Three side views of early stages in the development of StrongylocentrusFig. 250. Three side views of early stages in the development of Strongylocentrus.(From Agassiz.)a.anus (blastopore);d.stomach;o.œsophagus;c.rectum;w.vaso-peritoneal vesicle;v.ciliated ridge;r.calcareous rod.
Fig. 250. Three side views of early stages in the development of Strongylocentrus.(From Agassiz.)
a.anus (blastopore);d.stomach;o.œsophagus;c.rectum;w.vaso-peritoneal vesicle;v.ciliated ridge;r.calcareous rod.
Crinoidea. Antedon, the only Crinoid so far studied (Götte, No.549), presents some not inconsiderable variations from the usual Echinoderm type. The blastopore is placed on the somewhat flattened side of the oval blastosphere, and not, as is usual, at the hinder end.
An early larva of StrongylocentrusFig. 251. Dorso-ventral view of an early larva of Strongylocentrus.(From Agassiz.)a.anus;d.stomach;o.œsophagus;w.vaso-peritoneal vesicle;r.calcareous rod.
Fig. 251. Dorso-ventral view of an early larva of Strongylocentrus.(From Agassiz.)
a.anus;d.stomach;o.œsophagus;w.vaso-peritoneal vesicle;r.calcareous rod.
The blastopore completely closes, and is not converted into the permanent anus. The archenteron gives rise to the epithelioid lining of both body cavity and water-vascular system. These parts do not, however, appear as a single or paired outgrowth from the archenteron, but as three distinct outgrowths which are not formed contemporaneously. Two of them are firstformed, and become the future body cavity; but their lumens remain distinct. Originally appearing as lateral outgrowths, the right one assumes a dorsal position and sends a prolongation into the stalk (fig. 252,rp´), and the left one assumes first a ventral, and then an oral position (fig. 252,lp).
Section through an Antedon larvaFig. 252. Longitudinal section through an Antedon larva.(From Carpenter; after Götte.)al.mesenteron;wv.water-vascular ring;lp.left (oral) peritoneal vesicle;rp.right peritoneal vesicle;rp´.continuation of right vesicle into the stalk;st.stalk.
Fig. 252. Longitudinal section through an Antedon larva.(From Carpenter; after Götte.)
al.mesenteron;wv.water-vascular ring;lp.left (oral) peritoneal vesicle;rp.right peritoneal vesicle;rp´.continuation of right vesicle into the stalk;st.stalk.
The third outgrowth of the archenteron gives rise to the water-vascular vesicle. It first grows round the region of the future œsophagus and so forms the water-vascular ring. The wall of the ring then grows towards the body wall so as to divide the oral (left) peritoneal vesicle into two distinct vesicles, an anterior and a posterior, shewn infig. 253,lp´andlp. Before this division is completed, the water-vascular ring is produced in front into five processes—the future tentacles (fig. 252,wv)—which project into the cavity of the oral vesicle (lp). After the oral peritoneal space has become completely divided into two parts, the anterior dilates (fig. 253,lp´) greatly, and forms a large vestibule at the anterior end of the body. This vestibule (lp´) next acquires a communication with the mesenteron, shewn infig. 253at m. The anterior wall of this vestibule is finally broken through. By this rupture the mesenteron is placed in communication with the exterior by the opening atm, while at the same time the tentacles of the water-vascular ring (t) project freely to the exterior. Such is Götte’s account of the præ-oral body space, but, as he himself points out, it involves our believing that the lining of the diverticulum derived from the primitive alimentary vesicle becomes part of the external skin. This occurrence is so remarkable, that more evidence appears to me requisite before accepting it.
The formation of the anus occurs late. Its position appears to be the same as that of the blastopore, and is indicated by a papilla of the mesenteron attaching itself to the skin on the ventral side (fig. 253,an). It eventually becomes placed in an interradial space within the oral disc of the adult. The water-vascular ring has no direct communication with the exterior, but the place of the madreporic canal of other types appears to be taken in the larva by a single tube leading from the exterior into the body cavity, the external opening of which is placed on one of the oral plates (videp.571) in the next interradial space to the right of the anus, and a corresponding diverticulum of the water-vascular ring opening into the body cavity. The line of junction between the left and right peritoneal vesicles forms in the larva a ring-like mesentery dividing the oral from the aboral part of the bodycavity. In the adult[220]the oral section of the larval body cavity becomes the ventral part of the circumvisceral division of the body cavity, and the subtentacular canals of the arms and disc; while the aboral section becomes the dorsal part of the circumvisceral division of the body cavity, the cœliac canals of the arms, and the cavity of the centro-dorsal piece. The primitive distinction between the sections of the larval body cavity becomes to a large extent obliterated, while the axial and intervisceral sections of the body cavity of the adult are late developments.
Section of an advanced Pentacrinoid Antedon larvaFig. 253. Longitudinal section through the calyx of an advanced Pentacrinoid Antedon larva with closed vestibule.(From Carpenter; after Götte.)ae.epithelium of oral vestibule;m.mouth;al.mesenteron;an.rudiment of permanent anus;lp.posterior part of left (oral) peritoneal sack;lp´.anterior part of left (oral) peritoneal sack;wr.water-vascular ring;t.tentacle;mt.mesentery;rp.right peritoneal sack;rp´.continuation of right peritoneal sack into the stalk;r.roof of tentacular vestibule.
Fig. 253. Longitudinal section through the calyx of an advanced Pentacrinoid Antedon larva with closed vestibule.(From Carpenter; after Götte.)
ae.epithelium of oral vestibule;m.mouth;al.mesenteron;an.rudiment of permanent anus;lp.posterior part of left (oral) peritoneal sack;lp´.anterior part of left (oral) peritoneal sack;wr.water-vascular ring;t.tentacle;mt.mesentery;rp.right peritoneal sack;rp´.continuation of right peritoneal sack into the stalk;r.roof of tentacular vestibule.
The more important points in the development indicated in the preceding pages are as follows:
(1) The blastosphere is usually elongated in the direction of the axis of invagination, but in Comatula it is elongated transversely to this axis.
(2) The blastopore usually becomes the permanent anus, but it closes at the end of larval life (there being no anus in the adult) in Ophiuroids and some Asteroids, while in Comatula it closes very early, and a fresh anus is formed at the point where the blastopore was placed.
(3) The larval mouth always becomes the mouth of the adult.
(4) The archenteron always gives rise to outgrowths which form the peritoneal membrane and water-vascular systems. In Comatula there are three such outgrowths, two paired, which form the peritoneal vesicles, and one unpaired, which forms the water-vascular vesicle. In Asteroids and Ophiuroids there are two outgrowths. In Ophiuroids both of these are divided into a peritoneal and a water-vascular vesicle, but the right water-vascular vesicle atrophies. In Asteroids only one water-vascular vesicle is formed, which is derived from the left peritoneal vesicle. In Echinoids and Holothuroids there is a single vaso-peritoneal vesicle.
(5) The water-vascular vesicle grows round the larval œsophagus in Holothuroids, Ophiuroids, and Comatula; in these cases the larval œsophagus is carried on into the adult. In other forms the water-vascular vesicle forms a ring which does not enclose the œsophagus (Asteroids and Echinoids); in such cases a new œsophagus is formed, which perforates this ring.
Development of the larval appendages and metamorphosis.
Holothuroidea.The young larva of Synapta, to which J. Müller gave the name Auricularia (fig. 255), is in many respects the simplest form of Echinoderm larva. With a few exceptions the Auricularia type of larva is common to the Holothuria.
Larva of Holothuroid and of AsteroidFig. 254. A. The larva of a Holothuroid. B. The larva of an Asteroid.m.mouth;st.stomach;a.anus;l.c.primitive longitudinal ciliated band;pr.c.præ-oral ciliated band.
Fig. 254. A. The larva of a Holothuroid. B. The larva of an Asteroid.
m.mouth;st.stomach;a.anus;l.c.primitive longitudinal ciliated band;pr.c.præ-oral ciliated band.
It is (fig. 254A andfig. 255) bilaterally symmetrical, presenting a flattened ventral surface, and a convex dorsal one. The anus (an) is situated nearly at the hinder pole, and the mouth (m) about the middle of the ventral surface. In front of the mouth is a considerable process, the præ-oral lobe. Between the mouth and anus is a space, more or less concave according to the age of the embryo, interrupted by a ciliatedridge a little in front of the anus. A similar ciliated ridge is present on the ventral surface of the præ-oral lobe immediately in front of the mouth. The anal and oral ridges are connected by two lateral ciliated bands, the whole forming a continuous band, which, since the mouth lies in the centre of it (fig. 255), may be regarded as a ring completely surrounding the body behind the mouth, or more naturally as a longitudinal ring.
Evolution of an AuriculariaFig. 255. Diagrammatic figures representing the evolution of an Auricularia from the simplest Echinoderm larval form.(Copied from Müller.)The black line represents the ciliated ridge. The shaded part is the oral side of the ring, the clear part the aboral side.m.mouth;an.anus.
Fig. 255. Diagrammatic figures representing the evolution of an Auricularia from the simplest Echinoderm larval form.(Copied from Müller.)The black line represents the ciliated ridge. The shaded part is the oral side of the ring, the clear part the aboral side.
m.mouth;an.anus.
The bilateral Auricularia is developed from a slightly elongated gastrula with an uniform covering of cilia. The gastrula becomes flattened on the oral side. At the same time the cilia become specially developed on the oral and anal ridges, and then on the remainder of the ciliated ring, while they are simultaneously obliterated elsewhere; and so a complete Auricularia is developed. The water-vascular ring in the fully developed larva has already considerably advanced in the growth round the œsophagus (fig. 256w.v.r).
Most Holothurian larvæ, in their transformation from the bilateral Auricularia form to the radial form of the adult, pass through a stage in which the cilia form a number of transverserings, usually five in number, surrounding the body. The stages in this metamorphosis are shewn in figs. 256, 257, and 258.
Full-grown larva of SynaptaFig. 256. Full-grown larva of Synapta.(After Metschnikoff.)m.mouth;st.stomach;a.anus;p.v.left division of perivisceral cavity, which is still connected with the water-vascular system;w.v.r.water-vascular ring which has not yet completely encircled the œsophagus;l.c.longitudinal part of ciliated band;pr.c.præ-oral part of ciliated band.
Fig. 256. Full-grown larva of Synapta.(After Metschnikoff.)
m.mouth;st.stomach;a.anus;p.v.left division of perivisceral cavity, which is still connected with the water-vascular system;w.v.r.water-vascular ring which has not yet completely encircled the œsophagus;l.c.longitudinal part of ciliated band;pr.c.præ-oral part of ciliated band.
The primitive ciliated band, at a certain stage of the metamorphosis, breaks up into a number of separate portions (fig. 256), the whole of which are placed on the ventral surface. Four of these (fig. 257A and B) arrange themselves in the form of an angular ring round the mouth, which at this period projects considerably. The remaining portions of the primitive band change their direction from a longitudinal one to a transverse (fig. 257B), and eventually grow into complete rings (fig. 257C). Of these there are five. The middle one (257 B) is the first to develop, and is formed from the dorsal parts of the primitive ring. The two hinder rings develop next, and last of all the two anterior ones, one of which appears to be in front of the mouth (fig. 257C).
The later development of the mouth, and of the ciliated ridge surrounding it, is involved in some obscurity. It appears from Metschnikoff (No.560) that an invagination of the œsophagus takes place, carrying with it the ciliated ridge around the mouth. This ridge becomes eventually converted into the covering for the five tentacular outgrowths of the water-vascular ring (fig. 258), and possibly also forms the nervous system.
The opening of the œsophageal invagination is at first behind the foremost ciliated ring, but eventually comes to lie in front of it, and assumes a nearly terminal though slightly ventral position (fig. 258). No account has been given of the process by which this takes place, but the mouth is stated by Metschnikoff (thoughMüller differs from him on this point) to remain open throughout. The further changes in the metamorphosis are not considerable. The ciliated bands disappear, and a calcareous ring of ten pieces, five ambulacral and five interambulacral, is formed round the œsophagus. A provisional calcareous skeleton is also developed.
All the embryonic systems of organs pass in this case directly into those of the adult.
Three stages in the development of SynaptaFig. 257. Three stages in the development of Synapta.A and B are viewed from the ventral surface, and C from the side. (After Metschnikoff.)m.mouth;oe.œsophagus;pv.walls of the perivisceral cavity;wv.longitudinal vessel of the water-vascular system;p.dorsal pore of water-vascular system;cr.ciliated ring formed round the mouth from parts of the primitive ciliated band.
Fig. 257. Three stages in the development of Synapta.A and B are viewed from the ventral surface, and C from the side. (After Metschnikoff.)
m.mouth;oe.œsophagus;pv.walls of the perivisceral cavity;wv.longitudinal vessel of the water-vascular system;p.dorsal pore of water-vascular system;cr.ciliated ring formed round the mouth from parts of the primitive ciliated band.
The metamorphosis of most Holothuroidea is similar to that just described. In Cucumaria (Selenka) there is however no Auricularia stage, and the uniformly ciliated stage is succeeded by one with five transverse bands of cilia, and a præ-oral and an anal ciliated cap. The mouth is at first situated ventrally behind the præ-oral cap of cilia, but the præ-oral cap becomes gradually absorbed, and the mouth assumes a terminal position.
In Psolinus (Kowalevsky) there is no embryonic ciliated stage, and the adult condition is attained without even a metamorphosis. There appear tobe five plates surrounding the mouth, which are developed before any other part of the skeleton, and are regarded by P. H. Carpenter (No.548) as equivalent to the five oral plates of the Crinoidea. The larval condition with ciliated bands is often spoken of as the pupa stage, and during it the larvæ of Holothurians proper use their embryonic tube feet to creep about.
Late stage of SynaptaFig. 258. A late stage in the development of Synapta.(After Metschnikoff.)The figure shews the vestibular cavity with retracted tentacles; the ciliated bands; the water-vascular system, etc.p.dorsal pore of water-vascular system;pv.walls of perivisceral cavity;ms.amœboid cells.
Fig. 258. A late stage in the development of Synapta.(After Metschnikoff.)
The figure shews the vestibular cavity with retracted tentacles; the ciliated bands; the water-vascular system, etc.
p.dorsal pore of water-vascular system;pv.walls of perivisceral cavity;ms.amœboid cells.
Asteroidea.The commonest and most thoroughly investigated form of Asteroid larva is a free-swimming form known as Bipinnaria.
This form in passing from the spherical to the bilateral condition passes through at first almost identical changes to the Auricularian larva. The cilia become at an early period confined to an oral and anal ridge.
The anal ridge gradually extends dorsalwards, and finally forms a complete longitudinal post-oral ring (fig. 259A); the oral ridge also extends dorsalwards, and forms a closed præ-oral ring (fig. 259A), the space within which is left unshaded in my figure.
The presence of two rings instead of one distinguishes the Bipinnaria from the Auricularia. The two larvæ are shewn side by side infig. 254, and it is obvious that the two bands of the Bipinnaria are (as pointed out by Gegenbaur) equivalent to the single band of the Auricularia divided into two. Ontologically, however, the two bands of Bipinnaria do not appear to arise from the division of a single band.
As the Bipinnaria grows older, a series of arms grows out along lines of the two ciliated bands (fig. 259C), and, in many cases, three special arms are formed, not connected with the ciliated bands, and covered with warts. These latter arms areknown as brachiolar arms, and the larvæ provided with them as Brachiolaria (fig. 259D).
Diagrammatic representation of various forms of Asteroid LarvæFig. 259. Diagrammatic representation of various forms of Asteroid Larvæ. A, B, C, Bipinnaria; D, Brachiolaria.(Copied from Müller.)The black lines represent the ciliated bands; and the shading the space between the præ-oral and the post-oral bands.m.mouth;an.anus.
Fig. 259. Diagrammatic representation of various forms of Asteroid Larvæ. A, B, C, Bipinnaria; D, Brachiolaria.(Copied from Müller.)
The black lines represent the ciliated bands; and the shading the space between the præ-oral and the post-oral bands.
m.mouth;an.anus.
As a rule the following arms can be distinguished (fig. 259C and D), on the hinder ring (Agassiz’ nomenclature) a median anal pair, a dorsal anal pair, and a ventral anal pair, a dorsal oral pair, and an unpaired anterior dorsal arm; on the præ-oral ring a ventral oral pair, and sometimes (Müller) an unpaired anterior ventral arm.
The three brachiolar arms arise as processes from the base of the unpaired dorsal arm, and the two ventral oral arms. The extent of the development of the arms varies with the species.
The changes by which the Bipinnaria or Brachiolaria becomes converted into the adult starfish are very much more complicated than those which take place in Holothurians. For an accurate knowledge of them we are largely indebted to Alex. Agassiz (No.543). The development of the starfish takes place entirely at the posterior end of the larva close to the stomach.
On the right and dorsal side of the stomach, and externally to theright peritoneal space, are formed five radially situated calcareous rods arranged in the form of a somewhat irregular pentagon. The surface on which they are deposited has a spiral form, and constitutes together with its calcareous rods, theabactinal or dorsal surface of the future starfish. Close to its dorsal,i.e.embryonic dorsal, edge lies the dorsal pore of the water-vascular system (madreporic canal), and close to its ventral edge the anus. On the left and ventral side of the stomach is placed the water-vascular rosette, the development of which was described onp.549. It is situated on the actinal or ventral surface of the future starfish, and is related to the left peritoneal vesicle.
Metschnikoff (No.560) and Agassiz (No.543) differ slightly as to the constitution of the water-vascular rosette. The former describes and figures it as a completely closed rosette, the latter states that ‘it does not form a completely closed curve but is always open, forming a sort of twisted crescent-shaped arc.’
The water-vascular rosette is provided with five lobes, corresponding to which are folds in the larval skin, and each lobe corresponds to one of the calcareous plates developed on the abactinal disc. The plane of the actinal surface at first meets that of the abactinal at an acute or nearly right angle. The two surfaces are separated by the whole width of the stomach. The general appearance of the larva from the ventral surface after the development of the water-vascular rosette (i) and abactinal disc (A) is shewn infig. 260.
Bipinnaria larva of an AsteroidFig. 260. Bipinnaria larva of an Asteroid.(From Gegenbaur; after Müller.)b.mouth;a.anus;h.madreporic canal;i.ambulacral rosette;c.stomach;d. g. e.etc. arms of Bipinnaria;A.abactinal disc of young Asteroid.
Fig. 260. Bipinnaria larva of an Asteroid.(From Gegenbaur; after Müller.)
b.mouth;a.anus;h.madreporic canal;i.ambulacral rosette;c.stomach;d. g. e.etc. arms of Bipinnaria;A.abactinal disc of young Asteroid.
As development proceeds the abactinal surface becomes a firm and definite disc, owing to the growth of the original calcareous spicules into more or less definite plates, and to the development of five fresh plates nearer the centre of the disc andinterradial in position. Still later a central calcareous plate appears on the abactinal surface, which is thus formed of a central plate, surrounded by a ring of five interradial plates, and then again by a ring of five radial plates. The abactinal disc now also grows out into five short processes, separated by five shallow notches. These processes are the rudiments of the five arms, and each of them corresponds to one of the lobes of the water-vascular rosette. A calcareous deposit is formed round the opening of the water-vascular canal, which becomes the madreporic tubercle[221]. At about this stage the absorption of the larval appendages takes place. The whole anterior part of thelarva with the great præ-oral lobe has hitherto remained unchanged, but now it contracts and undergoes absorption, and becomes completely withdrawn into the disc of the future starfish. The larval mouth is transported into the centre of the actinal disc. In the larvæ observed by Agassiz and Metschnikoff nothing was cast off, but the whole absorbed.
According to Müller and Koren and Danielssen this is not the case in the larva observed by them, but part of the larva is thrown off, and lives for some time independently.
After the absorption of the larval appendages the actinal and abactinal surfaces of the young starfish approach each other, owing to the flattening of the stomach; at the same time they lose their spiral form, and become flat discs, which fit each other. Each of the lobes of the rosette of the water-vascular system becomes one of the radial water-vascular canals. It first becomes five-lobed, each lobe forming a rudimentary tube foot, and on each side of the middle lobe two fresh ones next spring out, and so on in succession. The terminal median lobe forms the tentacle at the end of the arm, and the eye is developed at its base. The growth of the water-vascular canals keeps pace with that of the arms, and the tube feet become supported at their base by an ingrowth of calcareous matter. The whole of the calcareous skeleton of the larva passes directly into that of the adult, and spines are very soon formed on the plates of the abactinal surface. The original radial plates, together with the spines which they have, are gradually pushed outwards with the growth of the arms by the continual addition of fresh rows of spines between the terminal plate and the plate next to it. It thus comes about that the original radial plates persist at the end of the arms, in connection with the unpairedtentacles which form the apex of the radial water-vascular tubes.
It has already been mentioned that according to Metschnikoff (No.560) a new œsophagus is formed which perforates the water-vascular ring, and connects the original stomach with the original mouth. Agassiz (No.543) maintains that the water-vascular ring grows round the primitive œsophagus. He says—“During the shrinking of the larva the long œsophagus becomes shortened and contracted, bringing the opening of the mouth of the larva to the level of the opening of the œsophagus, which eventually becomes the true mouth of the starfish.” The primitive anus is believed by Metschnikoff to disappear, but by Agassiz to remain. This discrepancy very possibly depends upon these investigators having worked at different species.
There is no doubt that the whole of the larval organs, with the possible exception of the œsophagus, and anus (where absent in the adult), pass directly into the corresponding organs of the starfish—and that the præ-oral part of the body and arms of the larva are absorbed and not cast off.
In addition to the Bipinnarian type of Asteroid larva a series of other forms has been described by Müller (No.561), Sars, Koren, and Danielssen (No.554) and other investigators, which are however very imperfectly known. The best known form is one first of all discovered by Sars in Echinaster Sarsii, and the more or less similar larvæ subsequently investigated by Agassiz, Busch, Müller, Wyville Thomson, etc. of another species of Echinaster and of Asteracanthion. These larvæ on leaving the egg have an oval form, and are uniformly covered by cilia. Four processes (or in Agassiz’ type one process) grow out from the body; by these the larvæ fix themselves. In the case of Echinaster the larvæ are fixed in the ventral concavity of the disc of the mother, between the five arms, where a temporary brood-pouch is established. The main part of the body is converted directly into the disc of the young starfish, while the four processes come to spring from the ventral surface, and are attached to the water-vascular ring. Eventually they atrophy completely. Of the internal structure but little is known; till the permanent mouth is formed, after the development of the young starfish is pretty well advanced, the stomach has no communication with the exterior.
A second abnormal type of development is presented by the embryo of Pteraster miliaris, as described by Koren and Danielssen[222]. The larvæ to the number of eight to twenty develop in a peculiar pouch on the dorsal surface of the body. The early stages are not known, but in the later ones the whole body assumes a pentagonal appearance with a mouth at one edgeof the disc. At a later stage the anus is formed on the dorsal side of an arm opposite the mouth. The stomach is surrounded by a water-vascular ring, from which the madreporic canal passes to the dorsal surface, but does not open. At a later stage the embryonic mouth and anus vanish, to be replaced by a permanent mouth and anus in the normal positions.
A third, and in some respects very curious, form is a worm like larva of Müller, which is without bands of cilia. The dorsal surface of the youngest larva is divided by transverse constrictions into five segments. On the under side of the first of these is a five-lobed disc, each lobe being provided with a pair of tube feet.
At a later period only three segments are visible on the dorsal surface, but the ventral surface has assumed a pentagonal aspect. The later stages are not known.
Diagrammatic figures shewing the evolution of an Ophiuroid PluteusFig. 261. Diagrammatic figures shewing the evolution of an Ophiuroid Pluteus from a simple Echinoderm larva.(Copied from Müller.) The calcareous skeleton is not represented.m.mouth;an.anus;d.anterior arms;d´.lateral arms;e´.posterior arms;g´.anterolateral arms.
Fig. 261. Diagrammatic figures shewing the evolution of an Ophiuroid Pluteus from a simple Echinoderm larva.(Copied from Müller.) The calcareous skeleton is not represented.
m.mouth;an.anus;d.anterior arms;d´.lateral arms;e´.posterior arms;g´.anterolateral arms.
Ophiuroidea.The full-grown larva of the Ophiuroids is known as a Pluteus. It commences with the usual more or less spherical form; from this it passes to a form closely resembling that of Auricularia with a rounded dorsal surface, and a flattened ventral one. Soon however it becomes distinguished by the growth of a post-anal lobe and the absence of a præ-oral lobe (fig. 261B). The post-anal lobe forms the somewhat rounded apex of the body. In front of the mouth, and between the mouth and anus, arise the anal and oral ciliated ridges, which soon become continued into a single longitudinal ciliated ring. At the same time the body becomes prolonged into a series ofprocesses along the ciliated band, which is continued to the extremity of each. The primitive ciliated ring never becomes broken up into two or more rings. A ciliated crown is usually developed at the extremity of the post-anal lobe. The arms are arranged in the form of a ring surrounding the mouth, and are all directed forwards.
The first arms to appear are two lateral ones, which usually remain the most conspicuous (fig. 261B and C,d´). Next arises a pair on the sides of the mouth, which may be called the mouth or anterior arms (C,d). A pair ventral to and behind the lateral arms is then formed, constituting the posterior arms (D,e´), and finally a pair between the lateral arms and the anterior, constituting the anterolateral arms (D,g´).
The concave area between the arms forms the greater part of the ventral surface of the body. Even before the appearance of any of the arms, and before the formation of the mouth, two calcareous rods are formed, which meet behind at the apex of the post-anal lobe, and are continued as a central support into each of the arms as they are successively formed. These rods are shewn at their full development infig. 262. The important points which distinguish a Pluteus larva from the Auricularia or Bipinnaria are the following:
(1) The presence of the post-anal lobe at the hind end of the body. (2) The slight development of a præ-oral lobe. (3) The provisional calcareous skeleton in the larval arms.
Pluteus larva of an OphiuroidFig. 262. Pluteus larva of an Ophiuroid.(From Gegenbaur; after Müller.)A.rudiment of young Ophiuroid;d´.lateral arms;d.anterior arms;e´.posterior arms.
Fig. 262. Pluteus larva of an Ophiuroid.(From Gegenbaur; after Müller.)
A.rudiment of young Ophiuroid;d´.lateral arms;d.anterior arms;e´.posterior arms.
Great variations are presented in the development of the arms and provisional skeleton. The presence of lateral arms is however a distinctive characteristic of the Ophiuroid Pluteus. The other arms may be quite absent, but the lateral arms never.
The formation of the permanent Ophiuroid takes place in much the same way as in the Asteroidea.
There is formed (fig. 262) on the right and dorsal side of stomach the abactinal disc supported by calcareous plates, at first only five in number and radial in position[223]. The disc is at first not symmetrical, but becomes so at the time of the resorption of the larval arms. It grows out into five processes—the five future rays. The original five radial plates remain as the terminal segments of the adult rays, and new plates are always added between the ultimate and penultimate plate (Müller), though it is probable that in the later stages fresh plates are added in the disc.
Diagrammatic Figures Shewing the Evolution of Echinoid PluteiFig. 263. Diagrammatic Figures Shewing the Evolution of Echinoid Plutei.(Copied from Müller.) The calcareous skeleton is not represented. E. Pluteus of Spatangus.m.mouth;an.anus;d.anterior arms;d´.point where lateral arms arise in the Ophiuroid Pluteus;e.anterointernal arms;e´.posterior arms;g´.anterolateral arms;g.anteroexternal arms.
Fig. 263. Diagrammatic Figures Shewing the Evolution of Echinoid Plutei.(Copied from Müller.) The calcareous skeleton is not represented. E. Pluteus of Spatangus.
m.mouth;an.anus;d.anterior arms;d´.point where lateral arms arise in the Ophiuroid Pluteus;e.anterointernal arms;e´.posterior arms;g´.anterolateral arms;g.anteroexternal arms.
The ventral surface of the permanent Ophiuroid is formed by the concave surface between the mouth and anus. Between this and the stomach is situated the water-vascular ring. It is at first not closed, but is horseshoe-shaped, with five blind appendages (fig. 262). It eventually grows round the œsophagus, which, together with the larval mouth, is retained in the adult. The five blind appendages become themselves lobed in the same way as in Asterias, and grow out along the five arms of the disc and become the radial canals and tentacles. All these parts of the water-vascular system are of course covered by skin, and probably also surrounded by mesoblast cells, in which at a later period the calcareous plates which lie ventral to the radial canal are formed. The larval anus disappears. As long as the larval appendages are not absorbed the ventral and dorsal discs of the permanent Ophiuroid fit as little as in the case of the Brachiolaria, but at a certain period the appendages are absorbed. The calcareous rods of the larval armsbreak up, the arms and anal lobe become absorbed, and the dorsal and ventral discs, with the intervening stomach and other organs, are alone left. After this the discs fit together, and there is thus formed a complete young Ophiuroid.
The whole of the internal organs of the larva (except the anus), including the mouth, œsophagus, the body cavity, etc. are carried on directly into the adult.
The larval skeleton is, as above stated, absorbed.
The viviparous larva of Amphiura squamata does not differ very greatly from the larvæ with very imperfect arms. It does not develop a distinct ciliated band, and the provisional skeleton is very imperfect. The absence of these parts, as well as of the anus, mentioned onp.549, may probably be correlated with the viviparous habits of the larva. With reference to the passage of this larva into the adult there is practically nothing to add to what has just been stated. When the development of the adult is fairly advanced the part of the body with the provisional skeleton forms an elongated rod-like process attached to the developing disc. It becomes eventually absorbed.
Two larvæ of StrongylocentrusFig. 264. Two larvæ of Strongylocentrus.(From Agassiz.)m.mouth;a.anus;o.œsophagus;d.stomach;c.intestine;v´.andv.ciliated ridges;w.water-vascular tube;r.calcareous rods.
Fig. 264. Two larvæ of Strongylocentrus.(From Agassiz.)
m.mouth;a.anus;o.œsophagus;d.stomach;c.intestine;v´.andv.ciliated ridges;w.water-vascular tube;r.calcareous rods.
Echinoidea.The Echinus larva (fig. 263) has a Pluteus form like that of the Ophiuroids, and in most points, such as the presence of the anal lobe, the ciliated band, the provisional skeleton, etc., develops in the same manner. The chief difference between the two Pluteus forms concerns the development of the lateral arms. These, which form the most prominent arms in the Ophiuroid Pluteus, are entirely absent in the EchinoidPluteus, which accordingly has, as a rule, a much narrower form than the Ophiuroid Pluteus.
A pair of ciliated epaulettes on each side of and behind the ciliated ring is very characteristic of some Echinoid larvæ. They are originally developed from the ciliated ring (fig. 266A and B,v´´). The presence of three processes from the anal lobe supported by calcareous rods is characteristic of the Spatangoid Pluteus (fig. 263E).
Lateral and ventral view of a larva of StrongylocentrusFig. 265. Lateral and ventral view of a larva of Strongylocentrus.(From Agassiz.) General references as in fig. 264.b.dorsal opening of madreporic canal;e´.posterior arms;e´´´.anterior arms;eIV.anterointernal arms.
Fig. 265. Lateral and ventral view of a larva of Strongylocentrus.(From Agassiz.) General references as in fig. 264.
b.dorsal opening of madreporic canal;e´.posterior arms;e´´´.anterior arms;eIV.anterointernal arms.
The first two pairs of arms to develop, employing the same names as in Ophiuroids, are the anterior attached to the oral process (fig. 263C,d) and the posterior pair (e´). A pair of anterolateral arms next becomes developed (g´). A fourth pair (not represented in Ophiuroids) appears on the inner side of the anterior pair forming an anterointernal pair (e), and in the Spatangoid Pluteus a fifth pair may be added on the external side of the anterior pair forming an anteroexternal pair (g).
Each of the first-formed paired calcareous rods is composed of three processes, two of which extend into the anterior and posterior arms; and the third and strongest passes into the anal lobe, and there meets its fellow (fig. 265). A transverse bar in front of the arms joins the rods of the two sides meeting them at the point where the three processes diverge. The process in the anterolateral arm (fig. 266B) is at first independent of this system of rods, but eventually unites with it. Although our knowledge ofthe Pluteus types in the different groups is not sufficient to generalise with great confidence, a few points seem to have been fairly determined[224]. The Plutei of Strongylocentrus (figs. 266and267) and Echinus have eight arms and four ciliated epaulettes. The only Cidaris-like form, the Pluteus of which is known, is Arbacia: it presents certain peculiarities. The anal lobe develops a pair of posterior (auricular) appendages, and the ciliated ring, besides growing out into the normal eight appendages, has a pair of short blunt anterior and posterior lobes. An extra pair of non-ciliated accessory mouth arms appears also to be developed. Ciliated epaulettes are not present. So far as is known the Clypeastroid larva is chiefly characterized by the round form of the anal lobe. The calcareous rods are latticed. In the Pluteus of Spatangoids there are (fig. 263) five pairs of arms around the mouth pointing forwards, and three arms developed from the anal lobe pointing backwards. One of these is unpaired, and starts from the apex of the anal lobe. All the arms have calcareous rods which, in the case of the posterior pair, the anterolateral pair, and the unpaired arm of the anal lobe, are latticed. Ciliated epaulettes are not developed.
Viviparous larvæ of Echinoids have been described by Agassiz[225].
The development of the permanent Echinus has been chiefly worked out by Agassiz and Metschnikoff.
In the Pluteus of Echinus lividus the first indication of the adult arises, when three pairs of arms are already developed, as an invagination of the skin on the left side, between the posterior and anterolateral arms, the bottom of which is placed close to the water-vascular vesicle (fig. 266B,w´). The base of this invagination becomes very thick, and forms the ventral disc of the future Echinus. The parts connecting this disc with the external skin become however thin, and, on the narrowing of the external aperture of invagination and the growth of the thickened disc, constitute a covering for the disc, called by Metschnikoff theamnion. The water-vascular vesicle adjoining this disc grows out into five processes, forming as many tube feet, which cause the surface of the involuted disc to be produced into the same number of processes. The external opening of the invagination of the disc never closes, and after the development of the tube feet begins to widen again, and the amnion to atrophy. Through the opening of the invagination the tube feet now project. The dorsal and right surface of the Pluteus, which extends so as to embrace the opening of the madreporic canal and the anus, forms the abactinal or dorsal surface of the future Echinus (fig. 267,a). This disc fits on to the actinal invaginated surface which arises on the left side of the Pluteus. On the right surface of the larva (dorsal of permanent Echinus) two pedicellariæ appear, and at a later period spines are formed, which are at first arranged in a ring-like form round the edge of the primitively flat test. While these changes are taking place, and the two surfaces of the future Echinus are gradually moulding themselves so as toform what is obviously a young Echinus, the arms of the Pluteus with their contained skeleton have been gradually undergoing atrophy. They become irregular in form, their contained skeleton breaks up into small pieces, and they are gradually absorbed.