Difference between revisions of "The Works of Francis Balfour 2-15"

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==CHAPTER XV. Chatognatha, MYZOSTOMEA AND GASTROTRICHA==
 +
 +
THE present chapter deals with three small isolated groups,
 +
which only resemble each other in that the systematic position of
 +
all of them is equally obscure.
 +
 +
Chatognatha.
 +
 +
The discoveries of Kowalevsky (No. 378) confirmed by
 +
Btitschli (No. 376) with reference to the development of Sagitta,
 +
though they have not brought us nearer to a knowledge of the
 +
systematic position of this remarkable form, are nevertheless of
 +
 +
 +
 +
 +
FIG. 164. THREE STAGES IN THE DEVELOPMENT OF SAGITTA. (A and C after
 +
Hiitschli and B after Kowalevsky.) The three embryos are represented in the same
 +
positions.
 +
 +
A. The gastrula stage.
 +
 +
li. A succeeding stage in which the primitive archenteron is commencing to be
 +
divided into three parts, the two lateral of which are destined to form the body
 +
cavity.
 +
 +
C. A later stage in which the mouth involution (/;/) has become continuous with
 +
the alimentary tract, and the blastopore has become closed.
 +
 +
m. mouth; al. alimentary canal ; ae. archenteron ; bl.p. blastopore; pv. perivisceral
 +
cavity; sf>. splanchnopleuric mesoblast; so. somatopleuric mesoblast ; ge. generative
 +
organs.
 +
 +
 +
 +
CH^TOGNATHA. 367
 +
 +
 +
 +
great value for the more general problems of embryology. The
 +
development commences after the eggs are laid. The segmentation is uniform, and a blastosphere, formed of a single layer of
 +
columnar cells, is the product of it. An invagination takes
 +
place, the opening of which narrows to a blastopore situated
 +
at the pole of the embryo opposite that at which the mouth
 +
subsequently appears (fig. 164 A). The simple archenteron soon
 +
becomes anteriorly divided into three lobes, which communicate
 +
freely with the still single cavity behind (fig. 164 B). The two
 +
lateral lobes are destined to form the body cavity, and the
 +
median lobe the alimentary tract of the adult. An invagination
 +
soon arises at the opposite pole of the embryo to the blastopore
 +
and forms the mouth and oesophagus (fig. 164 B and C, m).
 +
 +
At the gastrula stage there is formed a paired mass destined
 +
to give rise to the generative organs. It arises as a prominence
 +
of six cells, projecting from the hypoblast at the anterior pole of
 +
the archenteron, and soon separates itself as a mass, or probably
 +
a pair of masses, lying freely in the cavity of the archenteron
 +
(fig. 164 A. y ge). When the folding of the primitive cavity takes
 +
place the generative rudiment is situated at the hind end of the
 +
median lobe of the archenteron in the position represented
 +
in fig. 164 C, ge.
 +
 +
An elongation of the posterior end of the embryo now takes
 +
place, and the embryo becomes coiled up in the egg, and when
 +
eventually hatched sufficiently resembles the adult to be recognisable as a young Sagitta.
 +
 +
Before hatching takes place various important changes
 +
become manifest. The blastopore disappears after being carried
 +
to the ventral surface. The middle section of the trilobed region
 +
of the archenteron becomes separated from the unpaired
 +
posterior part, and forms a tube, blind behind, but opening
 +
in front by the mouth (fig. 165 A, al). It constitutes the permanent alimentary tract, and is formed of a pharyngeal epiblastic
 +
invagination, and a posterior hypoblastic section derived from
 +
the primitive archenteron. The anus is apparently not formed
 +
till comparatively late. After the isolation of the alimentary
 +
tract the remainder of the archenteron is formed of two cavities
 +
in front, which open freely into a single cavity behind (fig.
 +
165 A). The whole of it constitutes the body cavity and its walls
 +
 +
 +
 +
3 68
 +
 +
 +
 +
CH/ETOGNATHA.
 +
 +
 +
 +
f/ic mesoblast. The anterior paired part becomes partitioned off
 +
into a head section and a trunk section (fig. 165 A and B). The
 +
former constitutes a pair of distinct cavities (c.pv) in the head,
 +
and the latter two cavities opening freely into the unpaired
 +
portion behind. At the junction of the paired cavities with the
 +
unpaired cavity are situated the generative organs (ge). The
 +
inner wall of each of the paired cavities forms the splanchnopleuric mesoblast, and the outer wall of the whole the somatic
 +
mesoblast. The inner walls of the posterior cavities unite above
 +
and below the alimentary tract, and form the dorsal and ventral
 +
mesenteries, which divide the body cavity into two compartments
 +
in the adult. Before the hatching of the embryo takes place
 +
this mesentery is continued backwards so as to divide the
 +
primitively unpaired caudal part of the body cavity in the
 +
same way.
 +
 +
From the somatic mesoblast of the trunk is derived the
 +
single layer of longitudinal muscles of Sagitta, and part of the
 +
epithelioid lining of the body cavity. The anterior termination
 +
of the trunk division of the body cavity is marked in the adult
 +
by the mesentery dividing into two laminae, which bend outwards
 +
to join the body wall.
 +
 +
The cephalic section of the body cavity
 +
seems to atrophy, and
 +
its walls to become converted into the complicated system of muscles
 +
present in the head of
 +
the adult Sagitta.
 +
 +
In the presence of
 +
a section of the body
 +
cavity in the head the
 +
embryo of Sagitta re
 +
sembles Lumbricus,
 +
Spiders, etc.
 +
 +
The generative rudiment of each side
 +
divides into an anterior
 +
and a posterior part
 +
 +
 +
 +
 +
In;. [65. Two VIEWS OF A LATE EMBRYO OF
 +
SV.ITTA. A. from the dorsal surface. I?, from the
 +
tide. (After 15iitschli.)
 +
 +
m. mouth ; al. alimentary canal ; v.g. ventral
 +
ganglion (thickening of epiblast) ; rp. epiblast ; c.pv,
 +
cephalic section of body cavity; so. somatopleure ;
 +
s/>. splanchnopleure ; ,;v. generative
 +
 +
 +
 +
CH^ETOGNATHA. 369
 +
 +
 +
 +
(fig. 165, ge]. The former constitutes the ovary, and is situated
 +
in front of the septum dividing the tail from the body ; and the
 +
latter, in the caudal region of the trunk, forms the testis.
 +
 +
The nervous system originates from the epiblast. There is a
 +
ventral thickening (fig. 165 B, v.g) in the anterior region of the
 +
trunk, and a dorsal one in the head. The two are at first
 +
continuous, and on becoming separated from the epiblast remain
 +
united by thin cords.
 +
 +
The ventral ganglion is far more prominent during embryonic
 +
life than in the adult. Its position and early prominence in the
 +
embryo perhaps indicate that it is the homologue of the ventral
 +
cord of Chaetopoda 1 .
 +
 +
BIBLIOGRAPHY.
 +
 +
(376) O. Biitschli. "Zur Entwicklungsgeschichte der Sagitta." Zeitschrift f.
 +
wiss. Zoo!., Vol. xxni. 1873.
 +
 +
(377) C. Gegenbaur. " Uber die Entwicklung der Sagitta." Abhand. d. naturforschenden Gesellschaft in Halle, 1857.
 +
 +
(378) A. Kowalevsky. " Embryologische Studien an Wiirmern u. Arthropoden." Mem. Acad. Petersbourg, VII. ser., Tom. XVI., No. 12. 1871.
 +
 +
MYZOSTOMEA.
 +
 +
The development of these peculiar parasites on Crinoids has been
 +
investigated by Metschnikoff (No. 380), Semper (No. 381), and Graff
 +
(No. 379).
 +
 +
The segmentation is unequal, and would appear to be followed by an
 +
epibolic invagination. The outer layer of cells (epiblast) becomes covered
 +
with cilia, and the inner is transformed into a non-cellular (?) central yolk
 +
mass. At this stage the larva is hatched, and commences to lead a free
 +
existence. In the next stage observed by Metschnikoff, the mouth, oesophagus, stomach, and anus had become developed ; and two pairs of feet
 +
were present. In both of these feet Chaetopod-like setae were present, which
 +
in the hinder pair were simple fine bristles without a terminal hook. The
 +
papilliform portion of the foot is at first undeveloped. The feet become
 +
successively added, like Chaetopod segments, and the stomach does not
 +
become dendriform till the whole complement of feet (5 pairs) are present.
 +
 +
In the primitive covering of cilia, combined with a subsequent indication
 +
 +
1 Langerhans has recently made some important investigations on the nervous
 +
system of Sagitta, and identifies the ventral ganglion with the parieto-splanchnic ganglia of Molluscs, while he has found a pair of new ganglia, the development of which
 +
is unknown, which he calls the suboesophageal or pedal ganglia. The embryological facts do not appear to be in favour of these interpretations.
 +
 +
B. II. 24
 +
 +
 +
 +
3/0 MYZOSTOMEA.
 +
 +
 +
 +
of segments in the formation of the feet and setae, the larva of the Myzostomea shews an approximation to the Chaetopoda, and the group is
 +
probably to be regarded as an early Chactopod type specially modified in
 +
connection with its parasitic habits.
 +
 +
BIBLIOGRAPHY.
 +
 +
(379) L.Graff. Das Genus Myzostoma. Leipzig, 1877.
 +
 +
(380) E. Metschnikoff. "Zur Entwicklungsgeschichte d. Myzostomum."
 +
Zfit.f. wiss. Zool. y Vol. XVI. 1866.
 +
 +
(381) C. Semper. "Z. Anat. u. Entwick. d. Gat. Myzostomum." Ztit.f. wiss.
 +
Zool., Vol. ix. 1858.
 +
 +
GASTROTRICHA.
 +
 +
A few observations of Ludwig on the winter eggs of Ichthydium larus
 +
shew that the segmentation is a total and apparently a regular one. It
 +
leads to the formation of a solid morula. The embryo has a ventral
 +
curvature, and the caudal forks are early formed as cuticular structures. By
 +
the time the embryo leaves the egg, it has almost reached the adult state.
 +
The ventral cilia arise some little time prior to the hatching.
 +
 +
BIBLIOGRAPHY.
 +
 +
(382) H. Ludwig. " Ueber die Ordnung Gastrotricha Mctschn" Zeit. f. wiss.
 +
Zool., Vol. xxvi. 1876.
 +
 +
 +
 +
==CHAPTER XVI. NEMATELMINTHES AND ACANTHOCEPHALA==
 +
 +
NEM ATELMINTHES '.
 +
 +
Nematoidea. Although the ova of various Nematodes have
 +
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
 +
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.
 +
 +
Trichinidse. II. Gordioidea.
 +
 +
I. Nematoidea. , Filarid8B . m . Chaetosomoidea.
 +
 +
Mermithidae.
 +
[_ Anguillulidse.
 +
 +
2 The ova of Anguillula aceti are stated by Hallez to undergo a similar development to those of Cucullanus.
 +
 +
242
 +
 +
 +
 +
372
 +
 +
 +
 +
CUCULLANUS.
 +
 +
 +
 +
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,
 +
 +
 +
 +
 +
FIG. 166.
 +
 +
 +
 +
A.
 +
B.
 +
C.
 +
 +
 +
 +
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.
 +
 +
D. Embryo at the commencement of the gastrula stage.
 +
 +
E. Embryo when the blastopore is reduced to a mere slit.
 +
 +
F. Vermiform embryo after the division of the alimentary tract into oesophageal
 +
and glandular divisions.
 +
 +
m. mouth; ep. epiblast; hy. hypoblast; me. mesoblast; a?, oesophagus; bl.p. blastopore.
 +
 +
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).
 +
 +
The hypoblast of the embryo gives rise to the alimentary
 +
 +
 +
 +
NEMATELM1NTHES. 373
 +
 +
 +
 +
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
 +
body.
 +
 +
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.
 +
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.
 +
 +
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.
 +
 +
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.
 +
 +