Book - Outline of Comparative Embryology 1-9

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Richards A Outline of Comparative Embryology. (1931)
1931 Richards: Part One General Embryology 1 Historical Development of Embryology | 2 The Germ-Cell Cycle | 3 Egg and Cleavage Types | 4 Holoblastic Types of Cleavage | 5 Meroblastic Types of Cleavage | 6 Types of Blastulae | 7 Endoderm Formation | 8 Mesoderm Formation | 9 Types of Invertebrate Larvae | 10 Formation of the Mammalian Embryo | 11 Egg and Embryonic Membranes | Part Two Embryological Problems 1 The Origin And Development Of Germ Cells | 2 Germ-Layer Theory | 3 The Recapitulation Theory | 4 Asexual Reproduction | 5 Parthenogenesis | 6 Paedogenesis And Neoteny | 7 Polyembryony | 8 The Determination Problem | 9 Ecological Control Of Invertebrate Larval Types

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This historic 1931 embryology textbook by Richards was designed as an introduction to the topic. Currently only the text has been made available online, figures will be added at a later date. My thanks to the Internet Archive for making the original scanned book available.
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Part One General Embryology

Chapter IX Types of Invertebrate Larvae

The development of the eggs of the many invertebrate animals shows an amazing diversity in the methods by which the adult is reached after the period of germ-layer formation has been passed through. In some groups most complicated life histories involving adaptations to environments that are totally different from that in which the cleaving egg found itself are present, while in other groups development is simple and direct. Of these many types of invertebrate larvae it is possible to describe only a few in this chapter. No attempt is made here to describe the cases of direct development as they occ11r in the various phyla, but it is desired to explain how certain important invertebrate larval types undergo their metamorphoses and reach the adult condition. In the table showing the occurrence of the Various types of invertebrate larvae these forms which are selected for description are indicated by small capitals, while the others mentioned are of less general significance and are for reference.

I. THE PORIFERA The Amphiblastula

The free—swimming larva which occurs in all families of the Porifera is known as an amphiblastula. Its development has been investigated in a number of forms, among the best known of which is Sycandra raphanus (see fig. 64) studied by Schultze in 1875. Other important studies are the series of papers of Maas on various sponges, and the studies of Minehin on Leucosolenia.


The eggs are fertilized in the supporting jelly of the sponge wall, a position which they occupy when ripe and where the early stages of development are passed. (In Cliona, the boring sponge, the early stages of development take place externally.) In a cavity near a flagellated chamber which has a definite cellular lining the cleavage stages are undergone, forming an embryo of characteristic appearance known as an amphiblastula. At length the embryo emerges into the flagellated chamber and thence passes to the outside to become free swimming. After twenty—four to forty-eight hours of swimming about, it gradually finds lodgment and attachment to a substratum, a process which is spoken of as fixation, and which involves a complete and quite sudden metamorphosis. In different genera of sponges the events do not all take place in exactly the same order nor do the larvae reach the same degree of advancement at the time of emergence or of fixation. The essential principles of development are probably not dissimilar, however.


The free-swimming larva is at first an ovoid blastula in which there are cells of two obviously different sorts. The large anterior portion of the embryo is composed of flagellated cells and the posterior portion of larger, non-flagellated, granular cells. Originally open at both ends, the blastula soon grows so that the pointed end is closed and rounded out and the cells become columnar in form and flagellated. The opening at the other end is surrounded by the granular cells which upon its closure take the form of a flat layer, some of whose cells proliferate and presently push into the cavity. It is supposed that the flattening is due to pressure against the unyielding Inass of spicules, while the columnar cells produce a swelling into the space of the flagellated chamber adjacent. This is the condition in which the embryos are extruded to form free-swimming larvae.


In Leucosolenia, Minchin has described the columnar cells each as differentiated into two regions, a refractile portion at the internal end and a sharply marked-off granular part at the outer end‘. In living embryos the two.portions fiG 92‘ Amphiblasmln of are so distinct as to give superficially the Sycandra. (After Schultze.) appearance of an inner layer of refractile cells covered by an outer granular layer of cells; they are, however, merely parts of the same layer. Between the columnar and the posterior granular cells of this form is a zone of intermediate cells which are flagellated and entirely granular. The flagellated cells are much more numerous but much smaller than the granular cells. As development proceeds, the number of granular cells increases at the expense of the flagellated ones.

The larvae of Leucosolenia are transparent so that a mass of yellowishbrown pigment at the center may be easily seen. This is shown by sections to have the form of a tube open in front and behind, enclosing a lens-like body of gelatinous character which fills the central, blasteaoelic space. The central cells are so arranged that Minchin regarded this structure as a premature, light-perceiving organ of larval significance only. i

The free-swimming stage of the amphiblastulae is not of long duration.

They remain at the surface for perhaps a day, then sink to the bottom and after another twelve hours of swimming there they are ready for fixation. fixation takes place at the anterior pole, the granular cells growing about and enclosing the flagellated cells in the process of metamorphosis. This process involves in some genera also the active invagination of the flagellated cells. At this stage only the two kinds of cells may be distinguished, the central cells appearing to be thrown out with the pigment in metamorphosis (Minchin). The outer cells become the dermal or covering layer, the inner the gastral tissues.

Postlarval changes in both layers now set in by which the adult condition is reached. The dermal layer differentiates and two kinds of cells result, one superficial, the other migrating beneath the former. They differ also in the type of spicules which they are to secrete, the superficial producing the monaxones and the other the triradiate spicules. The gastral cells take on a radial ar~ rangement, a cavity appears in the center of the organism which


F10. 93.

fixation of the larx a of Sycandra

Iaphanus. (After Sehultze)

A, flagellated cells retreating into the interior; B, cup—shaped larva attached by amocboid processes of the outer granular layer, f.c., flagellated cells; g.c., granular cells.

presently (in five or six days) dcvelops an opening to the outside, the osculum, and becomes more complicated. The cells themselves elongate and assume the typical

collared appearance of choanocytes. The development of the pores and canals is a matter of slow growth, involving the formation of pouches and the rearrangement of cells, the secretion of spicules and gelatinous intercellular substance. Other types of larvae have been described for sponges but they may all be regarded as modifications of the amphiblastula type just described, as was shown by Maas (1898). Some sponges also reproduce by formation of gemmules, but this is a process of budding and no free-swimming larvae are developed. THE PLAN ULA 141

II. THE COELENTERATA

The larvae of the coelenterates exhibit a considerable variation in appearance in conformity to the wide range of adult conditions to be met with in the phylum. Like the adults, the embryos have, in spite of dissimilarities of form, a fundamentally common type of structure. Numerous experiments into developmental possibilities seem to have been tried out by the coelenterates with many diverse results, considering the paucity of material and restricted structural limitations. The structural simplicity is visible throughout the general similarities that may be discovered in the different kinds of embryos.

1. The Planula

The typical eoelenterate swimming larva is a planula. It occurs in nearly all_Hydroz0a (Tubularia is an exception) and in very many


and the character of the ciliated planula. (After Metschnikofi.)

Actinozoa and Seyphozoa. Because of its simple character and its widespread occurrence, an evolutionary significance as the ancestral coelenterate is suggested. It is the most important larval stage of this phylum.

The term planula was formerly applied to a blastula stage, but the usage is now obsolete. The blastula develops by the migration of cells inward, and by cell division into a two—layered, ciliated, swimming larva. The outer layer is in the form of a ciliated, columnar ectoderm; the inner is usually, in the Hydrozoa, a solid endoderm mass formed by many-celled, unipolar ingressions. There are numerous cases in both Actinozoa and Scyphozoa in which the endoderm is hollow. The gastral cavity does not appear in the hydroids until about the time of attachment of the swimming larvae. In Pennaria the hydranths are not covered by a hydrotheea; each has two circles of tentacles, one about the 142 TYPES OF INVERTEBRATE LARVAE

oral end and another at the aboral end. At the time of maturity a row of medusa buds appears around the middle of each zooid which gradually enlarges and differentiates into the somewhat reduced medusae characteristic of the genus. These p1'0duce the germ cells and do not become free until the sex products are shed. Sexes are separate. After fertilization inside the bell of the medusa, the eggs are immediately shed. The medusae are then free, and after a few hours of feeble swimming, they die. In this genus the Inedusa is the less important of the two generations. The fertilized egg as described by Hargitt goes through a very irregular cleavage, forming an irregular cell mass which differentiates into a solid planula of two layers, the outer ciliate(l. After perhaps twelve hours of swimming about, the planula begins to settle down and at length attaches to grow into a new hydroid colony.

2. The Actinula

In some forms, notably Tubularia, an intermediate larval form, the aetinula, intervenes between the planula and the attached stage. It is neither fixed nor swimming, but creeping, and is formed by the appearance upon the planula of blunt protuberances, the

Fm 95_ Enema, features of tentacle buds, which gradually grow longer. the various stages of develop- first appear the rudiments of the aboral

ment of the embryo of Tubularm _ - , mdmm (Rcdmwn fmmmlmen ) tentacles dnected toward the future aboral

A, plnnnln beginning to bud on end. The gastral cavity now is evident with ‘*b°ml t°"“1°1°5-B-CvA°“““l‘*e- thinned walls at one point where the mouth ab.t., aboral tentacles; o.t., oral . . n,nn,n1es_ opening Wlll presently break through about

the time when the buds of the oral tentacles appear. The aetinula now escapes and creeps about on the bottom, oral end downwards. It later tips over, attaches by the aboral end, elongates rapidly and gives rise to the buds which are to form the different zooids of the colony. Thus the aetinula larva is important as the ancestor hypothecated by Brooks for the origin of the diverse kinds of coelen terates.


3. Origin of Coelenterate Larvae

The grouping of the various members of this phylum depends upon the presence and degree of development of two primary structural forms, the polyp and the medusa. In the Hydrozoa both are present; in the Actinozoa the polyp form is especially developed, and in the Scyphozoa ORIGIN OF COELENTERATE LARVAE 143

the medusa. is the structural type. Within the Hydrozoa it is possible, as is commonly done, to form a series with Cumna and Lmope at one end,

fiG 96 A, gonophores of Tubularuz mdwzsa containing embryos B, creeping Actinula larvae, C, attached form of Tubularza, mdwzsa (Redmwn from Allman )

in which the hydranth generation is quite lacking, and Hydra at the other end, in which there is no medusa. These forms are as follows:

Cumna, Lmope (meduszi only) Gommwmus (very rndnnentary hydmnth ulternatmg with medusa generation) ‘ 144 TYPES OF INVERTEBRATE LARVAE

Obelia, Bougainvillea (medusa and hydranth alternating and of equal importance) Pennaria (medusa fixed during sex cell stage, and of reduced importance) Gonothryca

Tubularia (In these four cases the fixed medusa undergoes more and Claw, Hydfactinia more reduction in structure and importance.) Campanularia, female

Judendrium, Oampanularia, male (remnant of mcdusa bud) Hydra (no medusa, hydranth only)

It is evident for reasons that need not be gone into here that the evolution of this series cannot have been gone through from Hydra toward Cunina. On the other hand to assume the medusa. to be the more primitive form of the two types is hardly in keeping with obvious structural features, and for this reason it is not possible to regard this series as having begun with a form similar to Cunina and to have been evolved in the reverse order. Rather some such suggestion as that made long ago by Brooks (1885) seems better adapted to the facts. According to this suggestion the most primitive type is neither a medusa nor a hydranth, but some small generalized form like the actinula. From this it may be supposed that four, perhaps five, series of forms have been derived. first there is the hydroid colony like Obelia in which both medusae and hydranths are present in alternating fashion. The remainder of the series from Obelia through Pennaria to Eudendrium and possibly to Hydra may be derived from this hydroid by the gradual degeneration of the medusa. A second series radiating from the actinula-like ancestor produced Cunina and Aeginopsis in which no hydranth is present. Perhaps a third produced hydra-like forms at the other end with no medusae. The Scyphozoa and the Actinozoa are the fourth and fifth series respectively which owe their origin to this primitive form of coelenterate, the actinula—like form.

Of this form the planula is the larval type (fig. 94). It is the freeswimming larva common to all coelenterate groups. In some, the Hydrozoa and many Aetinozoa, it is a solid mass of endoderm with an ectodermal covering. In others, the Scyphozoa and some Actinozoa, the endoderm is a definite layer with a gastral cavity. The latter is held by some workers to be the more primitive of the two, for it is argued that as the organisms were evolved an animal with a solid internal mass of cells would be quite unable to support itself. An embryonic type developing from stored yolk material would find no such difliculty, and so the solid form of the planula was thought to be secondary.

The general plan of coelenterate embryology is now clear. The fertilized egg develops gradually into a planula which swims about for a time and, it may be after intervening larval stages have been passed SCYPHOZOAN LARVAE ' 145

through, settles down, attaches, and elongates into a hydra-like form representing the hydranth generation. This may grow into a hydroid colony or into the coral polyp or actinian, or go through further development processes to produce asexually a. rnedusa which in turn produces new germ cells.

4. Actinozoan Development

In the Aetinozoa the development of the sea anemones is very similar to that of the Seyphozoa up to a certain point. The planula is reached in the usual manner. The early stages of their development may be undergone in the gastral cavity of the mother in some cases, shedding of the embryos in the gastrula stage or thereabouts occurring through the mouth, or the eggs may be discharged from the mouth and fertilized among the tentacles or free in the sea water. After the free-swimming period the attachment at the broader end takes place. At the other end a narrow pit forms asthe beginning of thestomodaeum ; it deepens and its lower end breaks through, connecting the gastral cavity with the outside. The subsequent history is largely one of differentiation especially of the mesenteries. Some special forms of larvae occur in various subdivisions of this group but the general features are similar.

5. Scyphozoan Larvae

In the Scyphozoa the development of Aurelia may serve as a convenient type. F1 G. 97.

_ _ . A, free-swimming plaThe eggs are shed 1nto the digestive nula of Aurclia aurita; B. section

through s(-yplustoma having four

cavity’ pass out through the mouth after tentacles. (Redmwn from Hein.)

fertilization to pockets on the inside of

the oral arms where they undergo those processes which lead to the planula stages. Then they emerge and swim about freely. Histological differentiation continues, and the larvae are ready for attachment in four or five days. After attachment the larva elongates and broadens out, the mouth opening becomes widened and four tentacle buds appear between which the taenolae, endodermal ridges projecting into the gastral 146 TYPES OF INVERTEBRATE LARVAE

cavity, develop. This larva with its flattened, broadly open, oral disc is spoken of variously as a scyphistoma, scyphula, or hydra-tuba. Secondary tentacles develop between the original four and then continue to form until the appropriate number is reached. After further differentiation, the process sets in which distinguishes scyphozoan development from other types. It is the cutting off of its oral disc by means of a constriction around the scyphistonia to form an ephyra larva. This process is known as strobilization and iii a well—fed scyphistoma may take place repeatedly until a dozen or fifteen cphyrae are formed from the one larva.


1‘i(. ‘)5 iSti‘obiliz.it1on of Auiclia amila (Rednivm from Claus) t, tentacles of strobila t..ie taeniolac, g fiist tiiuisx crsc giome, I, lobes of cphvi 1

The more usual condition is for each scyphistoma to produce two 01 three ephyrae. The constrictions appear piogressively later as they pass down the lawn, and in the well—fed individuals the appearance of a pile of saucers is ieadily suggested

The ephyra now continues rapidly its development and differentiation leading to the form of the adult jellyfish. The scyphistoma tentacles aie resorbed and in their place the characteristic lobes of the ephyra appear. The oral cone lengthens into the manubrium and other changes occur which result in the metamorphosis of the ephyra into the jellyfish.

III. THE PLATYHELMINTHES

Development in the flatworms presents many diverse aspects. Some eggs undergo no metamorphosis but pass directly into the adult condiMULLER’S LARVA 147

tion. Others reach their final development only after passing through

‘the most complicated life histories, involving (lifferent hosts, totally

different environmental conditions, and totally different types of organization. Since it is not the purpose of this chapter to follow through life histories, but merely to show the relations of types of larvae to embryos and adults to which they are related, only meager reference can be made to the group as a whole. One well-known form is Mul1er’s larva, the

S J ’ mo. 1 . p’ . (ix

‘ W ‘ M s. ., _,4 . _,, < A r- *7 _ Irv-W ‘_ gr. , { * ' 1,1’; ~ ,. r.l. Q.” T‘. ls;-,’..\ (‘if :r Jicm « V’ 5, t'::7'\$\.«" ,‘ K3,,’ -* z ) ;,_ ‘ R» an, I ‘w 4 (' V 5.0. A B

ll(. ‘)0 \ smobilmd sn phisloxn L of Auului tlunla H uphv. 1 lll\ I. of Aurclirz mmta tftu lll)(‘I‘Iil()n (l{e(lrmn from ( lms)

gf Lttslrlt hl um ms mu nmnth rl, \ uious ruli ll lobes of H1111-lll\(‘ an! ‘I n m 11gm ll sense org ins

free-swimming larva of the polyelad Tuibellaiia named after its discoverer, Johannes Muller.

1. Mii1ler’s Larva

The development of the polyclads is known chiefly from an older study by Lang or Yungza and a later and more detailed investigation by Surface on the embryology of Planocera mquzlma. The cleavage is of the spiral type as found in such groups as gastropods and annelids‘. The cell lineage is similar in the early stages and the localization of formative substances surprisingly like that in the higher forms; the similarity is especially surprising when it is remembered that the Turbellaria agree closely with the etenophores in many points of structure Gastrulation is by the method characteristic of eggs with spiral 148 TYPES OF INVERTEBRATE LARVAE

cleavage, namely epiboly. An oval embryo with ciliated ectoderm and endoderm results. The stomodaeum opens into a rather broad gastral sac, in the wall of which are only small endoderm cells in place of the large endomeres of an earlier stage. Mcsodcrm, ectomesoderm, and ganglion cells are also present in the embryo of Planocera when it is ready for escape from the capsule. Eight ciliated lobes appear at this time just below the equator; these are the characteristic features of Mi'1ller’s larva.

Lang followed through the metamorphosis of Yungza in detail. The position of the mouth shifts from the pole to the ventral side of the


Fro 100 A, B, C, dorsal, ventral, and lateral view; of free-swimming larva of Yunma. (After Lang)

elongating larva. Symmetry is lost because of the inequality in the rate of growth of the two sides of the larva. Of the eight lobes one is dorsal, one ventral, and the others make up three pairs laterally placed, and all are connected by a continuous ciliated band. Growth in length with the gradual reduction of the lobes until the usual form of a flatworm is reached brings about the external changes. Among the internal changes involved are the development of eye-spots, the development of the parenchyma (which fills in the interior of the adult, and of which some cells differentiate into muscle cells), and the differentiation of the brain and genital organs. The most marked changes, however, are those MU LLER’ S LARVA 149

concerned with the development of the protusiblc pharynx. There is an invagination of the larval stomorlaeum, H] which the pharynx arises as a ridge-like thickening. The pharynx itself grows in length and the stomodaeum becomes everted to form the sheath of the pharynx. Surface records that the early cleavages as well as the maturation divisions require about one hour each. By forty—eight hours small ectodermal cells cover the entire embryo, which become ciliated, and during the third day rotation is set up in the capsule. Eye-spots appear about the fourth day. By the end of the fifth day the ciliated lobes appear, and


Tm 101 A, B, dorsal and xentrnl news of l‘lI‘\ are of Yungm aurrmtzaca. in process of metamorphosis (After Lang )

on the sixth the larva breaks from the egg membranes to become freeswimming. It swims for some time, eventually settling to the bottom and assuming the shape and habits of a typical polyclad.

Some of the turbellarian larvae, owing to disproportional development of some lobes, resemble fairly closely the pilidia of the nemerteans. There is resemblance between this larva in turn and the trochophore which Balfour thought was derived from it. Obviously, therefore, larval forms of this group are of importance in working out the ancestral history of invertebrate types, and by some the history is thought to run from the etenophores through the turbellarian forms to the pilidium and trochophore. 150 TYPES OF INVERTEBRATE LARVAE

2. Trematode Larvae

Trematodes are commonly divided into Monogenea and Digenea, depending upon whether one or more than one host is necessary for their development. Since only one host is necessary in the Monogenea, development is direct or nearly so, whereas in the Digenea there are often several intervening stages. In the sheep liver—fluke, which may be chosen as an example, the egg undergoes the early part of its development in a chitinous shell while it is passing through the alimentary canal of the


flo 102 A B, same as above but older

sheep. After it has been passed from the body of the host it at length escapes from the shell as a ciliated larva, the miracidium. This larva swims about in water or moves over damp vegetation until it comes in contact with a pond snail or perishes. It is said to be sensitive to the presence of the snails and to move toward them actively. At this time the larva is in many respects similar to a rhabdocoele turbellarian without gonads. It has nearly the shape of a slender cone with two eye-spots near the head end (the broader end). There is a brain, a pair of flame cells representing the excretory system, and an imperfect intestine. The rest of the body is filled with germ cells. After the miracidium has penetrated the body of the snail it loses its ciliated ectoderm, the enteron degenerates, and it elongates into a sporocyst, which is thus CESTODE LARVAE 151

merely a transformed miracidium.Within the sporocyst, germ balls* are budded ofl" and each begins a development similar to total cleavage to form a blastosphere which in its turn is converted into a gastrula. This passes then into the larval type known as the redia. The redia is usually looked upon as parthenogcnetically produced, and the alternation of generations thus involved differs from that seen in eoelenterates in that here a sexually developed sporocyst gives rise parthenogenetically to the redia. In the winter the rediae produce other rediae in a similar manner, but in summer they develop into another type of larvae, the cercaria. The rediae differ externally from miracidia in the absence of cilia, of eye-spots, and in the presence of a pharynx and a simple sac-like intestine, and their general shape is more cylindrical with a collar or circular ridge near the anterior end and short locomotor processes near the posterior. The cercariae have in an undeveloped condition all the organs which are found in the fluke and some other structures including a tail which are of brief duration and last only during larval life. The cercaria leaves the snail, encysts upon a blade of grass, and must be eaten by a sheep to continue its development. Its final metamorphosis consists in the loss of its tall, the rapid growth of the organs already present, and the development of its reproductive organs.

Variations from this case occur; in some forms stages are omitted and the cerearia may develop without a tail. The complication of two or more hosts and an intermediate larva which reproduces paedogenetically recalls the cases of polyembryony recorded in numbers of groups throughout the animal kingdom.

3. Cestode Larvae

Cestode larvae and life histories are only a little less complicated and involved than are those of the trematodes. The eggs of cestodes, like those of the trematodes, are surrounded in the uterus by a thin eggshell. In some forms there are also included “yolk cells” which contribute food to the developing embryo. Cleavage is irregular in type, although characteristic features are to be recognized. An outer layer of cells is formed from micromeres which separate from the larger cells early in cleavage. In most cases this layer of superficial cells gives rise to a chitinous coat which is thrown off ultimately. Only the central mass of cells which is enclosed by the superficial layer takes part in the formation of the embryo. This mass of cells develops a series of six hooks and becomes the hexacanth embryo. The development of the hexacanth

There is reason to doubt whether these germ balls are true germ cells involving a reduction of chromosomes in their development. If these are not germ cells then the rediae are asexually produced and a true alternation of generations exists. 152 TYPES OF INVERTEBRATE LARVAE

embryo (onchosphere or proscolex) and the formation of the chitinoid coat with its inclusions of yolk material take place within the uterus of the posterior proglottids, and in this condition the embryo is passed to the outside; it must await its appropriate intermediate host for further development. .

If it is ingested by the proper animal (often along with the entire proglottid in which it was produced) the digestive juices soften the coat of the embryo, which is set free and at once begins boring its way through the intestinal wall and passes into liver, mesentery, peritoneum, brain, or other organ. Here it encysts but continues its development. If the intermediate host is an invertebrate the next stage is a cysticercoid; if a vertebrate, a cysticercus. The difference is chiefly in the large bladder or caudal vesicle of the latter. The cysticercoid has typically three parts: scolex (the tapeworm head), a body, and a caudal vesicle, the latter forming an enclosure for the other developing parts inside it. In the final host the scolex is pushed out, attaches with its hooks to the wall of the alimentary tract and begins to grow.

The cysticercus is attained from the hexacanth embryo in the following manner. The hexacanth embryo, having reached its place of encystment, enlarges markedly and its interior becomes filled with fluid or with loose spongy tissue. For this reason it is called the bladder worm stage. A small invagination into one side appears which deepens and develops into the scolex, but in an inverted condition. An elevation at its bottom becomes the rostellum, and the suckers appear on the inside of the lateral walls near their base. Metamorphosis of such a larval form takes place after transference to the final host by the eversion of the scolex and very active growth in the region just behind it to form proglottids.

In a few instances instead of a single scolex many are produced by the cysticercus by budding from its interior. This is the case in Taenia coenurus, which produces as invaginations from its wall many scolices that are later to be everted. In Taenia echinococcus this polyembryonic development may go even farther, the ingrowths from the wall becoming separated off to drop into the cavity as secondary bladders. Each of these goes on actively producing new crops of scolices.

IV. THE NEMERTINEA

Of the four groups of the nemertean worms only one so far is known which develops by metamorphosis. In this (Heteronemertini, Schizonemertini) there are two families and the characteristic and important larval form is the pilidium. In one of the families, however, there occurs a creeping form which is really a pilidium modified for creeping in keepTHE PILIDIUM 153

ing with the similar habit of the adults. This is known as the larva of Desor and is that of the genus Lineus. The nemerteans, while showing a distinct advance over forms already considered in acquiring a second opening to the alimentary tract, the anus, exhibit many primitive features which relate them to the flatworms and also to the ctenophorcs. The pilidium itself also is prophetic in certain of its features of the

trochophore larvae of higher groups. Cerebmtulus has been most studied in America as a type of nemertean development.

The Pilidium

The eggs of Cerebratulus cleave spirally and form blastulae and gastrulae in the manner typical for such eggs. The free-swimming pilidium is a helmet-shaped larva with a spike of long cilia arising from the apical plate of cells. The cilia elsewhere are uniform in size and distribution except at the margins where a ciliary wreath may be seen. This is the locomotor organ of the embryo, the prototroch. The sides of the umbrella-like portion are prolonged downward into hanging lobes or lappets. The pilidium is produced from the symmetrical gastrula by the unequal development of certain parts so that the blastopore opening becomes oval in shape and the invaginated enteron grows in length and bends

over to one side, the bent sac-like portion becoming the stomach and the open funnel the oesophagus.

From two large cells which pass into the blastocoele the mesenehyme is produced. It consists of a mass of stellate cells whose amoeboid movements may be seen through the transparent outer covering. The pseudopodia which are put out enable the cells to attach and some are converted into muscle fibres. The pilidium is completed in this condition and swims about for a considerable period, perhaps two weeks, feeding at the surface of the sea upon microscopic organisms.

The metamorphosis of the pilidium has proven elusive since it is not yet possible to rear the animals under laboratory conditions and only fragmentary information has been gleaned from the material collected from the sea. We have to do here with the first of these transformations, numerously observed in the invertebrate groups, in which only part of the larval structures are included in the adult animal. In this case the larval ectoderm, prototroch, lappets, and apical sense organ, as well as the fused outer parts of the amniotic invaginations about to be described, form a double bell-shaped structure which is left behind like a cap after the little worm is differentiated and has constricted itself away. This

larval rudiment swims about for a time, and then, unable to feed and exhausted, it dies. 154 TYPES OF INVERTEBRATE LARVAE

The formation of the worin's body from the larval pilidium begins with four eetodermal invaginations which appear on the flattened lower surface. These amniotic invaginations form, in their deeper portions, the imaginal discs which contribute largely to the definitive body of the worm. They are distinguished as right and left anterior, in front of the

fir: 103 A. pilidium larva of Cerebralulus lactus (Redrawn from M:icBri(le, after C‘ B Wilson) lap , lappet, oes , oesophagus, st , stom ieh, II|("a0ll , iiie-seneliyme cells a p . apical plate B, a. pilidium Just before metamorphosis (Redr l.Wll from Mac Bride, ziftei Metchnikofi ) 9, p , apical plate, a im , anterior imagimil disc oes , oesophagiis, pr , anlage of proboscis; pt. im., posterior imaginal disc

mouth, and right and left posterior behind it. They continue to deepen and broaden, growing up over the already formed alimentary canal until they finally meet and coalesce. Their outer walls form a temporary larval covering known as the amnion, a delicate envelope separating the body of the worm from the covering of the larva, and the inner fused portion is the skin of the future worm. The intestine of the pilidium is THE PILIDIUM 155

thus included in the body of the new worm and becomes the endodermal portion of the latter.

Rudiments of the other organs of the worm are developing meanwhile. The proboscis develops as an ectodermal invagination projecting into a mesodermal mass which is to become the proboscis sheath. The brain

fiG 104 l‘ullv dt-xcloped pllulinm uitli young Il(_‘l11(‘ll\ Ill sutlun its amniotic mu‘ (ltcdumn flolu lunschnlt and Hculu, after liutsrhli )

rm, iunnion (l , inushnc of pilidium lama surrounded bv worm, cc, cctodmm of \\o1m, in . mouth of piluliuin, n , l)Q;£1l\lllng()f ncnous svstem, r , probosus so excrcton organ

is of compound origin, arising as an cctodermal proliferation from the anterior imaginal discs. The formation of the anus and certain other organs has not been followed through because of the lack of material. However, from the cndodermal portion of the pilidium and the inner part of the amniotic invaginations comes the material from which all the later structures are derived. The outer part of the fused invagina— tions, such as the lining of a cap of which the larval walls are the chief 156 TYPES OF INVERTEBRATE LARVAE

parts, is cast off, and the young animal which it covered is the young worm ready for life upon the sea bottom.

V. THE ANN ELII)A AND MOLLUSCA 1. The Trochophore Larva

The trochophore larva is typical of annelids and molluscs. Other smaller groups, among which are the Polyzoa, Sipunculoidea, and Echiuroidea, also have trochophore larvae, and still other larval forms show relationships to the trochophore which are more or less close. In some larvae simple modifications are present and in others the relationships are harder to trace. It is unquestionably the most important invertebrate type of larva, viewe(l from a phylogenetic standpoint.

The trochophore larva in its simple condition is ovoid and possesses certain characteristic features, most conspicuous of which is the prototroch, a ciliated band about the equator. The prototroch is the chief organ of locomotion and participates in getting food. A group of cells known from its position as the apical organ functions as a sense organ, and from these sensory cells apical cilia grow out as a tuft. In some cases a metatroch, a secondary belt of cilia, is present posterior to the prototroch and a second tuft of cilia known as a tclotroch is present at the pole opposite to the apical tuft. Departures from the typical arrangement of cilia occur in various species. Larvae with the primary ciliated band only are spoken of as monotrochal. In some, called atrochal, the prototroch is not developed, but cilia are present all over; in others, without a prototroch, a mctatroch only is developed. Polytrochal larvae also occur, but these are later stages in which secondary bands of cilia are formed about the developing post-trochal region.

Internally a trochophore has a complete digestive tract with mouth, large, bulb-like stomach, short intestine, and an anus. The mouth of annelids is the old blastopore or an opening developed at the same place as the blastopore after a very temporary closure. The intestine meets an ectodermal invagination, the proctodaeum, with which it fuses. and an opening breaks through completing the alimentary tract. An archinephridium is present, running from the ocsophageal region diagonally to the end of the intestine. Each contains a flame cell, or solenocyte, with a cavity and a tuft of cilia, and an excretory tube leads from it. In some trochophores there is an eye-spot containing red pigment cells; it is sensitive to light. The mcsoblast cells, or teloblasts, derived from 4d, are present. (or their descendants) and later elongate into two strings of cells, the mesoderm bands. THE ANNELID TROCHOPHORE 157

A typical trochophore as described is derived from the gastrula by seine simple changes, the chief of which is the shift of the blastopore from the vegetative pole to the lateral position. In this process several factors take part. One is the increase in rate of development and in the actual number of cells of the dorsal posterior cells, particularly the descendants of the 3d cell. This inequality of growth tends to push the opening sideways. In some forms the blastopore also becomes oval in shape, then constricts in the middle, and the portion in the a-b quadrants is pushed anteriorly by the growing cells of the d quadrant. Of these two openings the anterior one becomes the mouth and the posterior one after temporary closure reopens to become the anus. This procedure is made much of by some authorities who reason that in this manner must have come the separation of the original ingestive opening into two, one of which is ingestivc and the other egcstivc.

In the fully grown trochophore larvae of all the groups in which it occurs are found the points of structure as described. Indeed, the homologies between the various groups are complete even to the details of trochophore development. When one considers the remarkable array of organisms which in spite of total absence of agreement in adult features have this type of larval structure, he must find the fact of their common method of origin very surprising and significant. Nemerteans, rotifers, annclids, molluscs (except cephalopods), and bryozoa, form a series which certainly presents a complete gamut of adult variation, yet all come from larval forms with features that are in general the same. It would, however, be even more surprising if there were not minor differences to be found in the trochophores. There are variations in the relation of the mesoderm mother cells to the gut wall, of the persistence of the protonephridia, the presence of the anal vesicle, of a prototroch, of the gastropod shell gland, and in other special organs which are the peculiarities of particular groups. It is notthese differences, however, which deserve emphasis, but the common features of development which find expression in the widespread trochophore type.

2. The Annelid Trochophore

The metamorphosis of the trochophore into an adult annelid may next be considered. It consists chiefly in the growth in length of the post—troehal region and in the differentiation of certain elements found there. The essential features of the metamorphosis are (a) the elongation of the post—troehal region to form the body of the worm (post-cephalic portion) and its consequent segmentation; (b) the apparent reduction (which is not real, but simply relative) of the troehal and preoral regions 158 TYPES OF INVERTEBRATE LARVAE

and their resulting changes to form the head of the worm; and (c) the disappearance of the ciliated bands. The processes are of course accomplished gradually, there being no moment at which the trochophore ends and the adult begins. The post-trochal region is one of active growth, differentiation, and cell multiplication. As an example of the metamorphosis of the annelid trochophore, Polygordzus has been often studied,

fiG. 105 Troehophorc of P(ll_Ij(]0rdLu8. (After Woltereck) An , .irch1ncphr1d1um

11 p . apical plate, mt. intestine, mtt, mctatroeh, 0 , mouth. pr t. prototroch st stomach, ti 1: . telotroch.

and no other is probably better known. It has been figured by Hatschek, Agassiz, and others, but is best known from the work of Woltereck. The elongation of the post-troehal region in Polygordzus involves especially the growth and division of the cells of the intestinal walls, the multiplication of ectoderm cells and the growth and differentiation of the cells of the mesoderm bands. The descendants of the 4d cell form the mesoderm bands, symmetrically placed on the two sides of the intestine The pole cells from which they came are known as teloblasts. The bands multiply not only in length but in thickness and in each a series of cavities THE ANN ELID TROCHOPHORE 159

gradually appears. These are the mesoblastie somites, and they are marked off from each other by constrictions which are traced on the surface by transverse grooves in the ectodermal covering. Their cavities grow progressively, and as they expand they fill up the surrounding space which was the remnant of the blastocoele. Ultimately they meet above and below the intestine, their walls fuse and break through, making a continuous pouch in each segmfint instead of a pair. Evidently in the growing worm, segments are not a in the same degree of advancement.

The origin of the excretory organs shows two stages in development in the annelids. The archinephridium by the addition of other cells from the third quartette and by increase in number of its own cells becomes a protonephridium, a rather complicated structure whose basis is flame cells. The origin of the permanent nephridia has been the cause of controversy among embryologists, particularly as to their derivation from ectoderm or mesoderm. These posterior or permanent ncphridia or metanephridia grow from strings of cells lying in the wall of the embryo and running posteriorly. These break up into loops and a lumen appears which later becomes the tubular nephridrum. In N crezs, Wilson described these as of eetodermal origin except for the funnelshaped opening which he hsaid lils meso- P115;-orfilfiy agljlgrdliisziiigwlirégvftgi dermal. Others consider t at t e entire ‘fiolmeck end is mesodermal and secondarily at- a_ anus? 1,. posterlor b(,,d(., 0; tached to the eetoderm. Still others think h:*:‘:o““1i:E:SDI1 v I>F0t0neDh“d1«* theentirestructureisectodermal.Although ' D ' the details in Polygordzus are not fully known, Woltereek holds to the ectoderrnal origin.

The nervous system begins with the mass of nerve cells that lies under the apical tuft. This is the cerebral ganglion and from it radiate eight nerves to the cells of the prototroeh. Two, the lateral nerves, continue across the prototroch around the oesophagus to form the ventral nerve cords.

These are the beginnings of the more important organ systems of the transformed trochophore. For the details of the metamorphosis and for 160 TYPES OF INVERTEBRATE LARVAE

the conditions as found in special forms reference should be made to the monographs and special articles dealing with these matters.

3. The Molluscan Trochophore

The metamorphosis of the molluscan trochophore is more complicated and involves the interposition of another larval stage, the veliger. While in the trochophore the characteristic larval organs are present, in the veliger there are also the rudimentary stages of the adult organs. There are three characteristic organs of this stage: the velum, the shell gland,


fiG. 107. Young trochophore of Patclla cocrula. (After Wilson.)

a.p., apical plate; 111., mesodermnl cell; m.b., beginning of mesodermal band; 1no., mouth; s.g., shell gland; st., stomach.

and the foot. The velum is derived from the prototroch of the trochophore and is in the form of a bilobed, much enlarged projection abundantly provided with cilia. It serves as the locomotor organ of the voliger and disappears with metamorphosis. The shell gland is at first a simple invagination opposite the month which may open out to form a flattened disc or actually evert. Its cells secrete a thin, horny cuticle which is the shell of the trochophore. In the case of the bivalve molluscs the rapid extension of this area forms the mantle and calcareous as well as horny material is secreted, thus forming the two valves of the shell THE MOLLUSCAN TILOCHOPIIORIQ 161

connected by a region known as the hinge. The variations in the shape of the mantle lobes and the shell secreted by it and in the length and proportions of the hinge correspond to the various systematic groups. The foot in the earliest stages is a thickening of the ectoderm cells just ventral to the mouth. In the cavity of the slight hollow forming behind this bulging foot are mesenehyme cells which contribute to the differentiation of the organ. Other organs make their appearance in the developing veliger, including a larval kidney which disappears as the foot grows out, the muscles, the ganglia, gills, coelome, liver, and secondary structures as crystalline sac and otocyst. The details of their development are beyond the scope of the present account.

The metamorphosis of the veliger into an adult-like form is not a process of long duration. The appearance of the organism is surprisingly


Fm. 108. A, embryo of a heteropod. (llcilmwu from Balfour after Fol.)

u.. arelienteron; 0., body r-avity; f., foot; mo., mouth; s.g., shell gland; v., velum. B, young veliger of I’l(mr0l2runL'hidium. (Redrxuvn from Balfour after Lancaster.) in., loop of intestine; y., residual yolk spheres; n.g.. nerve ganglion; ot., otoeyst.

changed largely because of the sloughing off of the cells which formed the velum; this process and some others coincident with it allow the organism to assume the external shape of a small adult. The larval muscles disintegrate, the anterior region with the mouth shrinks, drawing closer together the mouth and anterior adductor and advancing the front end of the foot. As a result the position of the developing gills is changed and the intestine is shifted and straightened. Although now the organism has the recognizable form of an adult, it is of course very immature and numerous internal changes have yet to be undergone. Labial palps and gill filaments in the bivalves, adult kidneys, pericardium, the final relations of the mantle cavity, development of the gonads and their ducts, all these and other minor processes of differentiation are still to be undergone before the final condition is reached. 162 TYPES or INVERTEBRATE LARVAE

VI. THE CRUSTACEA Nauplius Larva

O. F. Mjiller long ago studied some forms of copepods which we now know are larval,‘ but he thought them adults and gave them the name Nauplms. In this way originated the term that now represents a stage common to all Crustacea. In the lower forms it is usually a free~swimming larva and in many. of the higher ones it is simply a stage passed through in the egg, but it IS represented in the development of all Crustacea


Fm. 100. A, ventral View of gastrula stage of .l.staru.s flzwialflis. (Redrzmn {min MacBride, after Reichenbach.) e. l., cephalic lobe; th. ab., thoracic abdominal thickening.

B. nauplius stage of the same. an., anus; at‘, rudiment of first antenna; at‘~’, rudiment of second antenna; car.. ridge making the first trace of the carapace; e.l., cephalic lobe; lah.. lahrium; m.. mouth; mn., rudiment of mandible; pr. c., protocerebrum; th. ah., rudiment of thorax and abdomen.

Crustacean eggs show a great range of developmental types, and the various stages show no uniformity throughout the different groups. Cleavage may be superficial or it may be total and nearly equal. A cleavage cavity is usually lacking because of the large amount of yolk, but some species have it in more or less restricted condition. Gastrulation is usually by epiboly and ingression, but here again conditions vary. Cross fertilization is usual, but there are cases of self—fertilization, and parthenogenetic development may alternate with normal fertilization. There are some cases in which only parthenogenesis has so far been discovered. But in all cases a nauplius stage is at length reached.

The form named nauplius by Muller actually had four pairs of appendages, whereas the stage now known as the nauplius has only three NAUPLIUS LARVA

163

pairs. It was Claus in 1858 who named the three-appendage stage which is of such widespread occurrence throughout the Crustaeea. A nauplius


Pro 110 Continuation of I15: 10‘)

at‘

at’

nm

In A the rudiments of muxill K‘ have appearul

and the caudal fork is VlSll)l0 In B thoracic appendages are seen and the abdomen Ill

segmen tmg

larva, therefore, has three pairs of appendages, first and second antennal,

and mandlbular. But the degree of development toward independent existence in the nauplius stage is by no means uniform in the difieient Crustacea. There is in fact a series of larval stages in the C1 ustacea, the most important of which are nauplius, metanauplius, protozoaca, zoaea, mysis, all leading to the adult. There are, also, others less generally important, but more specialized for pai tieular groups. Of those mentioned a completed series occurs in the higher crustaceans only, in some forms of which all are represented. For every stage listed there is some group of higher crustaceans in which the larva hatches from the egg at the corresponding time. That is, hatching, or the breaking out of the free-living’ form from the


fiG 111

Nnuphus of Cyclops (After Claus)

an‘, first antenna; an’, second antenna. man , mandibular appendage. lab , labrmm, ex , exopodlte. en , endopodxte

egg membrane, is a variable process and some certain species may be 164 TYPES OF INVERTEBRATE LARVAE

found to illustrate each stage as a newly hatched larva. Evidently those which hatch in the more advanced stages (for example, the decapods) do not show the nauplius in a full functioning condition as do those in which the nauplius is a free-swimming larva, as in the barnacles. In the

fiG. 112. Zones. of Penaeus, ventral view. (After Claus.)

uni, first antenna; an“, second antenna; e., eye stalk; mn., mandible; mx‘, mx’. first and second maxilla; mxp*, mxp’, mxp3. first, second. and third muxillipeds; pl., beginning of pleopods; ur, uropod; e.f.. caudal fin.

crayfish, for example, the nauplius condition is shown by three pairs of mere thickenings or buds instead of developed appendages and there are in the embryo in addition only two cephalic lobes, two thoracico-abdominal rudiments, and the endodermal rudiment; this is far from a free-swimming embryo.

The nauplius of Cyclops is an elliptical form with no external sign of NAUPLIUS LARVA 165

segmentation, but with an enlarged labrum, openings of the alimentary tract, a single median eye, and the three pairs of appendages. The first antennal appendage shows no sign of a biramous character, but both the second antennal and the mandibular have at least partially developed both exopod and endopod. Examples of forms hatching in this condition are the copepods as well as Penaeus, and Balanus. In Balanus this stage is preceded by one in which three primary segments are recognizable. These become subdivided into eight lobes, which are the labrum, right and left first antennal from the first primary segment, the right and left second antennal from the second segment, and the right and left mandibular and the telson from the third. Future growth takes place between the mandibular lobes and the telson. From this stage the next changes lead to the metanauplius.


Fm 113 Mysis larva of the lobster. Ilomarua amcncanus. lateral view. (After Herrick ) Exopoditcs of the walking legs are to be noted (ex.).

The metanauplius condition is reached at the end of the first molt in those forms which hatch as nauplii, but occurs before hatching in such forms as Branchipus and Lucifer. Two additional pairs of appendages are present, the first and second maxillae, and segmentation is beginning.

In the protozoaea, illustrated by a newly hatched Squzlla, the first and second maxillipeds are present as well as the appendages of the earlier types, the abdomen is partially segmented, but not the thorax, and the compound eyes are making their appearance.

The zoaea, the stage in which the crab hatches, possesses third maxillipeds, some of the thoracic appendages, the number varying with different forms, compound eyes, an abdomen completely segmented, but the thorax showing as yet incomplete indications.

The mysis stage, characteristic of the genus of that name, has all 166 TYPES' OF INVERTEBRATE LARVAE

five pairs of walking legs with exopods, six pairs of pleopods (although in some they are hardly developed at hatching), and segmentation is complete. The lobster also hatches in an advanced mysis stage.

finally in the fresh-water shrimp, Crangon, the condition of the adult is reached so far as external features are concerned at the time of hatching.

Of the less important larval types of Crustacea, no mention will be made here.

VII. THE INSECTA

Among the insects development may involve the formation of very complex larvae and more or less complete metamorphosis, or may be direct. The structure and cleavage of typical insect eggs have already been described in the section on superficial cleavage. The cleavage of the centrolecithal egg results in the formation of a blastoderm which may be of uniform thickness in development or may have early shown a differentiation on the ventral side of an aggregation of cells which will become the primitive streak.

Even if the blastoderm is at first of uniform thickness, it gradually becomes thicker in the ventral region by the multiplication of the cells, and thus in this case too the primitive streak or germ band is formed. Mesoderm is formed by one of three methods. (1) Along the medial line of the germ band an invaginating groove is presently to be seen in many embryos of which those of the Coleoptera may be taken as an example. From the material of this invaginating groove comes the inner layer from which is derived the mesoderm and endoderm. (2) In certain other insects the formation of the mesoderm may be accomplished without the formation of a tube but rather from a middle plate which remains nearly flat and is overgrown at the edges by the lateral folds. It occurs in certain Lepidoptera and other forms.

(3) Still another type of mesoderm formation takes on the character of a proliferation and invagination of cells from a median ventral blastoderm region. This is characteristic of the Orthoptera. At the edges of the germ band the blastoderm begins a folding process and at length the two folds meet over the band in the middle region. Their union produces two membranes, the outer one, known simply as a serosa, and the inner one, the amnion. The germ band continues its own growth and the marks of segmentation become apparent shortly as transparent grooves. An anterior pocket growing dorsally from the ventral germ band is the beginning of the stomodaeum and the foregut, and a similar one at the posterior end forms the proctodaeum. A pair of large prc—eephalic lobes at the anterior end of the germ band develop in time into the lateral THE INSECTA 167

eyes, and antennae and mouth parts appear as rudiments in proper serial fashion. The appendages likewise are outpocketings of the ectodermal germ bands. It is evident of course that the variety of structures found among the insects means great variability in details by which these steps are accomplished. The general body form of the insect is



Fm 114 Developing eggs of Bmchus quadnmaculatus (After Brauer )

A, tmrisverse section, showing completed blastoderm Dorsal side of egg toward top of page B, section of egg 9. little older than in A through anterior region Cells of veiitml blastoderm are crowded, but dorsal blastoderm is thin and flattened eh . chorion, v pl ventral plate.

completed by the growth of the margins of the germ band dorsally until they finally close over and form a dorsal wall. While this has been going on the progress and development of the internal structures has likewise been considerable and the embryo continues its development until finally ready to hatch into a larva.

During the larval stage the insect feeds, sometimes voraciously, storing up nutritive material for its subsequent development. In the more 168 TYPES OF INVERTEBRATE LARVAE

specialized insects this development is very complicated involving a complete metamorphosis with entire changes of form and often of function as well. The groups of insects which undergo this indirect type of development” or complete metamorphosis are spoken of as the Holometabola and include the Coleoptera, Strepsiptera, Neuroptera, Mecoptera, Trichoptera, Lepidoptera, Diptera, Siphonaptera, and Hymenoptera. The less specialized groups comprise the Heterometabola in which

fiG. 115. (‘ontinuation of 114.

A, Transverse section of egg after closure of gastral invagination. mes., mesoderm; am. ser.. amnio-serosal fold. B, Longitudinal section of an embryo of 32 hours. Am , amnion, ant. mes., anterior mesenteron anlage; caud. pl., caudal plate, mes., mesoderm; post. mes., posterior mesoderm anlage; ser., sorosa; st., stomodaeum.

development is direct and metamorphosis incomplete. These forms lack a true pupal period. They include Orthoptera, Dermaptera, Platyptera, Placoptera, Ephemeridae, Odonata, Thysanoptera, and Hemiptera. Since a distinction between larvae and pupae is lacking it is common to use the term nymph to apply to the young insects after hatching and several molts or ecdyses may be gone through among these forms. The terms, stages or stadia, are used to designate the intervals between the melts; and the term, instar, is frequently used to designate the insect at any particular stage. The Thysanura and the Colembola develop


F10. 116. Successive stages through the embryonic area. of Hydrophzlus. (From Korschelt and Hc-ider, after Helder)

A, Formation of invaginnted groove wluch 15 to produce the under layer B, Origin ol ammon. C. Amnion completed over embryonic area. Yolk has undergone cleavage D. Ongin of somxtes. E, Formatmn of tmchne F. Formatlon of coolome

Am.. amnion; c , coolomc, eat, octodcrm. end , ondodorm, mm, mesoderm. m ‘I. fit?!

m\I.4 hf mnundnum mu. m....\..n....u~. unvv\)Cn 4.. o...,.l.,... ..I .....l....I........ .. .... .,mo...,l 170 TYPES OF INVERTEBRATE LARVAE

without any metamorphosis and hence are often spoken of as the Ametabola.

Upon hatching, the larvae emerge with certain characteristics depending upon the group to which they belong; one is usually able to classify the insect into its order by mere reference to the type of larva. Common


FI0. 117. Types of larvae. (From Folsom.)

A, B, Thysanum; C, thysanuriform nymph; E—I, eruciform larvae; A. Fampadea; B, Lep7.'.9ma,' C, perlid nymph (Placoptera); D, Libellula (0donata); E, Tenthredopsis (Hymenoptera); F, Lachnostema, (Coleoptera); G. Melanotus (Coleopteru); H. Bombus (Hymenoptera); I, Hypoderma (Diptera).

names are given to the larvae, such as the caterpillar of the Lepidoptera, the grub of the Coleoptera, and the maggot of the Diptera. The types of larvae are known to entomologists under several terms but they are fundamentally of two groups, the campodeiform and the eruciform. The campodeiform are also spoken of as the thysanuriform in reference to their generalized structure. They are flattened, have a long body, often THE INSECTA 171

with long legs and antennae, hard body plates, caudal cerci with welldeveloped mandibles and active habits. These are characteristics of the adult thysanurans, especially of the genus Campodea, but in the insects which undergo metamorphosis they are of transitory importance only. The eruciform larva is illustrated by the caterpillar or maggot with the cylindrical body which may grow to become more or less spindle shaped, weak integument, and with mouth parts, antennae, legs, and caudal cerei reduced, even to complete disappearance; correspondingly it leads


110 118 Successive stages in the de\ clopment of a pluteus of the Ophiuroidea and Echmoidea (After Leuckart )

ad e , adoral ciliated band, :11 , alimentary canal, an. anus, ar, arms, mo . mouth, per 0 c . porioral cllrated band

a very inactive life. Between these two types transitional forms are easily recognized, for instance, in the genus M antzspa of the Neuroptcra the larva which hatches from the egg is distinctly campodeiform and active but it enters the egg sac of a spider and becomes sedentary, loses use of its legs and completely changes its form, that is, becomes an cruciform larva. For the changes which occur during the growth of the larvae, the frequency of the melts, and the great variety of adaptive structures that are assumed in the various larvae of the insects, the reader must refer to treatises on entomology. Likewise the formation of the pupa is a subject which varies too much in detail to be discussed in this work. 172 TYPES OF INVERTEBRATE LARVAE

The internal changes which accompany the metamorphosis of insects are only incompletely understood. In those forms in which metamorphosis is incomplete certain organs are of larval significance only and their tissues must give way and provide the materials for other struc— tures characteristic of the imago. This destruction of larval tissue, or

int.

fiG. 119. Successive stages in the development of u. hrpmnaria of the asteroids. (After Leuckart.)

ad. c., adoral ciliated band, int., intestine. mo., mouth. oes . oesophagus; pr. 0. c., pre oral ciliated band; pt. o.c., post-oral ciliated band. histolysis, takes place commonly during the pupal period through the activity of phagocytes and is followed by a period of the construction of imaginal tissues or histogenesis. At the end of this constructive period the animal is ready for the final metamorphosis which converts it into an active adult or image.

VIII. THE ECHINODERMATA

In the phylum Echinodermata are to be found some of the most remarkable forms of larvae of any group in the animal kingdom, and THE ECHINOIJERMATA 173

in this group metamorphosis has a peculiar and unusual significance. The larval stages are preparatory for the changes which are to adapt the adult forms to a great variety of living conditions, and they bridge the gap from a rather simple ancestral form to the diverse conditions found among the five classes of living echinoderms. One has only to consider the different types of adults found among the asteroids, echinoids, ophiuroids, the holothurians and the crinoids to appreciate the problem which confronts the simple type of ancestral larval form. Such extensive metamorphoses as are found here have for their special function the adapting of the organism to the very changed conditions in which it finds itself. Since the larval condition is retained to the last possible moment in development, although internal changes of the most


Fm. 120. Successive stages in the development of an auricularia of the holothuroldi (After Leuckart ) Letters as in 118 and 119.

profound character are taking place, metamorphosis is of a typo sometimes spoken of as cataelysmal.

The gastrula of all the echinoderms develops into a form similar to what was probably the hypotheticalancestor of all the classes of the group; it is known as a. dipleurula. From this dipleurula it is conceived that the echinopluteus of the sea-urchin, the ophiopluteus of the brittlestar, the bipinnaria of the starfish, and the auricularia of the holothurian are derived. Of these four larval forms the first two are more closely related to each other than to the other group, in which also affinities are discoverable to the larvae of the crinoids. It is not feasible to undertake here a description of the development of the larvae and their metamorphoses in all these cases. For particular reasons and also because it is the form most commonly studied, we choose to follow through the development of the starfish larva, the bipinnaria, from its origin from the gastrula to its metamorphosis.

1. The Bipinnaria

It is characteristic of echinoderm larvae that there should be a bilateral symmetry. Two important trends are noticeable as the transition from larva to’ adult takes place, namely the great development of the left side of the larva with the corresponding decrease of the right side, and the complete change of symmetry to a radial type whose axis is at right angles to the former larval axis. The egg of the starfish goes through a rapid and regular process of radial cleavage resulting in a blastula of about equal—sized cells. The cells become ciliated uniformly, the embryo bursts the egg membrane and, rising to the surface, begins its larval life. Gastrulation has in the meantime been going on, but the formation of mesenchyme is not completed until after the f ree—swimming stage is reached. The larva elongates, becomes somewhat flattened on the ventral side, the blastopore is metamorphosed into the anus, and the larval mouth breaks through anteriorly. The cilia disappear except for certain specialized ciliated _ bands which take on a shape characteristic of particular species. Gradually there appear as outgrowths certain special processes which as they grow carry out the ciliated bands to their borders


uc‘;:‘:)f‘;3O1‘;ngI‘l‘l‘l‘r°\fi:‘el and give the larva its bipinnatc form to which is of Asterms (Arm due the name “bipinnaria” given it by Sars. “3“““-) The earlier investigators were inclined to the al , alimentary - - _ ‘ mm mm” mouth; opinion that the starfish arose as a bud on the 1: ft

pr o 0., pre—oral Cllldtcd band; pt. o.c._ p 03 t- oral ciliated band.

side of the larva. It is now realized that this appearance is superficial only and that the larger part of the larval body is involved in the production of the adult. The earliest trace of the adult body appears as a five-lobed structure in the left posterior portion of the larva. By the growth and dilferentiation of this structure and the loss of the larval mouth and oesophagus and the formation of the new opening, the metamorphosis into the little starfish is accomplished. In the case of the starfish a temporary fixed stage which is not found in all types of echinoderm is passed through. During this stage certain other larval arms and processes are developed and the larva is spoken of as a brachiolaria. It is now desirable to follow in more detail the changes which have been thus briefly sketched.

2. The Dipleurula

All echinoderm larvae agree, while showing much variation from each other on other points, in the possession of the following features: they are bilaterally symmetrical, have a locomotor organ in the form of a curved and bent longitudinal ciliated band with preoral and anal loops. They possess a V—shaped adoral ciliated band; the alimentary canal

fiG 122 A, Bipinnai-1.1 of Astcmas (Frontal view) B, Side view of an older bipinnaria. Median brarhioliirian process beginning to grow out at b (After Agassiz )

a , anus, a d a . anterior dorsal arm, ad e , adoral eiliated band, int , intestine. L eoel . left coelome,m d a, median dorsal arm,mo , mouth, oes. oesophagus. pt d a . posterodorsiil arm, pt 1 a . postero-lateral arm, pt 0 a , post~oral arm, pt 0 e . post-oral band of fllld, pr .1, pre—oral arm, pr 0 c. pre-oral band of cilia, r eoel , right coelomic sac, st, stomach

consists of the typical four parts, namely, the mouth parts, oesophagus, enlarged stomach and intestine. The coelome is budded off from the apex of the archenteron as a pouch which divides into two lateral pockets and has communication with the exterior by a ciliated canal opening on the dorsal surface to the left of the median line. This canal is the stone canal. Since these features are common to all the different groups of echinoderms it is believed that they must have been the characteristics of an ancestral type from which the different classes of 176 TYPES OF INVERTEBRATE LARVAE

the echinoderm existing today have been derived. This hypothetical ancestor is known as a dipleurula. To this larva the characteristic features of the ophiuroids most faithfully conform, and of all echinoderm types it is the most nearly bilaterally symmetrical, for which reason, along with ‘others, it is probably to be regarded as the most primitive. In addition to the features ascribed to the hypothetical dipleurula, it is assumed to possess right and left coelomes more or less divided into three parts, anterior, middle, and posterior. However, a departure from the characteristic dipleurulan condition is seen in Asterias in that the coelome is divided into only two parts (unless the stone canal and its connections are to be regarded as the homologues of the anterior coelome of the dipleurula and the anterior coelome of the starfish is homologous to the middle portion of the dipleurulan structure). The middle part of



fiG. 123. Diagrammatie reconstruction of ancestral dipleurula.

u.. anus; an. coel., anterior coelome; l. hyd.. left hydrocoelc; l.pt. coel., left posterior coelome; m.p.. madroporic pore; rt. md. coel., right middle coelome; r.pt. coel., right posterior coelome; s.c., stone canal.

the body of dipleurula has lobe-like processes extending outwards and covered with ciliated epithelium into which the middle coelome extends. These processes permit a comparison with the lip of the braehiopods and so-called lophophore which bears ciliated tentacles. In both cases the organs function to direct food-bearing currents of water to the mouth. Resemblances have also been pointed out between dipleurula and a simple ctenophore plan of structure, and upon the basis of these suppositions it has been concluded by some that there once existed a class of simple marine animals one member of which was of the general character of a dipleurula and another the ctenophore-like ancestor of the annelids and molluscs. These speculations seem also to give a basis for consideration of the similarities that exist between the trochophore type of larva and those of the echinoderm and at the same time of the very great differences that are to be seen in the early development of these types.

As an example of the development of an echinoderin, although it is hardly to be regarded as a type of the other classes, the embryology of Asterzas is now to be traced in detail.

3. Metamorphosis of Asterias

By the end of the first day after hatching the free-swimining blastula has been converted into a gastrula which is distinguished from the usual type of invaginate gastrula in that the archenteron occupies only a small portion of the inner space, leaving a large blastocoele still present. Into this space are early budded mesenchymc cells which send out processes and make a network in a gelatinous fluid filling the blastocoele. The gradual leiigthening of the gastrula to take on the form of a cylinder with round ends, the differentiation of the ciliated apical plate and especially of the longitudinal eiliated band, involving a thickening of the epithelial cells where it is to form,with a general increase in the number of cilia on them and a corresponding decrease of the cilia elsewhere on the larval body, are the next characteristic changes through which the young larva passes (fig. 122). In the meantime the coelomic sacs are budded off pm 124 Larva of Asmm from the apex of the arclienteron as in the *1‘ days Old. from dorsal surface case of dipleurula. The gut grows forward, (Alflgjeflgiwftu) I m. becomes constricted into the intestine, (left), persicscoiiixiniidigpiiiiéiiiirg, stomach,and oesophagus, and the oesophagus ;f"‘v’~:‘1‘,l‘:l"t‘;1(;”’§:::,“‘ ' ii: 2 2 v turns forward and ventrally to meet an in- post,-()r.),l aimed band, smm: vagination from the ectoderm, the stomo- "‘°“‘°d“““"‘- 5“ 5‘°“"‘°hv °°‘Sv

_ _ _ _ oesophagus. daeuin, with which it fuses. The anus is the

blastopore of the gastrula stage. With the formation of the arms the bipinnaria attains the completion of its outer features. The structure of these arms varies in the different species. In Asterzas vulgarzs, there are borne by the preoral band two larval arms called the preoral and a median dorsal one, while in A. glaczalzs, A. rubens, and A. berylmus (fig. 122), European species, there are also on this preoral band a median ventral arm and a. median dorsal one which are directed forward; there are also in both cases a pair of antero—dorsals, and a pair of postero-dorsals, a pair of postero-laterals, and a pair of post-oral arms. When the time for metamorphosis has been reached and the larva begins to take on its temporarily attached condition there grows out from its anterior part a series of processes known as brachiolas, a median and two dorsals, and the larva is termed a brachiolaria. Into these arms extensions of the coelome push out and in this particular they differ from the arms of the bipinnaria. These arms as listed include all the lobe-like outgrowths which are found in other starfish larvae.

The Coelomé. The development of the structures connected with the coelome form the most important contribution to the starfish body. After a period of slow enlargement near the oesophagus where they originated, the coelomic sacs begin a remarkable series of transformations. The right sac in some of the larvae grows to form a canal leading to the outside through the right madroporic pore, which soon closes up and is lost; and in other larvae it is never found. The left coelomie pouch, however, always makes a short vertical growth to the dorsal surface, forming the pore canal and uniting with an eetodermal depression to form the primary madroporic pore. Through the beating of its cilia water is passed into the coelome. These structures of the left side are significant for the future of the FM 135_ 1.~m,,m1V;Cw(,;u1;mchio1,.,;,. larvae, for it is the left side which

(3°m°“’h“t 3"“ AB*‘5Si”)- will develop to the greater part of a., anus; a.d.a., anterior dorsal arm; the adult Starfish, While the right

br.a., anterior median brachiolar arm; _ _ _ int., intestine;l.p.c.,left posterior coelome; contributes only a minimum. Never pt.l.a., posterior lateral arm; pt.o.a., post- ‘ oral arm; pr.o.a.. pre-oral arm; pt.d.a., thelebs’ anccstrally the larva‘ was

posterior dorsal arm; st., stomach. symmetrical and the two sides originally produced similar organs.

The sacs now grow in length and the right and left fuse together in the pre-oral lobe, although not elsewhere. A partial constriction of the left sac just behind the madroporic pore divides it into the left anterior and posterior coelomes. The hinder part of the left anterior coelome swells out to form a rather circular five-lobed outgrowth. This is the beginning of the water vascular system and is known as the hydrocoele. In some forms (Asterina) a right hydrocoele and the right anterior and posterior coelomes are also formed, but this is not the ease in Asterias. A madroporic vesicle is formed near the median line from the right anterior coelome. Its origin and position have been variously described, but the best opinion now seems to derive it from the right coelome. These developments complete the structure of the free-swimming biMETAMORPHOSIS OF ASTERI AS 179

pinnaria. It is now ready to settle down, become attached, grow into the brachiolaria and begin the metamorphosis which will produce the adult starfish. Between the bases of the brachiolar arms a region of fixation appears by means of which the larva becomes attached (fig. 126B). It is a thickening of the ectoderm, which becomes glandular, enabling it to fix the little animal to the substratum. The attachment takes place near the anterior end of the larva and gives to that portion


Fm. 126. Lateral \ iews of bruoliiolnria ol'As(cr2as1uIgan~r in process of fixation. (Modified from Goto.)

A, Left side of earliest stage. temporary fixation bv the bruchioluriun arms; B, part of older brachiolaria corresponding to the base of A and showing the permanent fixation by means of the fixing disc, the pre—oral lob'e liming shrunk. 1-5, lobes of hydrocoelc growing out to corresponding enlargements of the body wall, I——V, the unlugen of the arms.

br.l., lateral braehiolar arm; fix, disc by which the larva attaches itself; 1.pr.o.a., left pro-oral arm; m.d.a., median dorsal arm; oes., oesophagus; p.d.u., posterior dorsal arm; p.l.a., posterior lateral arm; r.pr.o.a., right pre-oral arm; stom., stomodaeum.

of the body the function of a stalk for the enlarging posterior portion or disc. As development continues the stalk shortens and the disc grows. In the latter are the right and left coelomes, the hydroeocle, stomach, and intestine. In the former are the anterior coelomes, oesophagus, mouth, and madroporie vesicle.

While the larva is becoming attached and constrictions which divide both right and left coelomes into anterior and posterior parts are becoming more evident, the left posterior sends a process over to the right side around the gut. This at length fuses with the right anterior coelome, and as the left and right anteriors were already joined in the prc-oral lobe the entire co_1nplex presently becomes metamorphosed into the single anterior coelome. The right posterior coelome, however, is entirely c11t off from the right anterior and forms the epigastric coelome of the adult. During the time that the left anterior and posterior coelomes were divided from each other by a septum formed by their opposed portions, there appears a groove on the anterior side which is to form the stone canal, while one on the posterior side is the first sign of the perioral coelome. The stone canal becomes connected with the madroporic pore canal and runs down to the hydrocoele. The perioral coelome becomes a crescentic tube about a bud of the stomach which represents the adult stomach. The further development of the alimentary tract is as follows: The stomodaeum gradually disappears, having first become disconnected from the midgut. The oesophagus itself gradually shortens and atrophies as does the intestine, the larval anus having closed. It is stated, however, that a remnant of the intestine persists and from it the rectum of the adult arises. From the left side of the larval stomach the bud referred to above grows to form the cardiac stomach of the adult which may be cverted for the purposes of food getting. As it grows it is surrounded by the perioral coelome. At a point on the left side of the larva, which consequently is to be known as the oral side of the adult, the growing bud makes connections with the ectoderm and an opening appears forming the mouth. The rectum and adult anus do not appear until the body form of the young starfish is practically complete.

final Metamorphos1's. Since the left side of the larva becomes the oral side the right side is the future aboral and the first signs of the body form are seen as five thickened lobe-like elevations of the ectoderm which are the rudiments of the arms and indicate the position of the aboral disc. With the growth of the disc portion, the coelomes become displaced backward in the larva to occupy what will be their position in the new body region. The hydrocoele becomes entirely separated from the anterior coelome. It sends out lobes which are the ru(Iiments of the radial canals of the water vascular system and from which tube feet and outer parts of the system are later formed. The stalk portion, becoming less and less both in size and in importance owing to changes in the shape of its cells and to phagocytic action on some of the others, is reduced until it is included in the disc area and is covered by the oral disc. Only a small projection from the oral disc holds the larva fast to the substratum and at length it is broken as the little starfish completes its body form and wrenches itself free. The further changes which the organ system undergoes are gradually accomplished during the growth of the young animal.

Tornarza of Balanoglossus. The modes of development in Balanoglossus and its ielated genera present a great deal of variation. In the genus

fiG 127 New England tornarm. from left side (After l\Iorgan )

a . anus, a d l , axitenor dorsal loop of the longitudinal r-iliated bind up . apical plate; up m , apical muscle band, 011 long longitudinal minted band (-001 H first, second, third. pairs of (oelomie ca\ mes g p, gill pouches mt , intestine o mouth oes , oesophagus per , penoardml sac, pl 1 , posterior lateral ll‘!!! of the lomzitudm 11 mliatod band. prol, pre—oral loop of the longitudinal cihated bind, st, atom l(‘h ttr telotroch, wp , water—pore.

Dolzchoglossus there is a large yolky egg which by a mueh—abridged series of stages reaches the adult without the intervention of any independent larval form. In several of the species of Balanoglossus a type of development which is probably more primitive is found, involving the presence of a special larva known as the tornaria. Originally when this larva was first found i_n towings it was assumed to belong to some genus of the Asteroidea, but the works of Bateson, Morgan, Ritter, and Heider have left no doubt as to its relations to Balanoglossus. Strangely enough C

fiG. 128. Metamorphosis of tornaria from Bahamas. (After Morgan.)

A, just before metamorphosis; B, during metamorphosis; C, young balanoglossid worm with three pairs of gill slits. ap . apical plate, cil. long., degenerating longitudinal ciliated band; col., collar region, col.p, collar-pore; ex., excretory tissue in posterior wall of proboscis coelom, gp., endodermic gill pouch, g.s., gill-slit; pr.p., proboscis-pore; tb ,

tongue-bar dividing the gill-slit; ttr.. telotroch; a.ttr., accessory telotroch characteristic of the Bahamas larva.

not all of the types of tornaria which have been described agree in all points, and several different methods for example of inesoderm formation and development of the coelomic cavities have been recorded for this larva. The work of Heider, although the most recent, is the only one which deals with the early stages. He was able to follow through the cleavage and development of the larva up to the tornaria. stage, but could keep his specimens alive only eight days. Morgan’s account from species taken at Woods Hole begins about where Heider’s specimens died so a fairly complete history is available. The structure of the tornaria larva superficially resembles that of the young bipinnaria and it is possible to apply the designations used on the bipinnaria to the longitudinal ciliated band and the posterior transverse ciliated band. This latter is also known as atelotroch, corresponding roughly to the same structure of the annelid trochophore. From the anterior end of the growing archenteron a vesicle is budded off which becomes the anterior coelome or the coelomc of the proboscis of the adult worm. From the posterior wall of the anterior coelome are developed rudiments of the pericardium and the peculiar dorsal heart of the Balanoglossus, as well as the rudiments of the head kidney. The more posterior part of the archenteron proliferates two groups of cells which are the beginnings of the middle or collar coeloine and the posterior or trunk coeloine. At a level in c, anterior coelome; p.('., front of the collar coelome a constriction

, _ 0., mouth, 1., intestine.

appears which marks the separation of the proboscis from the collar region of the adult. These two posterior coelomes are formed by different methods in the tornaria taken by Morgan at Woods Hole and those later studied by him in the Bahamas. In the former a solid outgrowth of cells from the gut wall is the rudiment of the coelome. In the latter the wall of the body cavity is formed by the aggregation of scattered mesenchyme cells, a procedure which is rarely witnessed in the animal kingdom. At the time of the constriction dividing the proboscis region from that posterior to it a dorsal strip of ectoderm remains unconstricted and becomes depressed beneath the surface in the form of a groove. This is the neural groove which is folded off and forms a neural tube in the fashion characteristic for vertebrate embryos. Below it from the anterior part of the oesophagus a median dorsal pouch grows out to form the notochord. Gill pouches develop as several pairs of pockets from the sides of the oesophagus. The further development of the larva consists largely in the formation of the modifications of the coelome, the formation of the collar pores and of the genital organs. The metamorphosis is brought about by the development of a fold in the region between the collar and trunk coelomes and the general growth "in length of the trunk portion of the animal. No such complications of development and metamorphosis are found here as characterize the echino— derms, for the adult itself is of comparatively simple structure. Since the aim of this discussion is merely to show how the embryo is transformed into a young worm similar to the adult, it seems unnecessary to follow through further the details of the development of this very interesting animal.

Bibliographic Note

Among the more important accounts of the subjects contained in this chapter are the following: Schultze, Minchin, Maas, Metchnikoff, Géttc, MacBride, Brooks, Appelliif, Korschelt and Heider, Salensky, Lang, Surface, Woltereck, Balfour, Claus, Grave, Bury, field, Batcson, Morgan, Ritter, I-leider on Balanoglossus. These works are cited in full in the bibliography on page 406.


1931 Richards: Part One General Embryology 1 Historical Development of Embryology | 2 The Germ-Cell Cycle | 3 Egg and Cleavage Types | 4 Holoblastic Types of Cleavage | 5 Meroblastic Types of Cleavage | 6 Types of Blastulae | 7 Endoderm Formation | 8 Mesoderm Formation | 9 Types of Invertebrate Larvae | 10 Formation of the Mammalian Embryo | 11 Egg and Embryonic Membranes | Part Two Embryological Problems 1 The Origin And Development Of Germ Cells | 2 Germ-Layer Theory | 3 The Recapitulation Theory | 4 Asexual Reproduction | 5 Parthenogenesis | 6 Paedogenesis And Neoteny | 7 Polyembryony | 8 The Determination Problem | 9 Ecological Control Of Invertebrate Larval Types


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