641.
| |
| | |
| (570) E. Metschnikoff. " Untersuchungen iib d. Metamorphose, etc. (Tornaria)." Zeit.fiir wiss. ZooL, Bd. xx. 1870.
| |
| | |
| (571) J. M tiller. " Ueb. d. Larven u. Metamor. d. Echinodermen." Berlin
| |
| Akad., 1849 and 1850.
| |
| | |
| (572) J. W. Spengel. "Ban u. Entwicklung von Balanoglossus. Tagebl. d.
| |
| Naturf. Vers. Miinchen, 1877.
| |
| | |
| 1 It would be interesting to have further information about the fate of the thickening of epiblast in the vicinity of the eye-spots. The thickening should by rights be the
| |
| supra-oesophageal ganglion, and it does not seem absolutely impossible that it may give
| |
| rise to the dorso-median cord in the region of the collar, which constitutes, according
| |
| to Spengel, the main ganglion of the adult.
| |
| | |
| | |
| | |
| INDEX.
| |
| | |
| | |
| | |
| Abdominalia, 459, 493, 499
| |
| | |
| Acanthocephala, 379
| |
| | |
| Acanthosoma, 473, 474, 475
| |
| | |
| Acarina, 444, 454
| |
| | |
| Accipenser, 102
| |
| | |
| Achaeta, 319
| |
| | |
| Achelia, 538
| |
| | |
| Achtheres percarum, 490
| |
| | |
| Acineta, 7, 8
| |
| | |
| Acraspeda, 152, 165, 167, 178, 179, 182,
| |
| | |
| 185, 186
| |
| | |
| Actinia, 169, 171, 179
| |
| Actinophrys, 9
| |
| | |
| Actinotrocha, 315, 318, 363, 364
| |
| Actinozoa, 26, 102, 152, j66, 170, 171,
| |
| | |
| 172, 176, 178, 179, 181, 182, 186
| |
| Actinula, 155
| |
| Aculeata, 421
| |
| ^Egineta flavescens, 158
| |
| yEginidae, 156, 158
| |
| ^Eginopsis Mediterranea, 158
| |
| /Equorea Mitrocoma, 182
| |
| Agalma, 163
| |
| Agelena, 436, 450
| |
| Agelena labyrinthica, 119, 438
| |
| Alciope, 74
| |
| Alcippidae, 499
| |
| Alcyonaria, 152
| |
| Alcyonidse, 167, 168
| |
| Alcyonidium mytili, 297, 300, 302
| |
| Alcyonium palmatum, 119, 148, 167, 182
| |
| Alima, 484, 486
| |
| Amoeba, 19, 20
| |
| Amphibia, 22, 54, 56, 59, 60, 63, 66, 74,
| |
| | |
| 83, 102
| |
| | |
| Amphilina, 218
| |
| | |
| Amphioxus, 54, 56, 59, 61, 66, 93, 426
| |
| Amphipoda, 518
| |
| Amphiporus lactifloreus, 202
| |
| Amphistomum, 31
| |
| | |
| ,, subclavatum, 205
| |
| | |
| Amphitrochae, 330
| |
| Amphiura squamata, 565
| |
| | |
| | |
| | |
| Anchorella, 108, 492, 520
| |
| | |
| Anelasma squalicola, 499
| |
| | |
| Anguillulidse, 371
| |
| | |
| Annelida, 14, 25, 98, 503, 525
| |
| | |
| Anodon, 37, 38, 39, 100, 107, 259, 260,
| |
| | |
| 265, 266, 268
| |
| Anopla, 189, 202
| |
| Anura, 5
| |
| | |
| Antedon, 568, 573, 574
| |
| Aphides, 15, 16, 76, 79, 116, 428, 429
| |
| Aphrodite, 42
| |
| | |
| Apis, 402, 407, 408, 412, 413
| |
| Aplysia, 99, 226, 238, 252, 253
| |
| Aplysinidaa, 146
| |
| Apoda, 459, 493
| |
| Aptera, 395, 420
| |
| Apus, 1 6, 79, 460, 463
| |
| Arachnida, 22, 114, 119, 413, 4.51, 435,
| |
| | |
| 444, 454, 455, 458, 537, 539
| |
| Arachnitis, 171
| |
| Araneina, 50, 51, 436
| |
| Arbacia, 567
| |
| Area, 38
| |
| Archigetes, 218
| |
| Archizosea gigas, 494
| |
| Arenicola, 42
| |
| | |
| Argiope, 311, 312, 315, 317
| |
| Argonauta, 247, 248
| |
| Argulus, 492
| |
| Armata, 355
| |
| Arthropoda, 12, 16, 18, 22, 75, 77,79, 83,
| |
| | |
| 108, no, 221, 382, 383, 434, 448,503,
| |
| | |
| 5 2 5> 534 54', 54 2
| |
| Articulata, 311, 313, 316, 317
| |
| Ascaridiae, 371
| |
| Ascaris nigrovenosa, 16, 82
| |
| | |
| ,, lumbricoides, 375
| |
| Ascetta, 144
| |
| Ascidia canina, 53
| |
| Ascidians, 74, 102, 208, 426
| |
| Asellus aquaticus, 112,120, 516
| |
| Astacus, 66, 465, 477, 511, 512, 513,
| |
| | |
| 525
| |
| | |
| | |
| | |
| 586
| |
| | |
| | |
| | |
| INDKX.
| |
| | |
| | |
| | |
| Asteracanthion, 69, 70, 561
| |
| | |
| Asterias, 20, 68, 69, 71, 78, 80, 84, 549,
| |
| | |
| 564
| |
| Asteroidea, 35, 36, 544, 549, 557, 563,
| |
| | |
| 576
| |
| Astnea, 169
| |
| | |
| Astroides, 169
| |
| | |
| Atax Bonzi, 445
| |
| | |
| Atlanta, 231, 240
| |
| | |
| Atrochae, 330
| |
| | |
| Aurelia, 167
| |
| | |
| Auricularia, 553, 554, 562, 574
| |
| | |
| Autolytus cornutus, 319, 343
| |
| | |
| Aves, 56, 59, 61, 64, 107. 109
| |
| | |
| Axolotl, 1 6
| |
| | |
| Balanoglossus, 576, 579, 581
| |
| | |
| Balanus balanoides, 75, 493
| |
| | |
| Belemnites, 252, 253
| |
| | |
| Bipinnaria, 557, 563, 574, 576, 579
| |
| | |
| Blatta, 374, 395
| |
| | |
| Bojanus, organ of, 264, 282
| |
| | |
| Bonellia, 20, 43, 44, 98, 324, 355, 358,
| |
| | |
| 359
| |
| Bothriocephalus salmonis, 211
| |
| | |
| ,, proboscideus, 212
| |
| | |
| Brachiella, 492
| |
| Brachiolaria, 558, 564
| |
| Brachiopoda, 311, 317, 318
| |
| Brachyura, 466, 480, 483
| |
| Branchiobdella, 42, 43, 346
| |
| Branchiogasteropoda, 272
| |
| Branchiopoda, 79, 459, 523, 524
| |
| Branchipus, 463, 524
| |
| Branchiura, 459, 492
| |
| Branchionus urceolaris, 221
| |
| Braula, 396
| |
| Uuccinum, 237, 280
| |
| Bulimus citrinus, 229
| |
| Bunodes, 169, 171
| |
| Buthus, 431
| |
| | |
| Calcispongiae, 138, 148
| |
| | |
| Calopteryx, 402
| |
| | |
| Calycophoridce, 152, 159
| |
| | |
| Calyptoblastic Hydroids, 184, 185
| |
| | |
| Calyptraea, 223, 280
| |
| | |
| Campanularidse, 183, 184
| |
| | |
| Capitclla, 330, 332
| |
| | |
| Carabidae, 476
| |
| | |
| Carcinus Mcenas, 481, 483
| |
| | |
| Cardium, 260, 262
| |
| | |
| " pygmaeum, 262
| |
| | |
| Carinaria, 240
| |
| | |
| Caryophyllium, 168, 171
| |
| pea, 165, 167
| |
| | |
| Cecidomyia, 15, 79, 416, 417, 429
| |
| | |
| Cephalopoda, 20, 40, 41, 102, 108, 109,
| |
| 135. "5. 240, 242, 244, 250, 252, 253,
| |
| 270, 271, 272, 274, 279, 282, 287
| |
| | |
| Cephalothrix galatheae, 202
| |
| | |
| Ceratosponguc, 146
| |
| | |
| | |
| | |
| Cercariae, 207, 208, 209
| |
| | |
| Cerianthus, 168, 171
| |
| | |
| Cestodes, 14, 29, 31, 32, 33, 189, 210,
| |
| 212, 218, 313, 425, 541
| |
| | |
| Chsetogaster, 342
| |
| | |
| Chaetopoda, 5, 18, 23, 41, 43, 44 , 54,
| |
| 67, 209, 215, 270, 275, 307, 312, 317,
| |
| 318, 319, 320, 326, 334, 33<S, 342, 346,
| |
| 349, 350, 351, 364. 369. 33, 36, 408,
| |
| 448, 457, 458, 521, 576,582
| |
| | |
| ChiXitopteridte, 333
| |
| | |
| Cha^tosomoidea, 371
| |
| | |
| Chelifer, 434, 436, 442, 446, 454
| |
| | |
| Chermes, 15, 429
| |
| | |
| Chilognatha, 113, 387, 389, 391, 393,
| |
| | |
| 395
| |
| | |
| Chilopoda, 387, 392, 394
| |
| Chilostomata, 292, 297, 298, 304, 305
| |
| Chironomus, 15, 378, 401, 402, 415, 416,
| |
| | |
| 429
| |
| | |
| Chiton, 254, 256, 257, 273
| |
| Chordata, 5
| |
| Chrysaora, 165
| |
| Chthonius, 436
| |
| Cicada, 395
| |
| | |
| Cirripedia, 459, 492, 496,503, 509, 520
| |
| Cladocera, 459, 464, 519
| |
| Clausilia, 239
| |
| Clavella, 520
| |
| Clavularia crassa, 167
| |
| Cleodora, 241
| |
| Clepsine, 73, 346, 347, 349, 351, 352,
| |
| | |
| 353, 354
| |
| Clio, 242, 278
| |
| Clubione, 436
| |
| Clupeidae, 64
| |
| Cobitis barbatula, 378
| |
| Coccida;, 429
| |
| Coccus, 50
| |
| Ccelebogyne, 79
| |
| Coelenterata, 3, 5, 13, 18, 26, 27, 2S, 35,
| |
| | |
| 74, 93, 94, 126, 148, 170, 178,179, 1 80,
| |
| | |
| 181, 191, 342
| |
| | |
| Ccenurus cerebralis, 213, 214
| |
| Coleochaete, 1 1
| |
| | |
| Coleoptera, 396, 402, 409, 412, 420, 421,
| |
| ^5
| |
| | |
| Collembola, 395, 426
| |
| Comatula, 5, 552, 553
| |
| Condracanthus, ill, 120, 520
| |
| Conochilus volvox, 22 1
| |
| Convoluta, 32
| |
| Copepoda, 109, 120, 459, 460, 487, 489,
| |
| | |
| 493, 496, 503, 509, 519
| |
| Corallium rubrum, 168, 182
| |
| Corethra, 422, 423, 424
| |
| Crangoninoe, 476
| |
| Crnniiuhv, 311
| |
| Craniata, 5, 6, 19, 20, 54, 56, 59, 6l, 62,
| |
| | |
| 6 4 , 74, 102
| |
| | |
| Crinoidea, 35, 36, 544, 550, 568, 576
| |
| Criodilus, 321, 324, 328, 341
| |
| | |
| | |
| | |
| INDEX.
| |
| | |
| | |
| | |
| Crisia, 304
| |
| Crocodilia, 63
| |
| | |
| Crustacea, 5, 6, 18, 51, 66, 102, 109, 120,
| |
| 458, 4 6 5> 487* 5<>2, 521, 524, 537, 541
| |
| Cryptophialus, 499, 509
| |
| Crystalloides, 163
| |
| Ctenophora, 26, 93, 102, 152, 173, 175,
| |
| | |
| 177, 178, 179, 180, 181, 182
| |
| Ctenostomata, 292, 297, 298, 304, 305
| |
| Cucullarms elegans, 46, 75, 82, 371, 376
| |
| Cucumaria, 546, 556, 574
| |
| Cumaceae, 459, 465, 486, 506
| |
| Curculio, 421
| |
| | |
| Cyclas, 259, 260, 261, 265
| |
| Cyclops, 376, 377, 418, 489, 503
| |
| Cyclostomata, 102, -292, 304
| |
| Cymbulia, 241, 242
| |
| | |
| Cymothoa, 516, 517, 519, 520,524, 528
| |
| Cynipidae, 15, 421, 428
| |
| Cyphonautes, 297, 301, 304, 306, 308
| |
| Cypridina, 500, 502
| |
| Cysticercus cellulosce, 214, 217
| |
| | |
| ,, fasciolaris, 216
| |
| | |
| ,, limacis, 213
| |
| | |
| Daphnia, 79, 464
| |
| | |
| Dasychone, 331, 336
| |
| | |
| Decapoda, 66, 248, 459, 465, 469, 504,
| |
| | |
| 511
| |
| | |
| Dendroccela, 32, 33, 189, 195, 196
| |
| Dentalium, 258, 576
| |
| Desmacidon, 147
| |
| Desor, type of, 196, 197, 201, 202, 204,
| |
| | |
| 212, 424
| |
| Diastopora, 304
| |
| Dibranchiata, 225, 253
| |
| Dicyema, 9, 131, 134, 135, 136
| |
| Dimya, 225
| |
| Diphyes, 159
| |
| Diplozoon, 11, 209, 210
| |
| Diporpa, 210
| |
| Diptera, 49, 194, 204, 396, 401,402,407,
| |
| | |
| 409, 412, 416, 420, 429
| |
| Discina radiata, 317
| |
| Discinidse, 311
| |
| | |
| Discophora, 18, 42, 165, 346, 383
| |
| Distomese, 189, 205, 425
| |
| Distomum, 31
| |
| | |
| ,, cygnoides, 209
| |
| | |
| ,, globiparum, 207
| |
| | |
| ,, lanceolatum, 205
| |
| Dochmius duodenale, 375
| |
| | |
| ,, trigonocephalus, 375
| |
| Donacia, 401
| |
| Dracunculus, 376, 377
| |
| | |
| Echinaster fallax, 23
| |
| | |
| ,, Sarsii, 102, 561
| |
| Echinodermata, 5, 18, 24, 35, 74, 102,
| |
| | |
| 325, 424, 544, 573, 574, 57 6 > 5 82
| |
| Echinoidea, 35, 36, 544, 549, 565, 576
| |
| Echinorhyncus, 379, 380
| |
| | |
| | |
| | |
| Echinus lividus, 83, 84, 88
| |
| | |
| Echiurus, 44 , 357, 358
| |
| | |
| Ectoprocta, 297, 306
| |
| | |
| Edriophthalmata, 459, 465
| |
| | |
| Elaphocaris, 473
| |
| | |
| Elasmobranchii, 23, 56, 59, 61, 62, 64,
| |
| | |
| 67, 105, 106. 107, 108, 109
| |
| Enopla, 189, 202
| |
| Entoconcha mirabilis, 237
| |
| Entomophaga, 421
| |
| | |
| Entoprocta, 292, 298, 300, 302, 304, 306
| |
| Epeira, 436
| |
| | |
| Ephemera, 395, 409, 420, 422
| |
| Ephyra, 186
| |
| | |
| Epibulia auranliaca, 159, 165
| |
| Erichthus, 484, 507
| |
| Errantia, 319, 336
| |
| Esperia, 147
| |
| Estheria, 463, 464
| |
| Euaxes, lol, 322, 324, 341, 346,349
| |
| Eucharis, 178
| |
| | |
| ,, multicornis, 178
| |
| | |
| Eucopepoda, 459
| |
| Eucope polystyla, 23, 154
| |
| | |
| Eunice sanguinea, 319
| |
| | |
| Eupagurus prideauxii, 112, 113, 115, 511,
| |
| 520
| |
| | |
| Euphausia, 465, 468, 504, 505, 518, 523
| |
| | |
| Eurostomata, 176
| |
| | |
| Eurylepta auriculata, 192
| |
| | |
| Eurynome, 483
| |
| | |
| Euspongia, 146, 147
| |
| | |
| Filaria, 377
| |
| Filaridae, 371
| |
| Firoloidea, 240
| |
| Flagellata, 7, 8
| |
| Flustrella, 301, 303
| |
| Formica, 396
| |
| Fungia, 182, 186
| |
| Fusus, 275, 280, 284, 288
| |
| | |
| Gammarus, 122, 518
| |
| | |
| ,, fluviatilis, 117
| |
| ,, locusta, no, 112
| |
| Ganoids, 54, 102
| |
| Gasteropoda, 39, 41, 98, 225, 226, 229,
| |
| | |
| 230, 232, 233, 240, 258, 260, 261, 270,
| |
| | |
| 272, 275, 279, 283, 324
| |
| Gasterosteus, 64, 210
| |
| Gastrotricha, 370
| |
| Gasterotrochce, 330, 333
| |
| Gecarcinus, 465
| |
| Geophilus, 392, 393
| |
| Gephyrea, 5, 18, 24, 44, 54, 67, 102,
| |
| | |
| 318, 320, 325, 355, 357, 361, 364
| |
| Germogen, 134
| |
| Geryonia hastata, 156
| |
| Geryonidse, 156
| |
| Glochidia, 267, 268
| |
| Gnathobdellidas, 346, 349
| |
| Gordiacea, 94
| |
| | |
| | |
| | |
| 588
| |
| | |
| | |
| | |
| INDEX.
| |
| | |
| | |
| | |
| Cimlioidca, 371, 374, 378
| |
| | |
| ;nia, 168
| |
| Gorgonidce, 181
| |
| Gorgoninrc, 181
| |
| Gregarinidae, 8
| |
| Gryllotalpa, 401, 412, 413
| |
| Gunnnineiv, 147, 148
| |
| Gymnoblastic Hydroids, 184, 185
| |
| Gymnoloemata, 292
| |
| | |
| Gymnosomata, 225, 240, 241, 242, 270
| |
| Gyrodactylus, 210
| |
| | |
| Halichondria, 147
| |
| | |
| Ilalisarca, 22, 66, 145
| |
| | |
| Halistemma, 165
| |
| | |
| Helicidce, 238
| |
| | |
| Helioporidae, 182
| |
| | |
| Helix, 67, 229
| |
| | |
| Hemiptera, 395, 402, 403, 409, 420, 421
| |
| | |
| Hessia, 108, 492
| |
| | |
| Heterakis vermicularis, .374
| |
| | |
| Heteronereis, 343
| |
| | |
| Heteropoda, 71, 72, 225, 226, 231, 278
| |
| | |
| Hexacoralla, 152, 179, 182
| |
| | |
| Hippopodius gleba, 27, 159
| |
| | |
| Hirudinea, 74, 84
| |
| | |
| Hirudo, 350, 351, 352, 353, 354
| |
| | |
| Holometabola, 420, 422
| |
| | |
| Holostomum, 205
| |
| | |
| Holothuria, 19, 25, 35, 549, 558, 576
| |
| | |
| Holothuroidea, 35, 544, 553, 556
| |
| | |
| Homarus, 477
| |
| | |
| Hyaleacea, 273, 275
| |
| | |
| Hyaleidce, 241
| |
| | |
| Hydra, 21, 22, 26, 28, 29, 34, 152, 154,
| |
| | |
| 155. 179, 183
| |
| Hydractinia, 539
| |
| Hydrocoralla, 152, 181, 185
| |
| Hydroidea, 152
| |
| Hydromedusae, 152, 179, 182, 183, 184,
| |
| | |
| 185, 186, 187
| |
| Hydrophilus, 374, 396, 400, 401, 402,
| |
| | |
| 404, 408, 409
| |
| Hydrozoa, 14, 19, 26, 27, 67, 102, 152,
| |
| | |
| 155. 165. 179, 1 80, 181, 182, 539
| |
| Ilymenoptera, 396, 401, 402, 412, 420,
| |
| | |
| 421, 425
| |
| | |
| Ichneumon, 396
| |
| | |
| Inarticulata, 311, 316
| |
| | |
| Incrmi
| |
| | |
| Infusoria, 7, 8
| |
| | |
| Insecta, 5, 15, 18, 19, 25, 46, 395, 396,
| |
| | |
| 4^5, 455. 45
| |
| Intoshia gigas, 136
| |
| Isidimc, 181
| |
| Ixxlyctia, 147
| |
| Isopoda, 109, 515, 519, 521, 523, 527
| |
| | |
| Julus Moneletei, 387, 388, 389
| |
| Kochlorine, 499
| |
| | |
| | |
| | |
| Lacertilia, 64
| |
| Lacinularia, 221, 223
| |
| | |
| socialis, 75
| |
| Lamellibranchiata, 23, 25, 37, 39, 98,
| |
| | |
| 225, 241, 257, 258, 259, 269, 270, 271,
| |
| | |
| 273, 274, 288
| |
| Lepadkue, 498
| |
| | |
| Lepas fascicularis, 224, 493, 494, 495
| |
| Lepidoptera, 79, 396, 402, 407, 408, 412,
| |
| | |
| 413, 415, 417, 420, 421, 423, 415, 426.
| |
| | |
| 455
| |
| | |
| Leptodora, 16, 51
| |
| Leptoplana, 74, 189, 192, 193
| |
| Lernseopoda, 490, 492, 520
| |
| Leucifer, 507
| |
| | |
| Libellulidae, 402, 403, 409, 420
| |
| Limax, 229, 232, 239, 278, 280
| |
| Limnadia, 79, 524
| |
| Limulus, 534
| |
| Lina, 402
| |
| | |
| Lingulidae, 311, 316
| |
| Lithobius, 393
| |
| Lobatse, 178
| |
| | |
| Loligo, 242, 243, 244, 247, 253
| |
| Loricata, 507, 514
| |
| Lota, 105
| |
| | |
| Loxosoma, 292, 294, 296, 306, 307
| |
| Lucernaria, 185
| |
| Lumbricus, 341, 368
| |
| | |
| ,, agricola, 321
| |
| | |
| ,, rubellus, 324
| |
| | |
| trapczoides, 13, 321, 323
| |
| Lumbriconereis, 334
| |
| Lymnseus, 82, 98, 226, 227, 232, 238,
| |
| | |
| 281
| |
| Lycosa, 436
| |
| | |
| Macrostomum, 32, 34
| |
| | |
| Macrura, 476
| |
| | |
| Malacobdella, 203
| |
| | |
| Malacodermata, 171
| |
| | |
| Malacostraca, 66, 459, 462, 465, 504,
| |
| | |
| 505, 506, 511, 523
| |
| Mammalia, 56, 58, 59, 64, 66
| |
| Marsipobranchii, 59
| |
| Mastigopus, 473, 474
| |
| Medusoe, 27, 154, 157, i.^s, 16;, 164, 176,
| |
| | |
| 178, 181, 182, 183, 184, 185, 186
| |
| Megalopa, 482, 483, 484
| |
| Melolontha, 402, 421
| |
| Membranipora, 297, 303
| |
| Mermithido;, 371
| |
| Mesotrochoe, 330
| |
| Metachoetoe, 335
| |
| Metazoa, Q, 10, 12,67, 86, 125, 135, 14^,
| |
| | |
| ISO, 179
| |
| | |
| Millepora, 152, 181
| |
| | |
| Mitraria, 308, 337
| |
| | |
| Molgula, 102
| |
| | |
| Mollusca, 5, 18, 24, 66, 74, 84, 99, 225,
| |
| 247, 248, 251, 256, 257, 262, 271, 285,
| |
| 288, 307, 325, 333, 352, 576, 582
| |
| | |
| | |
| | |
| INDEX.
| |
| | |
| | |
| | |
| 589
| |
| | |
| | |
| | |
| Monomya, -225
| |
| Monostomum capitellum, 205
| |
| | |
| ,, mutabile, 205, 206
| |
| | |
| Monotrochse, 330
| |
| Montacuta, 260, 262
| |
| Musca, 396
| |
| Muscidae, 420, 423
| |
| Myobia, 444, 445
| |
| Myrianida, 343
| |
| Myriapoda, 22, iir, 113, 387, 394, 395,
| |
| | |
| 4i.3 458
| |
| Mynothela, 155
| |
| Myrmeleon, 396
| |
| | |
| Mysis, 120, 469, 472, 486, 504, 509, 525
| |
| Mytilus, 260, 261
| |
| Myxinoids, 5
| |
| Myxispongise, 145
| |
| Myzostomea, 369
| |
| | |
| Nais, 342
| |
| | |
| Nassa mutabilis, 101, 226, 227, 233, 262,
| |
| | |
| 278, 279, 288, 3^4
| |
| Natantia, 487
| |
| Natica, 237, 283
| |
| Nauplius, 5, 16, 460, 461, 463, 465, 466,
| |
| | |
| 469, 473, 490, 491, 493, 497
| |
| Nautilus pompilius, 253, 276
| |
| Nebaliadse, 459, 465, 486
| |
| Nematoda, 45, 46, 50, 66, 74, 75, 371,
| |
| | |
| 373. 374> 376
| |
| Nematogens, 131
| |
| Nematoidea, 18, 84, 94, 371, 374
| |
| Nematus ventricosus, 13, 427
| |
| Nemertea, 94, 189, 196, 202, 204
| |
| Nemertines, 30, 31, 33, 93, 136, 195,
| |
| | |
| 202, 328, 333, 424
| |
| Nephelis, 82, 346, 349, 350, 351, 352,
| |
| | |
| 354
| |
| | |
| Nereis, 343
| |
| | |
| ,, diversicolor, 319
| |
| | |
| ,, Dumerilii, 343
| |
| Neritina, 229, 237
| |
| Neuroptera, 396, 401, 420, 421
| |
| Neuroterus ventricularis, 428
| |
| Notonecta, 395
| |
| Nototrochse, 330, 353
| |
| Nudibranchiata, 229, 241
| |
| | |
| Ocellata, 184
| |
| | |
| Octocoralla, 152, 179
| |
| | |
| Octopus, 248
| |
| | |
| Odontophora, 225, 257, 271
| |
| | |
| Odontosyllis, 333
| |
| | |
| Oedogonium, 1 1
| |
| | |
| Oligochseta, 42, 319, 321, 325, 330, 338,
| |
| | |
| 346, 352
| |
| Olynthus, 144
| |
| Oniscus murarius, 107, 108, 109, 120,
| |
| | |
| 516, 520, 528
| |
| Opercula, 31
| |
| Ophiothryx, 36, 549
| |
| Ophidia, 64
| |
| | |
| | |
| | |
| Ophiuroidea, 136, 544, 553, 562, 565,
| |
| | |
| 576
| |
| | |
| Ophryotrochoe puerilis, 333
| |
| Opisthobranchiata, 225, 232, 237
| |
| Ornithodelphia, 109
| |
| Orthonectidae, 136
| |
| | |
| Orthoptera, 395, 414, 420, 421, 425, 426
| |
| Ostracoda, 459, 500, 510
| |
| Ostrea, 259, 260, 262
| |
| Oxyuridse, 46, 373, 374
| |
| Oxyurus ambigua, 374
| |
| ,, vermicularis, 375
| |
| | |
| PcEcilopoda, 534
| |
| Paguridse, 477
| |
| Pakemon, no
| |
| Palaemonetes, 476
| |
| Pakemoninre, 476, 511, 512
| |
| Palinurus, 478, 480
| |
| | |
| Paludina, 66, 227, 229, 235, 270, 278,
| |
| 280
| |
| | |
| ,, costata, 229
| |
| | |
| ,, vivipara, 226
| |
| Pandorina, n
| |
| Parasita, 489
| |
| Pedalion, 221
| |
| | |
| Pedicellina, 98, 292, 296, 299, 307
| |
| Pelagia, 167, 185
| |
| Penseinse, 476
| |
| Penaeus, no, 113, 465, 469, 473, 474,
| |
| | |
| 504, 518
| |
| | |
| Pennatulidae, 181
| |
| Pentacrinus, 5
| |
| Pentastomida, 539, 540
| |
| Pentastomum denticulatum, 540, 54!
| |
| | |
| tsenoicles, 539, 540, 541
| |
| Percidae, 64
| |
| PerennichaetcE, 335
| |
| Peripatus, 5, 386, 411, 412, 413, 542
| |
| Petromyzon, 61, 63, 64, 74, 83
| |
| Phalangella, 304
| |
| Phalangidse, 436
| |
| Phallusia, 83
| |
| | |
| Phascolosoma, 44, 355, 356, 361
| |
| Pholcus, 436, 442
| |
| Phoronis, 315, 355, 363, 364
| |
| Phoxinus laevis, 378
| |
| Phryganea, 396, 401, 409
| |
| Phylactokemata, 292, 294, 297, 305, 306
| |
| Phyllobothrium, 218
| |
| Phyllodoce, 329
| |
| Phyllopoda, 16, 459, 461, 505
| |
| Phyllosoma, 479, 480
| |
| Phylloxera, 429
| |
| Physophoridoe, 152, 16-2, 164
| |
| Pilidium, type of, 196, 200, 201, 202, 704,
| |
| | |
| 424
| |
| | |
| Pisces, 5
| |
| Piscicola, 20, 43
| |
| Pisidium, 259, 260, 262, 264
| |
| Planaria Neapolitana, 193
| |
| Planorbis, 273, 281, 325
| |
| | |
| | |
| | |
| 590
| |
| | |
| | |
| | |
| INDEX.
| |
| | |
| | |
| | |
| Platyelminthes, 18, 20, 24, 221, 424
| |
| Platygaster, 396, 416, 417, 418, 419
| |
| Pleurohrachia, 176, 177, 238
| |
| Pneumodermon, 242, 576
| |
| Podostomata, 292
| |
| Poduridce, 401, 405
| |
| Polychaeta, 42, 319, 325, 338
| |
| Polydesmus complanatus, 387, 388
| |
| Polygordius, 319, 325, 326, 327, 328,
| |
| | |
| 332, 357 386
| |
| Polynoe, 42, 331
| |
| Polyophthalmus, 328
| |
| Polyplacophora, 225, 254, 270, 271, 288
| |
| Polystomeas, 189, 205, 209
| |
| Polystomum, 209
| |
| | |
| ,, integerrimum, 30, 31, 210
| |
| | |
| Polytrochne, 330, 333
| |
| Polyxenia leucostyla, 158
| |
| Polyxenus lagurus, 387
| |
| Polyzoa, 98, 303, 305, 306. 30 8 > 3 ! 5. 3^
| |
| Porcellana, 483
| |
| Porifera, 102, 138, 148
| |
| Porthesia, 115
| |
| Prorhyncus, 32, 34
| |
| Prosobranchiata, 225, 237, 281
| |
| Prostomum, 32, 34, 38, 196
| |
| Protozoa, 8, 9, lo, n, 86, 135, 149
| |
| Protozoaea, 471
| |
| Protula Dysteri, 342
| |
| Pseudoneuroptera, 426
| |
| Pseudoscorpionid;e, 434
| |
| Psolinus, 556, 574
| |
| Psychidae, 16
| |
| Pteraster miliaris, 561
| |
| Pteropoda, 98, 225, 226, 229, 230, 232,
| |
| | |
| 240, 258, 270, 272, 279, 283
| |
| Pterotrachcea, 71, 229, 240
| |
| Pulex, 396
| |
| | |
| Pulmonata, 39, 225, 232, 238, 281, 282
| |
| I'urpura lapillus, 78
| |
| Pycnogonida, 538
| |
| Pyrosoma, 13, 53, 109
| |
| | |
| Rana temporaria, 210
| |
| Kaspailia, 147
| |
| Rcdia, 206, 207, 208, 209
| |
| Reniera, 147
| |
| | |
| Kcptilia, 56, 59, 60, 61, 62, 64, 109
| |
| Rhabditis dolichura, 82
| |
| Khabdoccela, 32, 33, iSy, ic/>
| |
| Khnbdopleura, 294, 306
| |
| Rhi/occphala, 459, 493, 499, 500
| |
| Klii/.ocrinus, 5
| |
| klii/.ostoma, 167
| |
| Rhomlx>gens, 131, 134
| |
| Khynchoncllidaj, 311
| |
| Rhyncdbddlkbe, 346
| |
| Rotifera, 5, 12, 18, 75, 76, 77, 79, 83,
| |
| 102, 221, 308, 325
| |
| | |
| Saccocirrus, 328, 329, 332, 340
| |
| Sacculina, 500
| |
| | |
| | |
| | |
| Sagartia, 169, 171
| |
| | |
| Sagitta, 33, 74, 94, 130, 366, 367, 368
| |
| | |
| Salmonidrc, 64
| |
| | |
| Salpa, 102
| |
| | |
| Sarcia, 164
| |
| | |
| Seaphopoda, 225, 257, 270, 271
| |
| | |
| Schistocephalus, 2 1 1
| |
| | |
| Schizopoda, 459, 465, 466
| |
| | |
| Scolopendra, 392
| |
| | |
| Scorpio, 120, 43 r, 44 6, 454, 455, 457
| |
| | |
| Scrobicularia, 38, 39
| |
| | |
| Scyllarus, 477
| |
| | |
| Scyphistoma, 179, 185, 186
| |
| | |
| Sedentaria, 319, 336
| |
| | |
| Sepia, 20, 40, 41, 242, 243, 244, 245,
| |
| | |
| 247> 2 49> 253
| |
| Sergestidce, 473, 507
| |
| Serpula, 319. 325, 331
| |
| Sertularia, 152, 183, 184
| |
| Silicispongia.', 147
| |
| Simulia, 401, 415
| |
| Siphonophora, 13, 77, 152, 159, 163,
| |
| | |
| 165, 179, 1 80, 182, 185
| |
| Sipunculida, 24
| |
| Sipunculus, 44
| |
| Sirex, 396
| |
| Sitaris, 42!
| |
| | |
| Spathegaster baccarum, 428
| |
| Spjo, 4 2 > 33 2 > 333
| |
| Spiroptera obtusa, 376
| |
| Spirorbis Pagenstecheri, 319
| |
| | |
| spirillum, 319, 336
| |
| Spirula, 252
| |
| Spirulirostra, 252
| |
| Spongelia, 147
| |
| Spongida, 138, 144, 148, 149
| |
| Spongilla, 147, 150
| |
| Sporocysts, 206, 207, 208, 209
| |
| Squilla, 66, 504, 507
| |
| Stephanomia pictum, 162, 165
| |
| Stomalopoda, 459, 465, 4X4
| |
| Stomodoeum, 413
| |
| Strongylidrc, 371, 375
| |
| Strongylocentrus, 567
| |
| Strongysoloma Guerinii, 3<S7, 388, 390
| |
| Stylasterictae, 152, r8r
| |
| Styliolidic, 24!
| |
| Stylochopsis ponticus, 193
| |
| Sycandra, 93, 138, 144, 145, 147, 150
| |
| | |
| ,, raphanus, i^S, 174
| |
| Syllis, 343
| |
| | |
| vivipara, 319
| |
| Sympodium coralloidcs, 168
| |
| | |
| Taeniatoe, 178
| |
| Tardigrada, 541
| |
| Teoenaria, 436
| |
| | |
| 'I'clcDsti'i, IS, 25, 5^), 59, C>4. 107, io<)
| |
| I'r].)troch;i.-, 330
| |
| Tcndra, 300
| |
| 'I '(.'nth reds, 396
| |
| Tcrcbdla concliilcga, 332, 333, 337
| |
| | |
| | |
| | |
| INDEX.
| |
| | |
| | |
| | |
| 591
| |
| | |
| | |
| | |
| Terebella nebulosa, 332, 333
| |
| Terebratula, 311, 315
| |
| Terebratulina, 311, 315, 316
| |
| | |
| ,, septentrionalis, 315, 316
| |
| | |
| Teredo, larva of, 262
| |
| Tergipes, 232, 238
| |
| | |
| ,, Edwardsii, 238
| |
| ,, lacinulatus, 238
| |
| Tethya, 147
| |
| Tetrabranchiata, 225
| |
| Tetranychus telarius, 116
| |
| Tetrastemma varicolor, 203
| |
| Thalassema, 44, 355, 357
| |
| Thalassinidae, 477
| |
| Thallophytes, n
| |
| Thecidium, 311, 312, 315, 316
| |
| Thecosomata, 225
| |
| Thoracica, 459, 493, 499, 500
| |
| Thysanozoon, 192, 193
| |
| Thysanura, 395, 408, 425, 458
| |
| Tichogonia, 39
| |
| Tipula, 396
| |
| Tipulidae, 420, 421
| |
| Toenia cosnurus, 214
| |
| | |
| ,, echinococcus, 215, 217
| |
| | |
| solium, 217
| |
| Tornaria, 579, 581
| |
| Toxopneustes, 22, 24, 35, 85, 88, 89
| |
| Tracheata, 385, 426, 432, 44 8, 455, 457,
| |
| | |
| 458, 538, 54i
| |
| | |
| Trachymedusae, 152, 156, 179, 185
| |
| Trematodes, 14, 16, 29, 30, 31, 32, 33,
| |
| | |
| 46, 94, 189, 205, 208, 210, 212, 216
| |
| | |
| | |
| | |
| Trichina, 377, 378
| |
| | |
| Trichinidse, 371
| |
| | |
| Trichocepha'lus affinis, 374
| |
| | |
| Trochosphsera aequatorialis, 221
| |
| | |
| TubiporidcE, 182
| |
| | |
| Tubularia, 34, 38, 152, 154, 158
| |
| | |
| Tubularidse, 29, 179, 183
| |
| | |
| Tunicata, 5, I 4 , 53
| |
| | |
| Turbellaria, 5, 30, 31, 33, 74, 98, 102,
| |
| | |
| 136, 179, 189, 193, 333
| |
| Tyroglyphus, 445
| |
| | |
| Unio, 37, 38, 39, 100, 101, 259, 260,265,
| |
| 266, 445
| |
| | |
| Vaginulus luzonicus, 229
| |
| | |
| Vermes, 5, 74, 102, 223, 324, 352
| |
| | |
| Verongia rosea, 146
| |
| | |
| Vertebrata, 14, 18, 19, 24, 59, 64, 83,
| |
| | |
| 272, 349. 397' 4^6
| |
| Vesiculata, 184
| |
| Vitrina, 229
| |
| Vorticella, 8, 9, 10
| |
| | |
| Wilsia, 164
| |
| Xiphoteuthis, 252
| |
| | |
| Zoantharia, 152, 168, 169
| |
| Zooea, 465, 468, 471, 474, 482, 483, 484,
| |
| 486, 504
| |
| | |
| | |
| | |
| BIBLIOGRAPHY.
| |
| | |
| | |
| | |
| THE OVUM.
| |
| | |
| General Works.
| |
| | |
| (1) } Ed. van Beneden. "Recherches sur la composition et la signification de
| |
| ,A T m ' cour ' d ' l Acad " r y- des Sci <<* de Belgique, Vol. xxxiv. 1870.
| |
| | |
| / '%, R- Leuckart. Artikel "Zeugung," R. WagMsfs Handworterbtek d. Physio
| |
| logte, Vol. iv. 1853.
| |
| | |
| (3 ^ Fr ' L/ydig- , " Die Dotterfurchung nach ihrem Vorkommen in d. Thierwelt
| |
| u. n. ihrer Bedeutung." Oken. Isis, 1848.
| |
| | |
| (4) Ludwig. "Ueber d. Eibildung im Thierreiche." Arbeiten a. d. zool.-zoot
| |
| Institiit. Wiirzburg, Vol. I. rSy^.
| |
| | |
| (5) AllenThomson. Article ' ' Ovum " in Todd's Cyclopedia of Anatomy and
| |
| Physiology, Vol. v. 1859.
| |
| | |
| (6) W. Waldeyer. Eierstock u. EL Leipzig, 1870.
| |
| | |
| THE OVUM OF CCELENTERATA.
| |
| | |
| (7) Ed. van Beneden. "De la distinction originelle d. testicule et de
| |
| 1'ovaire." Bull. Acad. roy. Belgique, f serie, Vol. xxxvu. 1874.
| |
| | |
| (8) R. and O. Hertwig. Der Organismus d. Medusen. Jena, 1878.
| |
| | |
| (9) N. Kleinenberg. Hydra. Leipzig, 1872.
| |
| | |
| THE OVUM OF PLATYELMINTHES.
| |
| | |
| (10) P. Hallez. Contributions a fHistoire naturelle des Turbellarih. Lille,
| |
| 1879.
| |
| | |
| (11) S. MaxSchultze. Beitrdge z. Naturgeschichte d. Turbellarien. Greifswald, 1851.
| |
| | |
| (12) C. Th. von Siebold. ' ' Helminthologische Beitrage." Miiller's Archiv,
| |
| 1836.
| |
| | |
| (13) C. Th. von Siebold. Lehrbuch d. vergleich. Anat.d. wirbellosen Thiere.
| |
| Berlin, 1848.
| |
| | |
| (14) E. Zeller. " Weitere Beitrage z. Kenntniss d. Polystomen." Zeit. f.
| |
| wiss. ZooL, Bd. xxvu. 1876.
| |
| | |
| [Vide also Ed. van Beneden (No. i).]
| |
| | |
| THE OVUM OF ECHINODERMATA.
| |
| | |
| (15) C. K. Hoffmann. " Zur Anatomic d. Echiniden u. Spatangen." Niederllindisch. Archiv f. Zoologie, Vol. I. 1871.
| |
| | |
| (16) C. K. Hoffmann. " Zur Anatomic d. Asteriden. Niederldndisch. Ardiiv
| |
| /. Zoologie, Vol. n. 1873.
| |
| | |
| (17) H. Ludwig. "Beitrage zur Anat. d. Crinoiden." Zeil. f. wiss. Zool.,
| |
| Vol. xxvin. 1877.
| |
| | |
| (18) Job. Miiller. "Ueber d. Canal in d. Eiern d. Holothurien." Miiller's
| |
| Archiv, 1854.
| |
| | |
| (19) C. Semper. Holothurien. Leipzig, 1868.
| |
| | |
| (20) E. Selenka. Befruchtung d. Eies v. Toxopneustes variegalits, 1878.
| |
| | |
| [Vide also Ludwig (No. 4), etc.]
| |
| | |
| 1 A very complete and critical account of the literature is contained in this paper.
| |
| B. II. a
| |
| | |
| | |
| | |
| BIBLIOGRAPHY.
| |
| | |
| | |
| | |
| THE OVUM OF MOLLUSC A.
| |
| Lamellibranchiata.
| |
| | |
| (21) II. Lacaze-Duthiers. " Organes genitaux des Acephales Lamellibranches." Ann. Set. Nat., 4 mc serie, Vol. 1 1. 1854.
| |
| | |
| (22) W. F lemming. " Ueb. d. er. Entwick. am Ei d. Teichmuschel." Archiv
| |
| f. mikr. Anat., Vol. x. 1874.
| |
| | |
| (23) W. Flamming. "Studien lib. d. Entwick. d. Najaden." Sitz. d. t: Akad.
| |
| Wiss. men, Vol. LXXI. 1875.
| |
| | |
| (24) Th. von Hassling. " Einige Bemerkungen, etc." Zeit. f. wiss. ZooL,
| |
| Bd. v. 1854.
| |
| | |
| (25) H. von Jhering. "Zur Kenntniss d. Eibildung bei d. Muscheln." Zeit.
| |
| f. wiss. ZooL, Vol. xxix. 1877.
| |
| | |
| (26) Keber. De Introihi Spermatozoorum in ovula, etc. Konigsberg, 1853.
| |
| | |
| (27) Fr. Leydig. " Kleinere Mittheilung etc." Miiller's Archiv, 1854.
| |
| | |
| Gasteropoda.
| |
| | |
| (28) C. Semper. "Beitrage z. Anat. u. Physiol. d. Pulmonaten." Zeit. f.
| |
| wiss. ZooL, Vol. vni. 1857.
| |
| | |
| (29) H. Eisig. " Beitrage z. Anat. u. Entwick. d. Pulmonaten." Zeit.f. wiss.
| |
| ZooL, Vol. xix. 1869.
| |
| | |
| (30) Fr. Leydig. " Ueb. Paludina vivipara." Zeit.f. wiss. ZooL, Vol. u. 1850.
| |
| | |
| Cephalopoda.
| |
| | |
| (31) Al. Kolliker. Entwicklungsgeschichte d. Cephalopoden. Zurich, 1844.
| |
| | |
| (32) E. R. Lankester. "On the Developmental History of the Mollusca."
| |
| Phil. Trans., 1875.
| |
| | |
| THE OVUM OF THE CHJETOPODA.
| |
| | |
| (33) Ed. Claparede. " Les Annelides Chaetopodes d. Golfe de Naples."
| |
| Mem.d. 1. Soctit. phys. eld 1 hist. nat. de Geneve, 1868 9 and 1870.
| |
| | |
| (34) E. Ehlers. Die Borstcnwiirmer nach system, und anat. Untersuchungen.
| |
| Leipzig, 186468.
| |
| | |
| (35) E. Selenka. " Das Gefass-System d. Aphrodite aculeata." Niedcrldndisches Archiv f. ZooL, Vol. n. 1873.
| |
| | |
| THE OVUM OF DISCOPHORA.
| |
| | |
| (36) H. Dorner. " Ueber d. Gattung Branchiobdella." Zeit.f. wiss. ZooL,
| |
| Vol. xv. 1865.
| |
| | |
| (37) R. Leuckart. Die menschlichen Parasiten.
| |
| | |
| (38) Fr. Leydig. "Zur Anatomie v. Piscicola eeometrica, etc." Zeit. f. wiss.
| |
| ZooL, Vol. I. 1849.
| |
| | |
| (30) C. O. Whitman. "Embryology of Clepsine." Quart. 7. of Alter.
| |
| Sci., Vol. xvin. 1878.
| |
| | |
| THE OVUM OF GEPHYREA.
| |
| | |
| (40) Keferstein u. Ehlers. Zoologische Beitrage. Leipzig, 1861.
| |
| | |
| (41) C. Semper. Holothurien, 1868, p. 145.
| |
| | |
| (42) J. W. Spengel. " Beitrage z. Kenntniss d Gephyreen." Beitriigc a. d.
| |
| zool. Stationz. Neapcl, Vol. I. 1879.
| |
| | |
| (43) J. W. Spengel. " Anatomische Mittheilungen lib. Gephyreen." Tagcbl.
| |
| d. Naturf. Vers. Munchen, 1877.
| |
| | |
| THE OVUM OF NEMATODA.
| |
| | |
| (44) Ed. Claparede. De la formation ct de la fccondaiiou dcs- n-uf.\ chcz Ics
| |
| I'crs Ntmatodcs. (ienevc, 1859.
| |
| | |
| (J- r )) K. I. (.-nek art. Hif nirnsf/i lichen Paras! ten.
| |
| | |
| | |
| | |
| BIBLIOGRAPHY. jjj
| |
| | |
| | |
| | |
| d.Nematoden."
| |
| | |
| ^' Nels0n * " On the reproduction of Ascaris mystax, etc." Phil.
| |
| (48) A.Schneider. Monographie d.' Nematoden. Berlin, 1866.
| |
| THE OVUM OF INSECT A.
| |
| | |
| | |
| | |
| Sm ' T? r u n d V Ueb ,?'* a5 Ei u ' seine Bildungsstdtte. Leipzig, 1 878.
| |
| (50) T. H. Huxley. " On the agamic reproduction and morphology of Aphis.
| |
| Ltnnean Trans., Vol. xxn. 1858. Vide also Manual of Invertebrate* Animals, 1877.
| |
| | |
| 1 * ^ ^ ^ *
| |
| | |
| | |
| | |
| (51)
| |
| bei den *,++,*
| |
| | |
| /-a\ ? r ',k ey< MS' Der Eierstock u. die Samentasche d. Insecten. Dresden, 1866.
| |
| tSl ~ ub . bock - " The ov a and pseudova of Insects." Phil. Trans. 1850.
| |
| (o4) Stem. Die weiblichen Geschlcchtsorgane d. Ktifer. Berlin, 1847.
| |
| [Conf. also Glaus, Landois, Weismann, Ludwig (No. 4).]
| |
| | |
| THE OVUM OF ARANEINA.
| |
| | |
| (55) Victor Cams. " Ueb. d. Entwick. d. Spinneneies." Zeit. f. wiss. Zool. t
| |
| Vol. ii. 1850.
| |
| | |
| (56) v. Wittich. "Die Entstehung d. Arachnideneies im Eierstock, etc."
| |
| Miiller s Archiv, 1849.
| |
| | |
| [Conf. Leydig, Balbiani, Ludwig (No. 4), etc.]
| |
| | |
| THE OVUM OF CRUSTACEA.
| |
| | |
| (57) Aug. Weismann. "Ueb. d. Bildung von Wintereiern bei Leptodora
| |
| hyalina." Zeit.f. wiss.ZooL, Vol. xxvn. 1876.
| |
| | |
| [For general literature vide Ludwig, No. 4, and Ed. van Beneden, No. i.]
| |
| | |
| THE OVUM OF CHORD ATA.
| |
| | |
| Urochorda (Tunicata).
| |
| | |
| (58) A. Kowalevsky. " Weitere Studien ii. d. Entwicklung d. Ascidien."
| |
| Archiv f. micr. Anat., Vol. VII. 1871.
| |
| | |
| (59) A. Kowalevsky. "Ueber Entwicklungsgeschichte d. Pyrosoma."
| |
| Arch.f. micr. Anat., Vol. xi. 1875.
| |
| | |
| (60) Kupffer. " Stammverwandtschaft zwischen Ascidien u. Wirbelthieren."
| |
| Arch. f. micr. Anat., Vol. VI. 1870.
| |
| | |
| (61) Giard. " Etudes critiques des travaux, etc. " Archives Zool. experiment.,
| |
| Vol. I. 1872.
| |
| | |
| (62) C. Semper. " Ueber die Entstehung, etc." Arbeiten a. d. zool.-zoot.
| |
| Institut Wiirzburg, Bd. II. 1875.
| |
| | |
| Cephalochorda.
| |
| | |
| (63) P. Langerhans. "Z. Anatomic d. Amphioxus lanceolatus," pp. 330 3.
| |
| Archiv f. mikr. Anat., Vol. xil. 1876.
| |
| | |
| Craniata.
| |
| | |
| (64) F. M. Balfour. "On the structure and development of the Vertebrate
| |
| Ovary." Quart. J. of Micr. Science, Vol. xvm. 1878.
| |
| | |
| (65) Th. Eimer. " Untersuchungen ii. d. Eier d. Reptilien." Arckiv f.
| |
| mikr. Anat., Vol. vni. 1872.
| |
| | |
| (66) Pfliiger. Die Eierstbcke d. Sdugethiere u. d. Menschen. Leipzig, 1863.
| |
| | |
| (67) J. Foulis. " On the development of the ova and structure of the ovary in
| |
| Man and other Mammalia." Quart. J. of Micr. Science, Vol. XVI. 1876.
| |
| | |
| (68) J. Foulis. " The development of the ova, etc." Journal of Anat. and
| |
| Phys., Vol. xni. 18789.
| |
| | |
| a 2
| |
| | |
| | |
| | |
| IV BIBLIOGRAPHY.
| |
| | |
| | |
| | |
| (69) C. Gegenbaur. " Ueb. d. Bau u. d. Entwicklung d. Wirbelthiereier mit
| |
| partieller Dottertheilung." Muller's Archiv, 1861.
| |
| | |
| (70) Alex. Gotte. Entwicklungsgeschichte d. Unke. Leipzig, 1875.
| |
| | |
| (71) W. His. Untersuchungen iib. d. Ei u. d. Eientwicklung bei Knochenfischcn.
| |
| Leipzig, 1873.
| |
| | |
| (72) A. Kolliker. Entwicklungsgeschichte d. Menschen u. hoherer Thicre,
| |
| Leipzig, 1878.
| |
| | |
| (73) J. Miiller. " Ueber d. zahlreichen Porenkanale in d. Eikapsel d. Fische."
| |
| Muller's Archiv, 1854.
| |
| | |
| (74) W. H. Ransom. " On the impregnation of the ovum in the Stickleback."
| |
| Pro. K. Society, Vol. vn. 1854.
| |
| | |
| (75) C. Semper. " Das Urogenitalsystem d. Plagiostomen etc." Arbeiten a.
| |
| d. zool.-zoot. Instit. Wiirzburg, Vol. II. 1875.
| |
| | |
| [Cf. Ludwig, No. 4, Ed. van Beneden, No. i, Waldeyer, No. 6, etc.]
| |
| | |
| | |
| | |
| MATURATION AND IMPREGNATION OF THE OVUM.
| |
| | |
| (76) Auerbach. Organologische Studien, Heft 2. Breslau, 1874.
| |
| | |
| (77) Bambeke. " Recherches s. Embryologie des Batraciens." Bull, de
| |
| royale de Belgique, 2me ser., T. LXI. 1876.
| |
| | |
| (78) E. van Beneden. " La Maturation de 1'CEufdes Mammiferes." Bull,
| |
| de fAcad. royale de Belgique, 2me ser., T. XL. No. 12, 1875.
| |
| | |
| (79) Id em. " Contributions a 1'Histoire de la Vesicule Germinative, &c." Bull,
| |
| de fAcad. royale de Belgique, sme ser., T. XLI. No. i, 1876.
| |
| | |
| (80) O. Biitschli. Eizelle, Zelltheilung, und Conjugation der Infusorien.
| |
| Frankfurt, 1876.
| |
| | |
| (81) F. M. Balfour. " On the Phenomena accompanying the Maturation and
| |
| Impregnation of the Ovum." Quart. J. of Micros. Science, Vol. xvm. 1878.
| |
| | |
| (82) Calberla. " Befruchtungsvorgang beim Ei von Petromyzon Planeri.*'
| |
| Zeit. f. iviss. Zool., Vol. xxx.
| |
| | |
| (83) W. Flemming. "Studien in d. Entwickelungsgeschichte der Najaden."
| |
| Sitz. d. k. Akad. Wiett, B. LXXI. 1875.
| |
| | |
| (84) H. Fol. "Die erste Entwickelung des Geryonideneies. " Jenaische
| |
| Zeitschrift, Vol. vn. 1873.
| |
| | |
| (85) Idem. " Sur le Developpement des Pte"ropodes." Archives de Zoologic
| |
| Experimental et Gtnerale, Vol. iv. and v. 1875 6.
| |
| | |
| (86) Idem. " Sur le Commencement de 1'Henog^nie." Archives des Sciences
| |
| Physiques et Naturelles. Geneve, 1877.
| |
| | |
| (87) Idem. Recherches s. I. Ftcondation etl. comrnen. d. rHcnogcnic. Geneve, 1879.
| |
| | |
| (88) R. Greeff. " Ueb. d. Bau u. d. Entwickelung d. Echinodermen." Sitzun.
| |
| der Gesellschaft z. Befonlerung d. gesammten Naturwiss. z. Marburg, No. 5, 1876.
| |
| | |
| (89) Oscar Hertwig. " Beit. z. Kenntniss d. Bildung, &c., d. thier. Eies."
| |
| Morphologisches Jahrbuch, Vol. I. 1876.
| |
| | |
| (90) Idem. Ibid. Morphologisches Jahrlntch, Vol. ill. Heft i, 1877.
| |
| | |
| (91) Idem. " Weitere Beitrage, &c." Morphologisches Jahrbuch, Vol. in. 1877.
| |
| Heft 3.
| |
| | |
| (92) Idem. "Beit. z. Kenntniss, &c." Morphologisches Jahrbuch, Vol. iv.
| |
| Heft i and 2. 1878.
| |
| | |
| (93) N. Kleinenberg. Hydra. Leipzig, 1872.
| |
| | |
| (94) C. Kupffer u. B. Benecke. Der Vorgang d. llcfnichtinig am Eie d.
| |
| Neunaugen. Konigsberg, 1878.
| |
| | |
| (95) J. Oellacher. "Beitrage zur Geschichte des Keimblaschens im Wirbelthicreie." Archiv f. micr. Anat., Bd. VIII. 1872.
| |
| | |
| (%) W. Salensky. " Befruchtung u. P^urchung d. Sterlets-Eies." Zoologischer Anzeigcr, No. 11, 1878.
| |
| | |
| (97) E. Selenka. Befruchtung des Eies von Toxopncustcs variegatus. Leipzig,
| |
| 1878.
| |
| | |
| fl Strasburger. Ucber Zclllnldu n- n. /.clltln ////;/;. Ji-na, 1876.
| |
| | |
| Idem. Utber Befrvehtung u. Zdlthdhing. Jena, 1X78.
| |
| | |
| (HiO) C. (). \V hi tin.in. "Tlic- Kniliryology of Clepsine." Quart. J. of A/i<r.
| |
| Science, Vol. xvm. 1878.
| |
| | |
| | |
| | |
| BIBLIOGRAPHY.
| |
| | |
| | |
| | |
| DIVISION OF NUCLEUS.
| |
| | |
| (101) W. Flamming. "Beitrage z. Kenntniss d. Xclle u. ihrcr Lcbun.scrschcinungen." Archiv f. mikr. Anat., Vol. xvi. 1878.
| |
| | |
| (102) E. Klein. "Observations on the glandular epithelium and division of
| |
| nuclei in the skin of the Newt." Quart, y. of Micr. Science, VoL XIX. 1879.
| |
| | |
| (103) Peremeschko. "Ueber d. Theilung d. thierischen Zellen." Archiv f.
| |
| mikr. Anat., Vol. xvi. 1878.
| |
| | |
| (104) E. Strasburger. "Ueber ein z. Demonstration geeignetes ZclltheilungsObject." Sitz. d. Jenaischen Gesell.f. Med. u. Naturwiss., July 18, 1879.
| |
| | |
| SEGMENTATION.
| |
| | |
| (105) E. Haeckel. "Die Gastrula u. Eifurchung." Jenaische Ztitschrift, Vol.
| |
| ix. 1877.
| |
| | |
| (106) Fr. Leydig. "Die Dotterfurchung nach ihrem Vorkommen in d. Thierwelt u. n. ihrer Bedeutung." Oken. /sis, 1848.
| |
| | |
| GENERAL WORKS ON EMBRYOLOGY.
| |
| | |
| (107) K. E. von Baer. " Ueb. Entwicklungsgeschichte d. Thiere." Konigsberg, 182837.
| |
| | |
| (108) C. Glaus. Grundziige d. Zoologie. Marburg und Leipzig, 1879.
| |
| | |
| (109) C. Gegenbaur. Grundriss d. vergleichenden Anatomie. Leipzig, 1878.
| |
| Vide also Translation. Elements of Comparative Anatomy. Macmillan and Co.,
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| 1 There is a Russian paper by the same author, containing a full account, with
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| clear illustrations, of his observations.
| |
| | |
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| BIBLIOGRAPHY, vii
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| (226) R. Leuckart. Die menschlichen Parasiten, Vol. I. Leipzig, 1863. Vide
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| ROTIFERA.
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| (232) F. Cohn. "Ueb. d. Fortpflanzung von Raderthiere." Zeit.f. wiss.
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| ^(233) F. Cohn. "Bemerkungen u. Raderthiere." Zeit.f. wiss. Zool., Vol. IX.
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| Zeit.f. wiss. Zool., Vol. xxn. 1872.
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| | |
| MOLLUSCA.
| |
| | |
| General.
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| (488) H. Reichenbach. " Die Embryoanlage u. erste Entwicklung d. Flusskrebses." Zeit. f. wiss. Zool., Vol. xxix., 1877.
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| (489) F. Richters. "Ein Beitrag zur Entwicklungsgeschichte d. Loricaten."
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| Zeit.f. wiss. Zool., Bd. xxiii., 1873.
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| (490) G. O. Sars. " Om Hummers postembryonale Udvikling." Vidcnsk Selsk.
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| Forh. Christiania, 1874.
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| (491) Sidney J. Smith. " The early stages of the American Lobster. " Tratts.
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| of the Connecticut Acad. of Arts and Sciences, Vol. II., Part i, 1873.
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| (492) R. v. Willemoes Suhm. " Preliminary note on the development of some
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| pelagic Decapoda." Proc. of Royal Society, 1876.
| |
| | |
| | |
| | |
| XX BIBLIOGRAPHY.
| |
| | |
| | |
| | |
| Stomatopoda.
| |
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| (41KI) \V. K. Brooks. " On the larval stages of Squilla empusa.'' Chesapeake
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| Zoological Laboratory^ Scientific results of the Session ^1878. Baltimore, 1879.
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| (494) C. Claus. "Die Metamorphose der Squilliden." Abhand. dcr konigl.
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| Gesell. der IViss. ztt Gottingen^ 1*7-.
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| ( 1 '.!">) Fr. M tiller. 4i Bruchstuck a. der Entwicklungsgeschichte d. Maulfiisser I.
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| und II." Archivf. Naturgeschichte, Vol. xxvni., 1862, and Vol. XXIX., 1863.
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| Cumacea*
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| ( 1%) A. Dohrn. " Ueber den Bau u. Entwicklung d. Cumaceen." Jenaische
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| Zeitschrift, Vol. v., 1870.
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| hopoda.
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| (497) Ed. van Beneden. " Recherches sur 1'Embryogenie des Crustaces. i.
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| Asellus aquaticus." BiuL de FAcad. roy. Belgique, 2me serie, Tom. XXVIII., No. 7,
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| 1869.
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| (498) N. Bobretzky. "Zur Embryologie des Oniscus murarius." Ztit. fur
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| wiss. Zool., Bd. xxiv., 1874.
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| (4119) J. F. Bullar. "On the development of the parasitic Isopoda." Phil.
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| Trans., Part n., 1878.
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| (500) A. Dohrn. " Die embryonale Entwicklung des Asellus aquaticus." Zeit.
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| f. wiss. Zool., Vol. xvii., 1867.
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| (501) II. Rathke. Untersuchungen iibcr die Bildung und Entwicklung der
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| VVasser-Assel. Leipzig, 1832.
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| (5u2) H. Rathke. Zur Morphologic. Reisebemerkungen aus J^aurien. Riga u.
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| Leipzig, 1837. (Bopyrus, Idothea, Ligia, lanira.)
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| | |
| . A mphipoda.
| |
| | |
| (503) Ed. van Beneden and E. Bessels. "Memoire sur la formation du
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| blastoderme chez les Amphipodes, les Lerneens et les Copepodes." Classe des Sciences
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| de F Acad. roy. de Belgiqtie, Vol. xxxiv., 1868.
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| | |
| (004) De la Valette St George. " Studien liber die Entwicklung der Amphipoden." Abhand. d. naturfor. Gesell. zu Halle, Bd. v., 1860.
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| | |
| Copcpoda.
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| (505) E. van Beneden and E. Bessels. "Memoire sur la formation du blastoderme chez les Amphipodes, les Lerneens et Copepodes." Classe des Sciences dc
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| FAcad. roy. de Bel^ique, Vol. xxxiv., 1868.
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| | |
| (">(Hi) E. van Beneden. " Recherches sur 1'Embryoge'nie des Crustaces I v. Anchorella, Lerneopoda, Branchiella, Hessia." Bull, de FAcad. roy. de Belgique^ sme
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| serie, T. xxix., 1870.
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| (507) C. Claus. Zur Anatomie u. Entwicklungsgeschichte d. Copepoden.
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| ("iii.S) C. Claus. " Untcrsuchungen iiber die Organisation u. N'crwaiulschaft d.
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| Copepoden." Witrzburger nalttnviss. Zeitschrift, Bd. ill., 1862.
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| | |
| (.")('.() C. Claus. ' Ueber den Bau u. d. Entwicklung von Achtheres percarum."
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| '/.cit.f. wiss. Zool., Bd. XI., 1862.
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| | |
| I ."> 1 ( i ) C . (' 1 a u s. Die frcilcbcnden Copepoden mit bcsonderer Beritcksichtigiing der
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| Fauna Dcutschlands, der Nordsec u. des Mitteltnecres. Leip/.i.^, 1863.
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| (511) C. C laus. " Ueber d. Entwicklung, Organisation u. systematische Stellung
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| d. Arguliihv." /.eit. f. wiss. tool., P>d. xxv., 1875.
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| | |
| (51^) P. P. C. Hoek. "Zur Entwicklungsgeschichte d. Entomostracen." Niederliindischcs Archiv, Vol. IV., 1877.
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| (513) N o rd m a n n. Mikrographische Beitrdge zur Naturgeschichte der ivirbcllosen
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| Thiert Z\\ cites Heft. 1832.
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| |.">M) Salensky. " Sphseronella Leuckartii." Archivf. Naturgcschichtc, 1868.
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| (515) F. Vejdovsky. "Untersuchnngen lib. d. Anat. u. Mctamorph. v. Trachcliastes polycolpus " Zcit.f. wiss. Zool., Vol. xxix., 1877.
| |
| | |
| | |
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| BIBLIOGRAPHY. xxi
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| | |
| | |
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| Cirripedia.
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| (516) C. Spence Bate. "On the development of the Cirripedia." Annals
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| and Mag. of Natur. History. Second Series, vm., 1851.
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| (517) E. van Beneden. " Developpement des Sacculines." Bull, de F Acad.
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| roy. de Belg., 1870.
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| (518) C. Claus. Die Cypris-dhnliehe Larve der Cirripedien. Marburg, 1869.
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| (519) Ch. Darwin. A monograph of the sub-class Cirripedia, i Vols., Kay
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| Society, 1851 4.
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| (520) A. Dohrn. " Untersuchungen iibcr Bau u. Entwicklung d. Arthropoden
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| ix. Eine neue Naupliusform (Archizoea gigas)." Zeit. f. wiss. Zool., Bd. XX.,
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| 1870.
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| | |
| (521) P. P. C. Hoek. "Zur Entwicklungsgeschichte der Entomostraken I.
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| Embryologie von Balanus." Wiederlandisches Archiv fur Zoologic, Vol. III., 1876 7.
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| (522) R. Kossmann. " Suctoria u. Lepadidze. Arbeiten a. d. zool.-zoot. Instituted. Univer. Wiirz., Vol. I., 1873.
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| (523) Aug. Krohn. " Beobachtungen iiber die Entwicklung der Cirripedien."
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| Wiegmanrfs Archiv fur Naturgesch., xxvi., 1860.
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| | |
| (524) E. Metschnikoff. Sitzungsberichte d. Versammlung deutscher Naturforscher zu Hannover ; 1865. (Balanus balanoides.)
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| | |
| (525) Fritz Muller. "Die Rhizocephalen." Archiv /. Naturgeschichte,
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| 18623.
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| (526) F. C. Noll. "Kochlorine hamata, ein bohrendes Cirriped." Zeit.f. wiss.
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| Zool., Bd. xxv., 1875.
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| | |
| (527) A. Pagenstecher. " Beitrage zur Anatomic und Entwicklungsgeschichte
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| von Lepas pectinata." Zeit.f. wiss. Zoo/., Vol. xin., 1863.
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| | |
| (52tt) J. V. Thompson. Zoological Researches and Illustrations, Vol. I., Part I.
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| Memoir iv. On the Cirripedes or Barnacles. 8vo. Cork, 1830.
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| (529) J. V. Thompson. " Discovery of the Metamorphosis in the second type
| |
| of the Cirripedes, viz. the Lepades completing the natural history of these singular
| |
| animals, and confirming their affinity with the Crustacea." Phil. Trans. 1835. P art
| |
| n.
| |
| | |
| (530) R. von Willemoes Suhm. "On the development of Lepas fascicularis."
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| Phil. Trans., Vol. 166, 1876.
| |
| | |
| Ostracoda.
| |
| | |
| (531) C. Claus. " Zur naheren Kenntniss der Jugendformen von Cypris ovum."
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| Zeit.f. wiss. Zool., Bd. XV., 1865.
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| | |
| (532) C. Claus. "Beitrage zur Kenntniss d. Ostracoden. Entwicklungsgeschichte von Cypris ovum." Schriften d. Gesell. zur Bejorderung d. gesamm. Naturwiss. zu Marburg, Vol. IX., 1868.
| |
| | |
| PCECILOPODA.
| |
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| (533) A. Dohrn. "Untersuch. lib. Bau u. Entwick. d. Arthropoden (Limulus
| |
| polyphemus)." Jcnaische Zeitschrift, Vol. VI., 1871.
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| | |
| (534) A. S. Packard. "The development of Limulus polyphemus." Mem.
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| Boston Soc. Nat. History, Vol. II., 1872.
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| | |
| PYCNOGONIDA.
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| (535) G. C a van n a. " Studie e ricerche sui Picnogonidi." PiMIicazioni del R.
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| Instittito di Studi super iori in Firenze, 1877.
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| (536) An. Dohrn. " Ueber Entwicklung u. Bau d. Pycnogoniden." Jenaische
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| Zeitschrift, Vol. v. 1870, and "Neue Untersuchungen ub. Pycnogoniden." Mittheil.
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| a. d. zoologischen Station zu Ncapel, Bd. I. 1878.
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| | |
| (537) G. Hodge. " Observations on a species of Pycnogon, etc." Annal. and
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| Mag. of Nat. Hist. Vol. ix. 1862.
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| (538) C. Semper. " Ueber Pycnogoniden u. ihre in Hydroiden schmarotzenden
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| Larvenformen. 1 ' Arbeiten a. d. zool.-zoot. Instit. IViiizburg, Vol. I. 1874.
| |
| | |
| | |
| | |
| xxii BIBLIOGRAPHY.
| |
| | |
| | |
| | |
| PENTASTOMIDA.
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| (539) P. T. van Ben e den. " Recherches s. 1'organisation et le developpement d.
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| Linguatules. Ann. d. Scien. Nat., 3 Ser., Vol. XI.
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| | |
| ("I'M R. Leuckart. " Bau u. Entwicklungsgeschichte d. Pentastomen." Leipzig
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| and Heidelberg. 1860.
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| | |
| TARDIGRADA.
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| (541) J. Kaufmann. " Ueber die Entwicklung u. systematische Stellung d.
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| Tardigraden." Zeit.f. iviss. Zool., Bd. ill. 1851.
| |
| | |
| ECHINODERMATA.
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| (542) Alex. Agassiz. Revision of the Echini. Cambridge, U.S. 1872 74.
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| | |
| (543) Alex. Agassiz. " North American Starfishes." Memoirs of the Museum
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| of Comparative Anatomy and Zoology at Harvard College, Vol. v., No. i. 1877
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| (originally published in 1864).
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| (544) J. Barrois. " Embryogenie de 1'Asteriscus verruculatus " Journal de
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| VAnat. et Phys. 1879.
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| (545) A. Baur. Beitrdge zur Naturgeschichte d. Synapta digitata. Dresden,
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| 1864.
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| (546) H. G. Bronn. Kiassen u. Ordnungen etc. Strahlenthiere, Vol. II. 1860.
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| (547) W. B. Carpenter. "Researches on the structure, physiology and development of Antedon." Phil. Trans. CLVI. 1866, and Proceedings of the Roy. Soc.,
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| No. 166. 1876.
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| (548) P. H. Carpenter. " On the oral and apical systems of the Echinoderms."
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| Quart. J. of Micr. Science, Vol. xvni. and xix. 1878 9.
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| (549) A. Gotte. " Vergleichende Entwicklungsgeschichte d. Comatula mediterranea." Arch.fiir micr. Anat., Vol. xn. 1876.
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| (550) R. Greeff. "Ueber die Entwicklung des Asteracanthion rubens vom Ei
| |
| bis zur Bipinnaria u. Brachiolaria." Schriftcn d. Gesellschaft zur Beforderung d. gesarnmlen Natnrwissenschaften zu Marburg, Bd. XII. 1876.
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| (551) R. Greeff. "Ueber den Bau u. die Entwicklung d. Echinodermen." Sitz.
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| d. Gesell. z. Beforderung d. gesam. Naturwiss. zu Marburg, No. 4. 1879.
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| (552) T. H. Huxley. "Report upon the researches of Mliller into the anat.
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| and devel. of the Echinoderms." Ann. and Mag. of Nat. Hist., 2nd Ser., Vol. vin.
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| (553) Koren and Danielssen. "Observations sur la Bipinnaria asterigera."
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| Ann. Scien. Nat., Ser. in., Vol. VII. 1847.
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| i-">l) Koren and Uanielssen. "Observations on the development of the Starfishes." Ann. and Mag. of Nat. Hist., Vol. XX. 1857.
| |
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| (."..",.",) A. Kowalevsky. "Entwicklungsgeschichte d. Holothurien." Aft m. Ac.
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| Petersburg, Ser. VII., Tom. XI., No. 6.
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| ("'"><') A. Krohn. "Beobacht. a. d. Entwick. d. Holothurien u. Seeigel."
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| M tiller's Archiv, 1851.
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| (Vi7) A. Krohn. " Ueb. d. Entwick. d. Seesterne u. Holothurien." Muller's
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| Ardiir,
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| A. Krohn. "Beobacht. lib. Echinodermenlarven." Miiller's Archiv,
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| 18(4.
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| ('>'>'.)} II. Ludwig. "Ueb. d. primar. Steinkanal d. Crinoideen, nebst vergl.
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| anat. Bemerk. ub. d. Echinodermen." Zeit.f. wiss. Zoo/., Vol. xxxiv. iSSo.
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| (">r,n) K. Metschnikoff. "Studien lib. d. Entwick. d. Echinodermen u.
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| Nemertinen." Mem. .!< . J', : /i'rsfa>nr?. Scries VII., Tom. XIV., No. 8. 1869.
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| (501)' Joh. Miillcr. " Ueb. d. Larven u. d. Metamorphose d. EchinodcM men.'
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| Alhandlnng,-n d. />>///'. Akad. (Five Memoirs), 1848, 49, 50. 52 (two Mnnoirs).
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| | |
| Joh. Miiller. " Allgemeincr Plan d. Entwicklung d. Echinodermen.
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| Abhandl. d. Berlin. Akad., 1853.
| |
| | |
| 1 The dates in this reference are the dates of publication.
| |
| | |
| | |
| | |
| BIBLIOGRAPHY. xxiil
| |
| | |
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| (563). E. Selenka. "Zur Entwicklung d. Holothurien." Ztit. f. wiss. Zoo/.,
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| Ed. xxvn. 1876.
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| (564) E. Selenka. "Keimblatter u. Organanlage bei Echiniden." Zeit.f.wiss.
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| Zool.t Vol. xxxni. 1879.
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| (565) Sir Wyville Thomson. " On the Embryology of the Echinodcrmata."
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| Natural History Review, 1864.
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| | |
| (566) Sir Wyville Thomson. "On the Embryogeny of Antedon rosacetw."
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| Phil. Trans. 1865.
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| | |
| ENTEROPNEUSTA.
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| (567) A. Agassiz. "Tornaria." Ann. Lyceum Nat. Hist. vin. New York,
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| 1866.
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| (568) A. Agassiz. "The History of Balanoglossus and Tornaria." Mem.
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| Amer. Acad. of Arts and Scien., Vol. IX. 1873.
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| (569) A. Gotte. " Entwicklungsgeschichte d. Comatula Mediterranea. " Archiv
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| fur mikr. Anat., Bd. XII., 1876, p. 641.
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| | |
| (570) E. Metschnikoff. " Untersuchungen lib. d. Metamorphose, etc. (Tornaria)." Zeit.fiir wiss. Zool., Bd. XX. 1870.
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| (571) J. M tiller. " Ueb. d. Larven u. Metamor. d. Echinodermen." Berlin.
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| Akad., 1849 and 1850.
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| (572) J. W. Spengel. "Bau u. Entwicklung von Balanoglossus." Tagebl.d.
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| Naturf. Vers. Mtinchen, 1877.
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| [[Category:Historic Embryology]][[Category:1800's]]
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