Three stages in the development of Limulus polyphemusFig. 245. Three stages in the development of Limulus polyphemus.(Somewhat modified from Packard.)A. Embryo in which the thoracic limbs and mouth have become developed on the ventral plate. The outer line represents what Packard believes to be the amnion.B. Later embryo from the ventral surface.C. Later embryo, just before the splitting of the chorion from the side. The full number of segments of the abdomen, and three abdominal appendages, have become established;m.mouth;I-IX.appendages.
Fig. 245. Three stages in the development of Limulus polyphemus.(Somewhat modified from Packard.)
A. Embryo in which the thoracic limbs and mouth have become developed on the ventral plate. The outer line represents what Packard believes to be the amnion.B. Later embryo from the ventral surface.C. Later embryo, just before the splitting of the chorion from the side. The full number of segments of the abdomen, and three abdominal appendages, have become established;m.mouth;I-IX.appendages.
Round the edge of the ventral plate there is a distinct ridge—the rudiment of the cephalothoracic shield.
With the further growth of the embryo the chorion becomes split and cast off, the embryo being left enclosed within the inner membrane. The embryo has a decided ventral flexure, and the abdominal region grows greatly and forms a kind of cap at the hinder end, while its vaulted dorsal side becomes divided into segments (fig. 245C). Of these there are according to Dohrn seven, but according to Packard nine, of which the last forms the rudiment of the caudal spine.
In the thoracic region the nervous system is by this stage formed as a ganglionated cord (Dohrn), with no resemblance to the peculiar œsophageal ring of the adult. The mouth is stated by Dohrn to lie between the second pair of limbs, so that, if the descriptions we have are correct, it must have by this stage changed its position with reference to the appendages. Between the thorax and abdomen two papillæ have arisen which form theso-called lower lip of the adult, but from their position and late development they can hardly be regarded as segmental appendages. In the course of further changes all the parts become more distinct, while the membrane in which the larva is placed becomes enormously distended (fig. 246A). The rudiments of the compound eyes are formed on the third (Packard) or fourth (Dohrn) segment of the cephalothorax, and the simple eyes near the median line in front. The rudiments of the inner process of the chelæ of the cephalothoracic appendages arise as buds. The abdominal appendages become more plate-like, and the rudiments of a third pair appear behind the two already present. The heart appears on the dorsal surface.
An ecdysis now takes place, and in the stage following the limbs have approached far more closely to their adult state (fig. 246A). The cephalothoracic appendages become fully jointed; the two anterior abdominal appendages (VII.) have approached, and begin to resemble the operculum of the adult, and on the second pair is formed a small inner ramus. The segmentation of the now vaulted cephalothorax becomes less obvious, though still indicated by the arrangement of the yolk masses which form the future hepatic diverticula.
Two stages in the development of Limulus polyphemusFig. 246. Two stages in the development of Limulus polyphemus.(After Dohrn.)A. An advanced embryo enveloped in the distended inner membrane shortly before hatching; from the ventral side.B. A later embryo at the Trilobite stage, from the dorsal side.I.,VII.,VIII.First, seventh, and eighth appendages.cs.caudal spine;se.simple eye;ce.compound eye.
Fig. 246. Two stages in the development of Limulus polyphemus.(After Dohrn.)
A. An advanced embryo enveloped in the distended inner membrane shortly before hatching; from the ventral side.B. A later embryo at the Trilobite stage, from the dorsal side.I.,VII.,VIII.First, seventh, and eighth appendages.cs.caudal spine;se.simple eye;ce.compound eye.
Shortly after this stage the embryo is hatched, and at about the time of hatching acquires a form (fig. 246B) in which it bears, as pointed out by Dohrn and Packard, the most striking resemblance to a Trilobite.
Viewed from the dorsal surface (fig. 246B) it is divided into two distinct regions, the cephalothoracic in front and the abdominal behind. The cephalothoracic has become much flatter and wider, has lost all trace of its previous segmentation, and has become distinctly trilobed. Thecentral lobe forms a well-marked keel, and at the line of insertion of the rim-like edge of the lateral lobes are placed the two pairs of eyes (seandce). The abdominal region is also distinctly trilobed and divided into nine segments; the last, which is merely formed of a median process, being the rudiment of the caudal spine. The edges of the second to the seventh are armed with a spine. The changes in the appendages are not very considerable. The anterior pair nearly meet in the middle line in front or the mouth; and the latter structure is completely covered by an upper lip. Each abdominal appendage of the second pair is provided with four gill lamellæ, attached close to its base.
Three weeks after hatching an ecdysis takes place, and the larva passes from a trilobite into a limuloid form. The segmentation of the abdomen has become much less obvious, and this part of the embryo closely resembles its permanent form. The caudal spine is longer, but is still relatively short. A fourth pair of abdominal appendages is established, and the first pair have partially coalesced, while the second and third pairs have become jointed, their outer ramus containing four and their inner three joints. Additional gill lamellæ attached to the two basal joints of the second and third abdominal appendages have appeared.
The further changes are not of great importance. They are effected in a series of successive moults. The young larvæ swim actively at the surface.
Our, in many respects, imperfect knowledge of the development of Limulus is not sufficient to shew whether it is more closely related to the Crustacea or to the Arachnida, or is an independent phylum.
The somewhat Crustacean character of biramous abdominal feet, etc. is not to be denied, but at the same time the characters of the embryo appear to me to be decidedly more arachnidan than crustacean. The embryo, when the appendages are first formed, has a decidedly arachnidan facies. It will be remembered that when the limbs are first formed they are allpost-oral. They resemble in this respect the limbs of the Arachnida, and it seems to be probable that the anterior pair is equivalent to the cheliceræ of Arachnida, which, as shewn in a previous section, are really post-oral appendages in no way homologous with antennæ[212].
The six thoracic appendages may thus be compared with the six Arachnidan appendages; which they resemble in their relation to the mouth, their basal cutting blades, etc.
The existence of abdominal appendages behind the six cephalothoracic does not militate against the Arachnidan affinities of Limulus, because in the Arachnida rudimentary abdominal appendages are always present in the embryo. The character of the abdominal appendages is probablysecondarily adapted to an aquatic respiration, since it is likely (for the reasons already mentioned in connection with the Tracheata) that if Limulus has any affinities with the stock of the Tracheata it is descended from air-breathing forms, and has acquired its aquatic mode of respiration. The anastomosis of the two halves of the generative glands is an Arachnidan character, and the position of the generative openings in Limulus is more like that in the Scorpion than in Crustacea.
A fuller study of the development would be very likely to throw further light on the affinities of Limulus, and if Packard’s view about the nature of the inner egg membrane were to be confirmed, strong evidence would thereby be produced in favour of the Arachnidan affinities.
(533)A. Dohrn.“Untersuch. üb. Bau u. Entwick. d. Arthropoden (Limulus polyphemus).”Jenaische Zeitschrift,Vol.VI., 1871.(534)A. S. Packard. “The development of Limulus polyphemus.”Mem. Boston Soc. Nat. History,Vol.II., 1872.
Pycnogonida.
The embryos, during the first phases of their development, are always carried by themalein sacks which are attached to a pair of appendages (the third) specially formed for this purpose. The segmentation of the ovum is complete, and there is in most forms developed within the eggshell a larva with three pairs of two-jointed appendages, and a rostrum placed between the front pair.
It will be convenient to take Achelia lævis, studied by Dohrn (No.536), as type.
The larva of Achelia when hatched is provided with the typical three pairs of appendages. The foremost of them is chelate, and the two following pairs are each provided with a claw. Of the three pairs of larval appendages Dohrn states that he has satisfied himself that the anterior is innervated by the supra-œsophageal ganglion, and the two posterior by separate nerves coming from two imperfectly united ventral ganglia. The larva is provided with a median eye formed of two coalesced pigment spots, and with a simple stomach.
The gradual conversion of the larva into the adult takes place by the elongation of the posterior end of the body into a papilla, and the formation there, at a later period, of the anus; while at the two sides of the anal papilla rudiments of a fresh pair of appendages—the first pair of ambulatory limbs of the adult—make their appearance. The three remaining pairs of limbs become formed successively as lateral outgrowths, and their development is accomplished in a number of successive ecdyses. As they are formed cæca from the stomach become prolonged into them. For each of them there appears a special ganglion. While the above changes are taking place the three pairs of larval appendages undergo considerable reduction. The anterior pair singly becomes smaller, the second loses its claw, and the third becomes reduced to a mere stump. In the adult thesecond pair of appendages becomes enlarged again and forms the so-called palpi, while the third pair develops in the male into the egg-carrying appendages, but is aborted in the female. The first pair form appendages lying parallel to the rostrum, which are sometimes called pedipalpi and sometimes antennæ.
The anal papilla is a rudimentary abdomen, and, as Dohrn has shewn, contains rudiments of two pairs of ganglia.
The larvæ of Phoxichilidium are parasitic in various Hydrozoa (Hydractinia, etc.). After hatching they crawl into the Hydractinia stock. They are at first provided with the three normal pairs of larval appendages. The two hinder of these are soon thrown off, and the posterior part of the trunk, with the four ambulatory appendages belonging to it, becomes gradually developed in a series of moults. The legs, with the exception of the hindermost pair, are fully formed at the first ecdysis after the larva has become free. In the genus Pallene the metamorphosis is abbreviated, and the young are hatched with the full complement of appendages.
The position of the Pycnogonida is not as yet satisfactorily settled. The six-legged larva has none of the characteristic features of the Nauplius, except the possession of the same number of appendages.
The number of appendages (7) of the Pycnogonida does not coincide with that of the Arachnida. On the other hand, the presence of chelate appendages innervated in the adult by the supra-œsophageal ganglia rather points to a common phylum for the Pycnogonida and Arachnida; though as shewn above (p.455) all the appendages in the embryo of true Arachnida are innervated by post-oral ganglia. The innervation of these appendages in the larvæ of Pycnogonida requires further investigation. Against such a relationship the extra pair of appendages in the Pycnogonida is no argument, since the embryos of most Arachnida are provided with four such extra pairs. The two groups must no doubt have diverged very early.
Bibliography.
(535)G. Cavanna.“Studie e ricerche sui Picnogonidi.”Pubblicazioni del R. Instituto di Studi superiori in Firenze,1877.(536)An. Dohrn.“Ueber Entwicklung u. Bau d. Pycnogoniden.”Jenaische Zeitschrift,Vol.V.1870, and“Neue Untersuchungen üb. Pycnogoniden.”Mittheil. a. d. zoologischen Station zu Neapel,Bd.I.1878.(537)G. Hodge. “Observations on a species of Pycnogon, etc.”Annal. and Mag. of Nat. Hist.Vol.IX.1862.(538)C. Semper.“Ueber Pycnogoniden u. ihre in Hydroiden schmarotzenden Larvenformen.”Arbeiten a. d. zool.-zoot. Instit. Würzburg,Vol.I.1874.
Pentastomida.
The development and metamorphosis of Pentastomum tænoides have been thoroughly worked out by Leuckart (No.540) and will serve as type for the group.
In the sexual state it inhabits the nasal cavities of the dog. The early embryonic development takes place as the ovum gradually passes down the uterus. The segmentation appears to be complete; and gives rise to an oval mass in which the separate cells can hardly be distinguished. This gradually differentiates itself into a characteristic embryo, divided into a tail and trunk. The tail is applied to the ventral surface of the trunk, and on the latter two pairs of stump-like unsegmented appendages arise, each provided with a pair of claws. At the anterior extremity of the body is formed the mouth, with a ventral spine and lateral hook, which are perhaps degenerated jaws. The spine functions as a boring apparatus, and an apparatus with a similar function is formed at the end of the tail. A larval cuticle now appears, which soon becomes detached from the embryo, except on the dorsal surface, where it remains firmly united to a peculiar papilla. This papilla becomes eventually divided into two parts, one of which remains attached to the cuticle, while the part connected with the embryo forms a raised cross placed in a cup-shaped groove. The whole structure has been compared, on insufficient grounds, to the dorsal organ of the Crustacea.
The eggs, containing the embryo in the condition above described, are eventually carried out with the nasal slime, and, if transported thence into the alimentary cavity of a rabbit or hare, the embryos become hatched by the action of the gastric juice. From the alimentary tract of their new host they make their way into the lungs or liver. They here become enveloped in a cyst, in the interior of which they undergo a very remarkable metamorphosis. They are, however, so minute and delicate that Leuckart was unable to elucidate their structure till eight weeks after they had been swallowed. At this period they are irregularly-shaped organisms, with a most distant resemblance to the earlier embryos. They are without their previous appendages, but the alimentary tract is now distinctly differentiated. The remains of two cuticles in the cyst seem to shew that the above changes are effected in two ecdyses.
In the course of a series of ecdyses the various organs of the larval form known as Pentastomum denticulatum continue to become differentiated. After the first (=third) ecdysis the œsophageal nerve-ring and sexually undifferentiated generative organs are developed. At the fourth (=sixth) ecdysis the two pairs of hooks of the adult are formed in pockets which appeared at a somewhat earlier stage; and the body acquires an annulated character. At a somewhat earlier period rudiments of the external generative organs indicate the sex of the larva.
After a number of further ecdyses, which are completed in about six months after the introduction of the embryos into the intermediate host, the larva attains its full development, and acquires a form in which it has long been known as Pentastomum denticulatum. It now leaves its cyst and begins to move about. It is in a state fit to be introduced into its final host; but if it be not so introduced it may become encysted afresh.
If the part of a rabbit or hare infected by a Pentastomum denticulatum be eaten by a dog or wolf, the parasite passes into the nasal cavity of thelatter, and after further changes of cuticle becomes a fully developed sexual Pentastomum tænioides, which does not differ to any very marked extent from P. denticulatum.
In their general characters the larval migrations of Pentastomum are similar to those of the Cestodes.
The internal anatomy of the adult Pentastomum, as well as the characters of the larva with two pairs of clawed appendages, are perhaps sufficient to warrant us in placing it with the Arthropoda, though it would be difficult to shew that it ought not to be placed with such a form as Myzostomum (videp.369). There do not appear to be any sufficient grounds to justify its being placed with the Mites amongst the Arachnida. If indeed the rings of the body of the Pentastomida are to be taken as implying a true segmentation, it is clear that the Pentastomida cannot be associated with the Mites.
Bibliography.
(539)P. J. van Beneden.“Recherches s. l’organisation et le développement d. Linguatules.”Ann. d. Scien. Nat.,3Ser., Vol.XI.(540)R. Leuckart.“Bau u. Entwicklungsgeschichte d. Pentastomen.”Leipzig and Heidelberg. 1860.
Tardigrada.
Very little is known with reference to the development of the Tardigrada. A complete and regular segmentation (von Siebold, Kaufmann,No.541) is followed by the appearance of a groove on the ventral side indicating a ventral flexure. At about the time of the appearance of the groove the cells become divided into an epiblastic investing layer and a central hypoblastic mass.
The armature of the pharynx is formed very early at the anterior extremity, and the limbs arise in succession from before backwards.
The above imperfect details throw no light on the systematic position of this group.
Tardigrada.
(541)J. Kaufmann.“Ueber die Entwicklung u. systematische Stellung d. Tardigraden.”Zeit. f. wiss. Zool.,Bd.III.1851.
Summary of Arthropodan Development.
The numerous characters common to the whole of the Arthropoda led naturalists to unite them in a common phylum, but the later researches on the genealogy of the Tracheata and Crustacea tend to throw doubts on this conclusion, while there is not as yet sufficient evidence to assign with certainty a definite position in either of these classes to the smaller groups described in the present chapter. There seems to be but littledoubt that the Tracheata are descended from a terrestrial Annelidan type related to Peripatus. The affinities of Peripatus to the Tracheata are, as pointed out in a previous chapter (p.386), very clear, while at the same time it is not possible to regard Peripatus simply as a degraded Tracheate, owing to the fact that it is provided with such distinctly Annelidan organs as nephridia, and that its geographical distribution shews it to be a very ancient form.
The Crustacea on the other hand are clearly descended from a Phyllopod-like ancestor, which can be in no way related to Peripatus.
The somewhat unexpected conclusion that the Arthropoda have a double phylum is on the whole borne out by the anatomy of the two groups. Without attempting to prove this in detail, it may be pointed out that the Crustacean appendages are typically biramous, while those of the Tracheata are never at any stage of development biramous[213]; and the similarity between the appendages of some of the higher Crustacea and those of many Tracheata is an adaptive one, and could in no case be used as an argument for the affinity of the two groups.
The similarity of many organs is to be explained by both groups being descendants of Annelidan ancestors. The similarity of the compound eye in the two groups cannot however be explained in this way, and is one of the greatest difficulties of the above view. It is moreover remarkable that the eye of Peripatus[214]is formed on a different type to either the single or compound eyes of most Arthropoda.
The conclusion that the Crustacea and Tracheata belong to two distinct phyla is confirmed by a consideration of their development. They have no doubt in common a centrolecithal segmentation, but, as already insisted on, the segmentation is no safe guide to the affinities.
In the Tracheata the archenteron is never, so far as we know, formed by an invagination[215], while in Crustacea theevidence is in favour of such an invagination being the usual, and, without doubt, the primitive, mode of origin.
The mesoblast in the Tracheata is formed in connection with a median thickening of the ventral plate. The unpaired plate of mesoblast so formed becomes divided into two bands, one on each side of the middle line.
In both Spiders and Myriopods, and probably Insects, the two plates of mesoblast are subsequently divided into somites, the lumen of which is continued into the limbs.
In Crustacea the mesoblast usually originates from the walls of the invagination, which gives rise to the mesenteron.
It does not become divided into two distinct bands, but forms a layer of scattered cells between the epiblast and hypoblast, and does not usually break up into somites; and though somites are stated in some cases to be found they do not resemble those in the Tracheata.
The proctodæum is usually formed in Crustacea before and rarely later[216]than the stomodæum. The reverse is true for the Tracheata. In Crustacea the proctodæum and stomodæum, especially the former, are very long, and usually give rise to the greater part of the alimentary tract, while the mesenteron is usually short.
In the Tracheata the mesenteron is always considerable, and the proctodæum is always short. The derivation of the Malpighian bodies from the proctodæum is common to most Tracheata. Such diverticula of the proctodæum are not found in Crustacea.
[210]The nature of the inner membrane is obscure. It is believed by Packard to be moulted after the formation of the limbs, andto be equivalent to the amnion of Insects, while by Dohrn it is regarded as a product of the follicle cells.[211]Dohrn finds at first only five appendages, but thinks that the sixth (the anterior one) may have been present but invisible.[212]Dohrn believes that he has succeeded in shewing that the first pair of appendages of Limulus is innervated in the embryo from the supra-œsophageal ganglia. His observations do not appear to me conclusive, and arguing from what we know of the development of the Arachnida, the innervation of these appendages in the adult can be of no morphological importance.[213]The biflagellate antennæ of Pauropus amongst the Myriapods can hardly be considered as constituting an exception to this rule.[214]I hope to shew this in a paper I am preparing on the anatomy of Peripatus.[215]Stecker’s description of an invagination in the Chilognatha cannot be accepted without further confirmation;videp.388.[216]This is stated to be the case in Moina (Grobben).
[210]The nature of the inner membrane is obscure. It is believed by Packard to be moulted after the formation of the limbs, andto be equivalent to the amnion of Insects, while by Dohrn it is regarded as a product of the follicle cells.
[211]Dohrn finds at first only five appendages, but thinks that the sixth (the anterior one) may have been present but invisible.
[212]Dohrn believes that he has succeeded in shewing that the first pair of appendages of Limulus is innervated in the embryo from the supra-œsophageal ganglia. His observations do not appear to me conclusive, and arguing from what we know of the development of the Arachnida, the innervation of these appendages in the adult can be of no morphological importance.
[213]The biflagellate antennæ of Pauropus amongst the Myriapods can hardly be considered as constituting an exception to this rule.
[214]I hope to shew this in a paper I am preparing on the anatomy of Peripatus.
[215]Stecker’s description of an invagination in the Chilognatha cannot be accepted without further confirmation;videp.388.
[216]This is stated to be the case in Moina (Grobben).
The development of the Echinodermata naturally falls into two sections:—
(1) The development of the germinal layers and of the systems of organs; (2) the development of the larval appendages and the metamorphosis.
The Development of the Germinal Layers and of the Systems of Organs.
The development of the systems of organs presents no very important variations within the limits of the group.
Holothuroidea.The Holothurians have been most fully studied (Selenka,No.563), and may be conveniently taken as type.
The segmentation is nearly regular, though towards its close, and in some instances still earlier, a difference becomes apparent between the upper and the lower poles.
At the close of segmentation (fig. 247A) the egg has a nearly spherical form, and is constituted of a single layer of columnar cells enclosing a small segmentation cavity. The lower pole is slightly thickened, and the egg rotates by means of fine cilia.
An invagination now makes its appearance at the lower pole (fig. 247B), and simultaneously there become budded offfrom the cells undergoing the invaginationamœboid cells, whicheventually form the muscular system and the connective tissue. These cells very probably have a bilaterally symmetrical origin. This stage represents the gastrula stage which is common to all Echinoderms. The invaginated sack is the archenteron. As it grows larger one side of the embryo becomes flattened, and the other more convex. On the flattened side a fresh invagination arises, the opening of which forms the permanent mouth, the opening of the first invagination remaining as the permanent anus (fig. 248A).
Two stages in the development of Holothuria tubulosaFig. 247. Two stages in the development of Holothuria tubulosa viewed in optical section.(After Selenka.)A. Blastosphere stage at the close of segmentation. B. Gastrula stage.mr.micropyle;fl.chorion;s.c.segmentation cavity;bl. blastoderm;ep.epiblast;hy.hypoblast;ms.amœboid cells derived from hypoblast;a.e.archenteron.
Fig. 247. Two stages in the development of Holothuria tubulosa viewed in optical section.(After Selenka.)
A. Blastosphere stage at the close of segmentation. B. Gastrula stage.mr.micropyle;fl.chorion;s.c.segmentation cavity;bl. blastoderm;ep.epiblast;hy.hypoblast;ms.amœboid cells derived from hypoblast;a.e.archenteron.
These changes give us the means of attaching definite names to the various parts of the embryo. It deserves to be noted in the first place that the embryo has assumed a distinctly bilateral form. There is present a more or less concave surface extending from the mouth to near the anus, which will be spoken of as the ventral surface. The anus is situated at the posterior extremity. The convex surface opposite the ventral surface forms the dorsal surface, which terminates anteriorly in a rounded præ-oral prominence.
It will be noticed infig. 248A that in addition to the primitive anal invagination there is present a vesicle (v.p.). This vesicle is directly formed by a constriction of the primitivearchenteron (fig. 249Vpv.), and is called by Selenka the vaso-peritoneal vesicle. It gives origin to the epithelioid lining of the body cavity and water-vascular system of the adult[218]. In the parts now developed we have the rudiments of all the adult organs.
The mouth and anal involutions (after the separation of the vaso-peritoneal vesicle) meet and unite, a constriction indicating their point of junction (fig. 248B). Eventually the former gives rise to the mouth and œsophagus, and the latter to the remainder of the alimentary canal[219].
Three stages in the development of Holothuria tubulosaFig. 248. Three stages in the development of Holothuria tubulosa viewed from the side in optical section.(After Selenka.)m.mouth;oe.œsophagus;st.stomach;i.intestine;aanus;l.c.longitudinal ciliated band;v.p.vaso-peritoneal vesicle;p.v.peritoneal vesicle;p.r.right peritoneal vesicle;pl.left peritoneal vesicle;w.v.water-vascular vesicle;p.dorsal pore of water-vascular system;ms.muscle cells.
Fig. 248. Three stages in the development of Holothuria tubulosa viewed from the side in optical section.(After Selenka.)
m.mouth;oe.œsophagus;st.stomach;i.intestine;aanus;l.c.longitudinal ciliated band;v.p.vaso-peritoneal vesicle;p.v.peritoneal vesicle;p.r.right peritoneal vesicle;pl.left peritoneal vesicle;w.v.water-vascular vesicle;p.dorsal pore of water-vascular system;ms.muscle cells.
The vaso-peritoneal vesicle undergoes a series of remarkable changes. After its separation from the archenteron it takes up a position on the left side of this, elongates in an antero-posterior direction, and from about its middle sends a narrow diverticulum towards thedorsalsurface of the body, where anopening to the exterior becomes formed (fig. 248B,p.). The diverticulum becomes the madreporic canal, and the opening the dorsal pore.
The vaso-peritoneal vesicle next divides into two, an anterior vesicle (fig. 248B,w.v.), from which is derived the epithelium of the water-vascular system, and a posterior (fig. 248B,p.v.), which gives rise to the epithelioid lining of the body cavity. The anterior vesicle (fig. 248C,w.v.) becomes five-lobed, takes a horseshoe-shaped form, and grows round the œsophagus (fig. 256,w.v.r). The five lobes form the rudiments of the water-vascular prolongations into the tentacles. The remaining parts of the water-vascular system are also developed as outgrowths of the original vesicle. Five of these, alternating with the original diverticula, form the five ambulacral canals, from which diverticula are produced into the ambulacral feet; a sixth gives rise to the Polian vesicle. The remaining parts of the original vesicle form the water-vascular ring.
We must suppose that eventually the madreporic canal loses its connection with the exterior so as to hang loosely in the interior, though the steps of this process do not appear to have been made out.
Section through an embryo of Cucumaria doliolumFig. 249. Longitudinal section through an embryo of Cucumaria doliolum at the end of the fourth day.Vpv.vaso-peritoneal vesicle;ME.mesenteron;Blp.,Ptd.blastopore, proctodæum.
Fig. 249. Longitudinal section through an embryo of Cucumaria doliolum at the end of the fourth day.
Vpv.vaso-peritoneal vesicle;ME.mesenteron;Blp.,Ptd.blastopore, proctodæum.
The original hinder peritoneal vesicle grows rapidly, and divides into two (fig. 248C,pl.andpr.), which encircle the two sides of the alimentary canal, and meet above and below it. The outer wall of each of them attaches itself to the skin, and the inner one to the alimentary canal and water-vascular system; in both cases the walls remain separated from the adjacent parts by a layer of the amœboid cells already spoken of. The cavity of the peritoneal vesicles becomes the permanent body cavity. Where the walls ofthe two vesicles meet on the dorsal side, a mesentery, suspending the alimentary canal and dividing the body cavity longitudinally, is often formed. In other parts the partition walls between the two sacks appear to be absorbed.
The amœboid cells, which were derived from the invaginated cells, arrange themselves as a layer round all the organs (fig. 249). Some of them remain amœboid, attach themselves to the skin, and form part of the cutis; and in these cells the calcareous spicula of the larva and adult are formed. Others form the musculature of the larval alimentary tract, while the remainder give rise to the musculature and connective tissue of the adult.
The development of the vascular system is not known, but the discovery of Kowalevsky, confirmed by Selenka, that from the walls of the water-vascular system corpuscles are developed, identical with those in the blood-vessels, indicates that it probably develops in connection with the water-vascular system. The observations of Hoffmann and Perrier on the communication of the two systems in the Echinoidea point to the same conclusion. Though nothing very definite is known with reference to the development of the nervous system, Metschnikoff suggests that it develops in connection with the thickened bands of epiblast which are formed by a metamorphosis of the ciliated bands of the embryo, and accompany the five radial tubes (videp.555). In any case its condition in the adult leaves no doubt of its being a derivative of the epiblast.
From the above description the following general conclusions may be drawn:—
(1) The blastosphere stage is followed by a gastrula stage.
(2) The gastrula opening forms the permanent anus, and the mouth is formed by a fresh invagination.
(3) The mesoblast arises entirely from the invaginated cells, but in two ways:—
(a) As scattered amœboid cells, which give origin to the muscles and connective tissue (including the cutis) of the body wall and alimentary tract.
(b) As a portion separated off from the archenteron, which gives rise both to the epithelioid lining of the body cavity, and of the water-vascular system.
(4) The œsophagus is derived from an invagination of the epiblast, and the remainder of the alimentary canal from the archenteron.
(5) The embryonic systems of organs pass directly into those of the adult.
The development of Synapta diverges, as might be expected, to a very small extent from that of Holothuria.
Asteroidea.In Asterias the early stages of development conform to our type. There arise, however, two bilaterally symmetrical vaso-peritoneal diverticula from the archenteron. These diverticula give rise both to the lining of the body cavity and water-vascular system. With reference to the exact changes they undergo there is, however, some difference of opinion. Agassiz (543) maintains that both vesicles are concerned in the formation of the water-vascular system, while Metschnikoff (560) holds that the water-vascular system is entirely derived from the anterior part of the larger left vesicle, while the right and remainder of the left vesicle form the body cavity. Metschnikoff’s statements appear to be the most probable. The anterior part of the left vesicle, after separating from the posterior, grows into a five-lobed rosette (fig. 260,i), and a madreporic canal (h) with a dorsal pore opening to the exterior. The rosette appears not to grow round the œsophagus, as in the cases hitherto described. But the latter is stated to disappear, and a new œsophagus to be formed, which pierces the rosette, and places the old mouth in communication with the stomach. Except where the anus is absent in the adult, the larval anus probably persists.
Ophiuroidea.The early development of the Ophiuroidea is not so fully known as that of other types. Most species have a free-swimming larva, but some (Amphiura) are viviparous.
The early stages of the free-swimming larvæ have not been described, but I have myself observed in the case of Ophiothrix fragilis that the segmentation is uniform, and is followed by the normal invagination. The opening of this no doubt remains as the larval anus, and there are probably two outgrowths from this to form the vaso-peritoneal vesicles. Each of these divides into two parts, an anterior lying close to the œsophagus, and a posterior close to the stomach. The anterior on the right side aborts; that on the left side becomes the water-vascular vesicle, early opens to the exterior, and eventually grows round the œsophagus, which, as in Holothurians, becomes the œsophagus of the adult. The posterior vesicles give rise to the lining of the body cavity, but are stated by Metschnikoff to be at first solid, and only subsequently to acquire a cavity—the permanent body cavity. The anus naturally disappears, since it is absent in the adult. In the viviparous type the first stages are imperfectly known, but it appears that the blastopore vanishes before the appearance of the mouth. The development of the vaso-peritoneal bodies takes place as in the free-swimming larvæ.
Echinoidea.In the Echinoidea (Agassiz,No.542, Selenka,No.564) there is a regular segmentation and the normal invagination (fig. 250A). The amœboid mesoblast cells arise as two laterally placed masses, and give rise to the usual parts. The archenteron grows forward and bends towardsthe ventral side (fig. 250B). It becomes (fig. 250C) divided into three chambers, of which the two hindermost (dandc) form the stomach and intestine; while the anterior forms the œsophagus, and gives rise to the vaso-peritoneal vesicles. These latter appear as a pair of outgrowths (fig. 251), but become constricted off asa single two-horned vesicle, which subsequently divides into two. The left of these is eventually divided, as in Asteroids, into a peritoneal and water-vascular sack, while the right forms the right peritoneal sack. An oral invagination on the flattened ventral side meets the mesenteron after its separation from the vaso-peritoneal vesicle. The larval anus persists, as also does the larval mouth, but owing to the manner in which the water-vascular rosette is established the larval œsophagus appears to be absorbed, and to be replaced by a fresh œsophagus.