The Works of Francis Balfour 2-20
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Foster M. and Sedgwick A. The Works of Francis Balfour Vol. II. A Treatise on Comparative Embryology 1. (1885) MacMillan and Co., London.
- The Ovum and Spermatozoon | The Maturation and Impregnation of the Ovum | The Segmentation of the Ovum | Dicyemae and Orthonectidae Dicyema | Porifera | Coelenterata | Platyhelminthes | Rotifera | Mollusca | Polyzoa | Brachiopoda | Chilopoda | Discophora | Gephyrea | Chaetognatha | Nemathelminthes | Tracheata | Crustacea | Pcecilopoda | Echinodermata | Enteropneusta | Bibliography
The Works of Francis Balfour Foster M. and Sedgwick A. The Works of Francis Balfour Vol. I. Separate Memoirs (1885) MacMillan and Co., London.
Foster M. and Sedgwick A. The Works of Francis Balfour Vol. II. A Treatise on Comparative Embryology 1. (1885) MacMillan and Co., London.
Foster M. and Sedgwick A. The Works of Francis Balfour Vol. III. A Treatise on Comparative Embryology 2 (1885) MacMillan and Co., London.
Foster M. and Sedgwick A. The Works of Francis Balfour Vol. IV. Plates (1885) MacMillan and Co., London.
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Vol II. A Treatise on Comparative Embryology (1885)
Chapter XX. Echinodermata
THE development of the Echinodermata naturally falls into two sections:
(i) The development of the germinal layers and of the systems of organs; (2) the development of the larval appendages and the metamorphosis.
The Development of the Germinal Layers and of 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. 247 A) the egg has a nearly spherical form, and is constituted of a single layer of columnar cells enclosing a small segmentation cavity. The lower pole is slightly thickened, and the egg rotates by means of fine cilia.
An invagination now makes its appearance at the lower pole (fig. 247 B), and simultaneously there become budded off from the cells undergoing the invagination amoeboid cells, which eventually form the muscular system and the connective tissue. These cells very probably have a bilaterally symmetrical origin. This stage represents the gastrula stage which is common to all Echinoderms. The invaginated sack is the archenteron. As it grows larger one side of the embryo becomes flattened, and the other more convex. On the flattened side a fresh invagination arises, the opening of which forms the permanent mouth, the opening of the first invagination remaining as the permanent anus (fig. 248 A).
1 The following classification of the Echinodermata is employed in this chapter.
I. Holothuroidea. IV. Echinoidea.
II. Asteroidea. V. Crinoidea.
FIG. 247. TWO STAGES IN THE DEVELOPMENT OF HOLOTHURIA TUBULOSA VIEWED IN OPTICAL SECTION. (After Selenka.)
A. Blastosphere stage at the close of segmentation. B. Gastrula stage. mr. micropyle ; //. chorion; s.c. segmentation cavity; bl. blastoderm; ep. epiblast; hy. hypoblast; ms. amoeboid cells derived from hypoblast ; a.e. archenteron.
These changes give us the means of attaching definite names to the various parts of the embryo. It deserves to be noted in the first place that the embryo has assumed a distinctly bilateral form. There is present a more or less concave surface extending from the mouth to near the anus, which will be spoken of as the ventral surface. The anus is situated at the posterior extremity. The convex surface opposite the ventral surface forms the dorsal surface, which terminates anteriorly in a rounded prse-oral prominence.
It will be noticed in fig. 248 A that in addition to the primitive anal invagination there is present a vesicle (?/.). This vesicle is directly formed by a constriction of the primitive B. II. 35
archenteron (fig. 249 Vpv.), and is called by Selenka the vasoperitoneal vesicle. It gives origin to the epithelioid lining of the body cavity and water-vascular system of the adult 1 . In the parts now developed we have the rudiments of all the adult organs. The mouth and anal involutions (after the separation of the vaso-peritoneal vesicle) meet and unite, a constriction indicating their point of junction (fig. 248 B). Eventually the former gives
FIG. 248. THREE STAGES IN THE DEVELOPMENT OF HOLOTHURIA TUBULOSA
VIEWED FROM THE SIDE IN OPTICAL SECTION. (After Selenka.) tn. mouth; oe. oesophagus; st. stomach; i. intestine; a. anus; I.e. longitudinal ciliated band; v.p. vaso-peritoneal vesicle; p.v. peritoneal vesicle; p.r. right peritoneal vesicle ; //. left peritoneal vesicle ; w.v. water- vascular vesicle ; p. dorsal pore of water- vascular system ; ms. muscle cells.
rise to the mouth and cesophagus, and the latter to the remainder of the alimentary canal 2 .
The vaso-peritoneal vesicle undergoes a series of remarkable changes. After its separation from the archenteron it takes up a position on the left side of this, elongates in an anteroposterior direction, and from about its middle sends a narrow diverticulum towards the dorsal surface of the body, where an
1 The origin of the vaso-peritoneal vesicle is not quite the same in all the species. In Holothuria tubulosa it is separated from the csecal end of the archenteron; the remainder of which then grows towards the oral invagination. In Cucumaria the archenteron forks (fig. 249) ; and one fork forms the vaso-peritoneal vesicle, and the other the major part of the mesenteron.
2 There appears to be some uncertainty as to how much of the larval cesophagus is derived from the stomodaeal invagination.
opening to the exterior becomes formed (fig. 248 B, /.). The diverticulum becomes the madreporic canal, and the opening the dorsal pore.
The vaso-peritoneal vesicle next divides into two, an anterior vesicle (fig. 248 B, w.v.), from which is derived the epithelium of the water-vascular system, and a posterior (fig. 248 B, /.?;.), which gives rise to the epithelioid lining of the body cavity. The anterior vesicle (fig. 248 C, w.v.) becomes fivelobed, takes a horseshoe-shaped form, and grows round the oesophagus (fig. 256, w.v.r). The five lobes form the rudiments of the water-vascular prolongations into the tentacles. The remaining parts of the water-vascular system are also developed as outgrowths of the original vesicle. Five of these, alternating with the original diverticula, form the five ambulacral canals, from which diverticula are produced into the ambulacral feet ; a sixth gives rise to the Polian vesicle. The remaining parts of the original vesicle form the water-vascular ring.
We must suppose that eventually the madreporic canal loses its connection with the exterior so as to hang loosely in the interior, though the steps of this process do not appear to have been made out.
The original hinder peritoneal vesicle grows rapidly, and divides into two (fig. 248 C, pi. and pr.}, which encircle the two sides of the alimentary canal, and meet above and below it. The outer wall of each of them attaches itself to the skin, and the inner one to the alimentary canal and watervascular system ; in both cases the walls remain separated from the adjacent parts by a layer of the amoeboid cells already spoken of. The cavity of the peritoneal vesicles becomes the permanent body cavity. Where the walls of
FIG. 249. LONGITUDINAL SECTION
THROUGH AN EMBRYO OF CUCUMARIA DOLIOLUM AT THE END OF THE FOURTH DAY.
Vpv. vaso-peritoneal vesicle; ME. mesenteron; Blp., Ptd. blastopore, proctodaeum.
the two vesicles meet on the dorsal side, a mesentery, suspending the alimentary canal and dividing the body cavity longitudinally, is often formed. In other parts the partition walls between the two sacks appear to be absorbed.
The amoeboid cells, which were derived from the invaginated cells, arrange themselves as a layer round all the organs (fig. 249). Some of them remain amoeboid, attach themselves to the skin, and form part of the cutis; and in these cells the calcareous spicula of the larva and adult are formed. Others form the musculature of the larval alimentary tract, while the remainder give rise to the musculature and connective tissue of the adult.
The development of the vascular system is not known, but the discovery of Kowalevsky, confirmed by Selenka, that from the walls of the watervascular system corpuscles are developed, identical with those in the bloodvessels, indicates that it probably develops in connection with the watervascular system. The observations of Hoffmann and Perrier on the communication of the two systems in the Echinoidea point to the same conclusion. Though nothing very definite is known with reference to the development of the nervous system, Metschnikoff suggests that it develops in connection with the thickened bands of epiblast which are formed by a metamorphosis of the ciliated bands of the embryo, and accompany the five radial tubes (vide p. 555). In any case its condition in the adult leaves no doubt of its being a derivative of the epiblast.
From the above description the following general conclusions may be drawn :
(1) The blastosphere stage is followed by a gastrula stage.
(2) The gastrula opening forms the permanent anus, and the mouth is formed by a fresh invagination.
(3) The mesoblast arises entirely from the invaginated cells, but in two ways :
(a) As scattered amoeboid cells, which give origin to the muscles and connective tissue (including the cutis) of the body wall and alimentary tract.
(&) As a portion separated off from the archenteron, which gives rise both to the epithelioid lining of the body cavity, and of the water-vascular system.
(4) The oesophagus is derived from an invagination of the epiblast, and the remainder of the alimentary canal from the archenteron.
(5) The embryonic systems of organs pass directly into those of the adult.
The development of Synapta diverges, as might be expected, to a very small extent from that of Holothuria.
Asteroidea. In Asterias the early stages of development conform to our type. There arise, however, two bilaterally symmetrical vaso-peritoneal diverticula from the archenteron. These diverticula give rise both to the lining of the body cavity and water-vascular system. With reference to the exact changes they undergo there is, however, some difference of opinion. Agassiz (543) maintains that both vesicles are concerned in the formation of the water-vascular system, while Metschnikoff (560) holds that the watervascular system is entirely derived from the anterior part of the larger left vesicle, while the right and remainder of the left vesicle form the body cavity. MetschnikofFs statements appear to be the most probable. The anterior part of the left vesicle, after separating from the posterior, grows into a five-lobed rosette (fig. 260, /), and a madreporic canal (h] with a dorsal pore opening to the exterior. The rosette appears not to grow round the oesophagus, as in the cases hitherto described. But the latter is stated to disappear, and a new oesophagus to be formed, which pierces the rosette, and places the old mouth in communication with the stomach. Except where the anus is absent in the adult, the larval anus probably persists.
Ophiuroidea. The early development of the Ophiuroidea is not so fully known as that of other types. Most species have a free-swimming larva, but some (Amphiura) are viviparous.
The early stages of the free-swimming larvae have not been described, but I have myself observed in the case of Ophiothrix fragilis that the segmentation is uniform, and is followed by the normal invagination. The opening of this no doubt remains as the larval anus, and there are probably two outgrowths from this to form the vaso-peritoneal vesicles. Each of these divides into two parts, an anterior lying close to the oesophagus, and a posterior close to the stomach. The anterior on the right side aborts ; that on the left side becomes the water-vascular vesicle, early opens to the exterior, and eventually grows round the oesophagus, which, as in Holothurians, becomes the oesophagus of the adult. The posterior vesicles give rise to the lining of the body cavity, but are stated by Metschnikoff to be at first solid, and only subsequently to acquire a cavity the permanent body cavity. The anus naturally disappears, since it is absent in the adult. In the viviparous type the first stages are imperfectly known, but it appears that the blastopore vanishes before the appearance of the mouth. The development of the ^vaso-peritoneal bodies takes place as in the free-swimming larvae.
Echinoidea. In the Echinoidea (Agassiz, No. 542, Selenka, No. 564) there is a regular segmentation and the normal invagination (fig. 250 A). The amoeboid mesoblast cells arise as two laterally placed masses, and give rise to the usual parts. The archenteron grows forward and bends towards
the ventral side (fig. 250 B). It becomes (fig. 250 C) divided into three chambers, of which the two hindermost (d and c) form the stomach and intestine ; while the anterior forms the oesophagus, and gives rise to the
FIG. 250. THREE SIDE VIEWS OF EARLY STAGES IN THE DEVELOPMENT OF
STRONGYLOCENTRUS. (From Agassiz.)
a, anus (blastopore) ; d. stomach ; o. oesophagus ; c . rectum ; w. vaso-peritoneal vesicle ; v. ciliated ridge ; r. calcareous rod.
vaso-peritoneal vesicles. These latter appear as a pair of outgrowths (fig. 251), but become constricted off as a single two-horned vesicle, which subsequently divides into two. The left of these is eventually divided, as in Asteroids, into a peritoneal and water-vascular sack, while the right forms the right peritoneal sack. An oral invagination on the flattened ventral side meets the mesenteron after its separation from the vaso-peritoneal vesicle. The larval anus persists, as also does the larval mouth, but owing to the manner in which the water-vascular rosette is established the larval oesophagus appears to be absorbed, and to be replaced by a fresh oesophagus.
Crinoidea. Antedon, the only Crinoid so far studied (Gotte, No. 549), presents some not inconsiderable variations from the usual Echinoderm type. The blastopore is placed on the somewhat flattened side of the oval blastosphere, and not, as is usual, at the hinder end.
The blastopore completely closes, and is not converted into the permanent anus. The archenteron gives rise to the epithelioid lining of both body cavity and water-vascular system. These parts do not, however, appear as a single or paired outgrowth from the archenteron, but as three distinct outgrowths which are not formed contemporaneously. Two of them are first
FIG. -251. DORSO-VENTRAL VIEW OF AN EARLY LARVA OF STRONGYLOCENTRUS. (From Agassiz.)
a. anus ; d. stomach ; o. oesophagus ; w. vaso-peritoneal vesicle; r. calcareous rod.
formed and become the future body cavity; but their lumens remain distinct. Jngmally appearing as lateral outgrowths, the right one assumes a dorsal position and sends a prolongation into the stalk (fig. 252 rp'\ and the left one assumes first a ventral, and then an oral position (fur 252 lp\
The third outgrowth of the archenteron gives rise to the water-vascular vesicle. It first grows round the region of the future oesophagus and so forms the water-vascular ring. The wall of the ring then grows towards the body wall so as to divide the oral (left) peritoneal vesicle into two distinct vesicles, an anterior and a posterior, shewn in fig. 253, lp' and lp. Before this division is completed, the water-vascular ring is produced in front into five pro
FIG. 252. LONGITUDINAL SECTION THROUGH AN ANTEDON LARVA. (From Carpenter: after Gotte.)
al. mesenteron ; -wv. water- vascular ring ; lp. left (oral) peritoneal vesicle; rp. right peritoneal vesicle ; rp'. continuation of right vesicle into the stalk ; st. stalk.
cessesthe future tentacles (fig. 252, wv) which project into the cavity of the oral vesicle (lp\ After the oral peritoneal space has become completely divided into two parts, the anterior dilates (fig. 253, //) greatly, and forms a large vestibule at the anterior end of the body. This vestibule (lp'} next acquires a communication with the mesenteron, shewn in fig. 253 at m. The anterior wall of this vestibule is finally broken through. By this rupture the mesenteron is placed in communication with the exterior by the opening at m, while at the same time the tentacles of the water-vascular ring (/) project freely to the exterior. Such is Gotte's account of the prge-oral body space, but, as he himself points out, it involves our believing that the lining of the diverticulum derived from the primitive alimentary vesicle becomes part of the external skin. This occurrence is so remarkable, that more evidence appears to me requisite before accepting it.
The formation of the anus occurs late. Its position appears to be the same as that of the blastopore, and is indicated by a papilla of the mesenteron attaching itself to the skin on the ventral side (fig. 253, an). It eventually becomes placed in an interradial space within the oral disc of the adult. The water-vascular ring has no direct communication with the exterior, but the place of the madreporic canal of other types appears to be taken in the larva by a single tube leading from the exterior into the body cavity, the external opening of which is placed on one of the oral plates (vide p. 571) in the next interradial space to the right of the anus, and a corresponding diverticulum of the water-vascular ring opening into the body cavity. The line of junction between the left and right peritoneal vesicles forms in the larva a ring-like mesentery dividing the oral from the aboral part of the body
cavity. In the adult 1 the oral section of the larval body cavity becomes the ventral part of the circumvisceral division of the body cavity, and the subtentacular canals of the arms and disc ; while the aboral section becomes the dorsal part of the circumvisceral division of the body cavity, the cceliac canals of the arms, and the cavity of the centro-dorsal piece. The primitive
FIG. 253. LONGITUDINAL SECTION THROUGH THE CALYX OF AN ADVANCED PENTRACRINOID ANTEDON LARVA WITH CLOSED VESTIBULE.
(From Carpenter ; after Gotte.)
ae. epithelium of oral vestibule; ;//. mouth; al. mesenteron; an. rudiment of permanent anus; lp. posterior part of left (oral) peritoneal sack; lp' '. anterior part of left (oral) peritoneal sack; wr. water-vascular ring; /. tentacle; mt. mesentery; rp. right peritoneal sack; rp '. continuation of right peritoneal sack into the stalk; r. roof of tentacular vestibule.
distinction between the sections of the larval body cavity becomes to a large extent obliterated, while the axial and intervisceral sections of the bodycavity of the adult are late developments.
The more important points in the development indicated in the preceding pages are as follows :
(i) The blastosphere is usually elongated in the direction of the axis of invagination, but in Comatula it is elongated transversely to this axis.
1 Vide P. H. Carpenter, "On the genus Actinometra." Linnean Trans., and Series, Zoology, Vol. n., Part I., 1879.
(2) The blastopore usually becomes the permanent anus, but it closes at the end of larval life (there being no anus in the adult) in Ophiuroids and some Asteroids, while in Comatula it closes very early, and a fresh anus is formed at the point where the blastopore was placed.
(3) The larval mouth always becomes the mouth of the adult.
(4) The archenteron always gives rise to outgrowths which form the peritoneal membrane and water-vascular systems. In Comatula there are three such outgrowths, two paired, which form the peritoneal vesicles, and one unpaired, which forms the water-vascular vesicle. In Asteroids and Ophiuroids there are two outgrowths. In Ophiuroids both of these are divided into a peritoneal and a water-vascular vesicle, but the right watervascular vesicle atrophies. In Asteroids only one water-vascular vesicle is formed, which is derived from the left peritoneal vesicle. In Echinoids and Holothuroids there is a single vaso-peritoneal vesicle.
(5) The water- vascular vesicle grows round the larval oesophagus in Holothuroids, Ophiuroids, and Comatula ; in these cases the larval oesophagus is carried on into the adult. In other forms the water-vascular vesicle forms a ring which does not enclose the cesophagus (Asteroids and Echinoids); in such cases a new oesophagus is formed, which perforates this ring.
Development of the larval appendages and metamorphosis.
Holothuroidea. The young larva of Synapta, to which J. Muller gave the name Auricularia (fig. 255), is in many respects the simplest form of Echinoderm larva. With a few exceptions the Auricularia type of larva is common to the Holothuria.
It is (fig. 254 A and fig. 255) bilaterally symmetrical, presenting a flattened ventral surface, and a convex dorsal one. The anus (an) is situated nearly at the hinder pole, and the mouth (m) about the middle of the ventral surface. In front of the mouth is a considerable process, the prae-oral lobe. Between the mouth and anus is a space, more or less concave according to the age of the embryo, interrupted by a ciliated
A similar ciliated ridge is A E
ridge a little in front of the anus, present on the ventral surface of the prae-oral lobe immediately in front of the mouth. The anal and oral ridges are connected by two lateral ciliated bands, the whole forming a continuous band, which, since the mouth lies in the centre of it (fig. 255), may be regarded as a ring completely surrounding the body behind the mouth, or more naturally as a longitudinal ring.
The bilateral Auricularia is developed from a slightly elongated gastrula with an uniform covering of cilia. The gastrula becomes flattened on the oral side. At the same time the cilia become specially developed on the oral and anal ridges, and then on the remainder of the ciliated ring, while they are
FIG. 254. A. THE LARVA OF A HOLOTHUROID. B. THE LARVA OF AN ASTEROID.
- //. mouth; st. stomach; a. anus; l.c>
primitive longitudinal ciliated band; pr.c. prae-oral ciliated band.
FIG. 155. DIAGRAMMATIC FIGURES REPRESENTING THE EVOLUTION OF AN AURICULARIA FROM THE SIMPLEST ECHINODERM LARVAL FORM. (Copied from MUller.)
The black line represents the ciliated ridge. The shaded part is the oral side of the ring, the clear part the aboral side.
/;;. mouth; an. anus.
simultaneously obliterated elsewhere ; and so a complete Auricularia is developed. The water-vascular ring in the fully-developed larva has already considerably advanced in the growth round the oesophagus (fig. 256 w.v.r).
Most Holothurian larvae, in their transformation from the bilateral Auricularia form to the radial form of the adult, pass through a stage in which the cilia form a number of transverse
rings, usually five in number, surrounding the body. The stages in this metamorphosis are shewn in figs. 256, 257, and 258.
The primitive ciliated band, at a certain stage of the metamorphosis, breaks up into a number of separate portions (fig. 256), the whole of which are placed on the ventral surface. Four of these (fig. 257 A and B) arrange themselves in the form of an angular ring round the mouth, which at this period projects considerably. The remaining portions of the primitive band change their direction from a longitudinal one to a transverse (fig. 257 B), and eventually grow into complete rings (fig. 2570). Of these there are five. The middle one (257 B) is the first to develop, and is formed from the dorsal parts of the primitive ring. The two hinder rings develop next, and last of all the two anterior ones, one of which appears to be in front of the mouth (fig. 257 C).
The later development of the mouth, and of the ciliated ridge surrounding it, is involved in some obscurity. It appears from Metschnikoff (No. 560) that an invagination of the oesophagus takes place, carrying with it the ciliated ridge around the mouth. This ridge becomes eventually converted into the covering for the five tentacular outgrowths of the water- vascular ring (fig. 258), and possibly also forms the nervous system.
The opening of the cesophageal invagination is at first behind the foremost ciliated ring, but eventually comes to lie in front of it, and assumes a nearly terminal though slightly ventral position (fig. 258). No account has been given of the process by which this takes place, but the mouth is stated by Metschnikoff (though
FIG. 256. FULL-GROWN LARVA OF SYNAPTA. (After Metschnikoff.)
m. mouth ; st. stomach ; a. anus ; p.v. left division of perivisceral cavity, which is still connected with the watervascular system ; w.v.r. water-vascular ring which has not yet completely encircled the oesophagus; I.e. longitudinal part of ciliated band ; pr.c. prae-oral part of ciliated band.
Miiller differs from him on this point) to remain open throughout. The further changes in the metamorphosis are not considerable. The ciliated bands disappear, and a calcareous ring of ten pieces, five ambulacral and five interambulacral, is formed round the oesophagus. A provisional calcareous skeleton is also developed.
All the embryonic systems of organs pass in this case directly into those of the adult.
The metamorphosis of most Holothuroidea is similar to that just described. In Cucumaria (Selenka) there is however no Auricularia stage, and the uniformly ciliated stage is succeeded by one with five transverse
FIG. 257. THREE STAGES IN THE DEVELOPMENT OF SYNAPTA. A and B are viewed from the ventral surface, and C from the side. (After Metschnikoff.)
m. mouth; oe. oesophagus; pv. walls of the perivisceral cavity; wv. longitudinal vessel of the water- vascular system; p. dorsal pore of water-vascular system; cr. ciliated ring formed round the mouth from parts of the primitive ciliated band.
bands of cilia, and a prae-oral and an anal ciliated cap. The mouth is at first situated ventrally behind the prse-oral cap of cilia, but the prae-oral cap becomes gradually absorbed, and the mouth assumes a terminal position.
In Psolinus (Kowalevsky) there is no embryonic ciliated stage, and the adult condition is attained without even a metamorphosis. There appear to
be five plates surrounding the mouth, which are developed before any other part of the skeleton, and are regarded by P. H. Carpenter (No. 548) as equivalent to the five oral plates of the Crinoidea. The larval condition with ciliated bands is often spoken of as the pupa stage, and during it the larvae of Holothurians proper use their embryonic tube feet to creep about.
Asteroidea. The commonest and most thoroughly investigated form of Asteroid larva is a free swimming form known as Bipinnaria.
This form in passing from the spherical to the bilateral condition passes through at first almost identical changes to the Auricularian larva. The cilia become at an early period confined to an oral and anal ridge.
The anal ridge gradually extends dorsalwards, and finally forms a complete longitudinal post-oral ring (fig. 259 A) ; the oral ridge also extends dorsalwards, and forms a closed prae-oral ring (fig. 259 A), the space within which is left unshaded in my figure.
The presence of two rings instead of one distinguishes the Bipinnaria from the Auricularia. The two larvae are shewn side by side in fig. 254, and it is obvious that the two bands of the Bipinnaria are (as pointed out by Gegenbaur) equivalent to the single band of the Auricularia divided into two. Ontologically, however, the two bands of Bipinnaria do not appear to arise from the division of a single band.
As the Bipinnaria grows older, a series of arms grows out along lines of the two ciliated bands (fig. 259 C), and, in many cases, three special arms are formed, not connected with the ciliated bands, and covered with warts. These latter arms are
FlG. 258. A LATE STAGE IN THE DEVELOPMENT OF SYNAPTA. (After Metschnikoff.)
The figure shews the vestibular cavity with retracted tentacles ; the ciliated bands ; the water-vascular system, etc.
p. dorsal pore of water-vascular system ; pv. walls of perivisceral cavity; ms. amoeboid cells.
known as brachiolar arms, and the larvae provided with them as Brachiolaria (fig. 259 D).
As a rule the following arms can be distinguished (fig. 259 C and D), on the hinder ring (Agassiz' nomenclature) a median anal pair, a dorsal anal pair, and a ventral anal pair, a dorsal oral pair, and an unpaired anterior dorsal arm ; on the prae-oral ring a ventral oral pair, and sometimes (Miiller) an unpaired anterior ventral arm.
The three brachiolar arms arise as processes from the base of the unpaired dorsal arm, and the two ventral oral arms. The extent of the development of the arms varies with the species.
FIG. 259. DIAGRAMMATIC REPRESENTATION OF VARIOUS FORMS OF ASTEROID LARWE. A, B, C, BIPINNARIA; D, BRACHIOLARIA. (Copied from Muller.) The black lines represent the ciliated bands ; and the shading the space between the prae-oral and the post-oral bands.
m. mouth; an. anus.
The changes by which the Bipinnaria or Brachiolaria becomes converted into the adult starfish are very much more complicated than those which take place in Holothurians. For an accurate knowledge of them we are largely indebted to Alex. Agassiz (No. 543). The development of the starfish takes place entirely at the posterior end of the larva close to the stomach.
On the right and dorsal side of the stomach, and externally to the rig/it peritoneal space, are formed five radially situated calcareous rods arranged in the form of a somewhat irregular pentagon. The surface on which they are deposited has a spiral form, and constitutes together with its calcareous rods, the
abactinal or dorsal surface of the future starfish. Close to its dorsal, i.e. embryonic dorsal, edge lies the dorsal pore of the water-vascular system (madreporic canal), and close to its ventral edge the anus. On the left and ventral side of the stomach is placed the water-vascular rosette, the development of which was described on p. 549. It is situated on the actinal or ventral surface of the future starfish, and is related to the left peritoneal vesicle.
Metschnikoff (No. 560) and Agassiz (No. 543) differ slightly as to the constitution of the water- vascular rosette. The former describes and figures it as a completely closed rosette, the latter states that ' it does not form a completely closed curve but is always open, forming a sort of twisted crescent-shaped arc.'
The water-vascular rosette is provided with five lobes, corresponding to which are folds in the larval skin, and each lobe corresponds to one of the calcareous plates developed on the abactinal disc. The plane of the actinal surface at first meets that of the abactinal at an acute or nearly right angle. The two surfaces are separated by the whole width of the stomach. The general appearance of the larva from the ventral surface after the development of the water-vascular rosette (i) and abactinal disc (A) is shewn in fig. 260.
As development proceeds the abactinal surface becomes a firm and definite disc, owing to the growth of the original calcareous spicules into more or less definite plates, and to the development of five fresh plates nearer the centre of the disc and interradial in position. Still later a central calcareous plate appears on the abactinal surface, which is thus formed of a central plate, surrounded by a ring of five interradial plates, and then again by a ring of five radial plates. The abactinal disc now also grows out into five short processes, separated by five shallow notches. These processes are the rudiments of the five arms, and each of them corresponds to one of the lobes of the water-vascular rosette. A calcareous deposit is formed round the opening of the water-vascular canal, which becomes the madreporic tubercle 1 . At about this stage the absorption of the larval appendages takes place. The whole anterior part of the
1 The exact position of the madreporic tubercle in relation to the abactinal plates does not seem to have been made out. It might have been anticipated that it would be placed in one of the primary interradial plates, but this does not seem to be the case. The position of the anus is also obscure.
larva with the great prae-oral lobe has hitherto remained unchanged, but now it contracts and undergoes absorption, and becomes completely withdrawn into the disc of the future starfish. The larval mouth is transported into the centre of the actinal disc. In the larvae observed by Agassiz and Metschnikoff nothing was cast off, but the whole absorbed.
According to M tiller and Koren and Danielssen this is not the case in the larva observed by them, but part of the larva is thrown off, and lives for some time independently.
After the absorption of the larval appendages the actinal and abactinal surfaces of the young starfish approach each other, owing to the flattening of the stomach ; at the same time they lose their spiral form, and become flat discs, which fit each other. Each of the lobes of the rosette of the watervascular system becomes one of the radial water-vascular canals. It first becomes five-lobed, each lobe forming a rudimentary tube foot, and on each ^ d ctinal disc of youn Aste ' side of the middle lobe two fresh ones
next spring out, and so on in succession. The terminal median lobe forms the tentacle at the end of the arm, and the eye is developed at its base. The growth of the water-vascular canals keeps pace with that of the arms, and the tube feet become supported at their base by an ingrowth of calcareous matter. The whole of the calcareous skeleton of the larva passes directly into that of the adult, and spines are very soon formed on the plates of the abactinal surface. The original radial plates, together with the spines which they have, are gradually pushed outwards with the growth of the arms by the continual addition of fresh rows of spines between the terminal plate and the plate next to it. It thus comes about that the original radial plates persist at the end of the arms, in connection with the unpaired
FIG. 260. BIPINNARIA LARVA OF AN ASTEROID. (From Gegenbaur ; after Miiller.)
b. mouth ; a. anus ; h. madreporic canal ; t. ambulacral rosette ; c . stomach ; d. g. e. etc. arms of Bipinnaria ; A.
tentacles which form the apex of the radial water-vascular tubes.
It has already been mentioned that according to Metschnikoff (No. 560) a new oesophagus is formed which perforates the water-vascular ring, and connects the original stomach with the original mouth. Agassiz (No. 543) maintains that the water-vascular ring grows round the primitive oesophagus. He says " During the shrinking of the larva the long oesophagus becomes " shortened and contracted, bringing the opening of the mouth of the larva " to the level of the opening of the oesophagus, which eventually becomes "the true mouth of the starfish." The primitive anus is believed by Metschnikoff to disappear, but by Agassiz to remain. This discrepancy very possibly depends upon these investigators having worked at different species.
There is no doubt that the whole of the larval organs, with the possible exception of the oesophagus, and anus (where absent in the adult), pass directly into the corresponding organs of the starfish and that the prae-oral part of the body and arms of the larva are absorbed and not cast off.
In addition to the Bipinnarian type of Asteroid larva a series of other forms has been described by Miiller (No. 561), Sars, Keren, and Danielssen (No. 554) and other investigators, which are however very imperfectly known. The best-known form is one first of all discovered by Sars in Echinaster Sarsii, and the more or less similar larvae subsequently investigated by Agassiz, Busch, Miiller, Wyville Thomson, etc. of another species of Echinaster and of Asteracanthion. These larvae on leaving the egg have an oval form, and are uniformly covered by cilia. Four processes (or in Agassiz' type one process) grow out from the body ; by these the larvae fix themselves. In the case of Echinaster the larvae are fixed in the ventral concavity of the disc of the mother, between the five arms, where a temporary brood-pouch is established. The main part of the body is converted directly into the disc of the young starfish, while the four processes come to spring from the ventral surface, and are attached to the water- vascular ring. Eventually they atrophy completely. Of the internal structure but little is known ; till the permanent mouth is formed, after the development of the young starfish is pretty well advanced, the stomach has no communication with the exterior.
A second abnormal type of development is presented by the embryo of Pteraster miliaris, as described by Koren and Danielssen 1 . The larvae to the number of eight to twenty develop in a peculiar pouch on the dorsal surface of the body. The early stages are not known, but in the later ones the whole body assumes a pentagonal appearance with a mouth at one edge
1 The following statements are taken from the abstract in Bronn's Thierreichs. B. II. 36
of the disc. At a later stage the anus is formed on the dorsal side of an arm opposite the mouth. The stomach is surrounded by a water-vascular ring, from which the madreporic canal passes to the dorsal surface, but does not open. At a later stage the embryonic mouth and anus vanish, to be replaced by a permanent mouth and anus in the normal positions.
A third, and in some respects very curious, form is a worm like larva of Miiller, which is without bands of cilia. The dorsal surface of the youngest larva is divided by transverse constrictions into five segments. On the under side of the first of these is a five-lobed disc, each lobe being provided with a pair of tube feet.
At a later period only three segments are visible on the dorsal surface, but the ventral surface has assumed a pentagonal aspect. The later stages are not known.
Ophiuroidea. The full-grown larva of the Ophiuroids is known as a Pluteus. It commences with the usual more or less spherical form ; from this it passes to a form closely resembling
FIG. 261. DIAGRAMMATIC FIGURES SHEWING THE EVOLUTION OK AN OPHIUROID PLUTEUS FROM A SIMPLE ECHINODERM LARVA. (Copied from Miiller.) The calcareous skeleton is not represented.
///. mouth; an. anus; d. anterior arms; d'. lateral arms; e'. posterior arms; tf. anterolateral arms.
that of Auricularia with a rounded dorsal surface, and a flattened ventral one. Soon however it becomes distinguished by the growth of a post-anal lobe and the absence of a prae-oral lobe (fig. 261 B). The post-anal lobe forms the somewhat rounded apex of the body. In front of the mouth, and between the mouth and anus, arise the anal and oral ciliated ridges, which soon become continued into a single longitudinal ciliated ring. At the same time the body becomes prolonged into a series of
processes along the ciliated band, which is continued to the extremity of each. The primitive ciliated ring never becomes broken up into two or more rings. A ciliated crown is usually developed at the extremity of the post-anal lobe. The arms are arranged in the form of a ring surrounding the mouth, and are all directed forwards.
The first arms to appear are two lateral ones, which usually remain the most conspicuous (fig. 261 B and C, cf\ Next arises a pair on the sides of the mouth, which may be called the mouth or anterior arms (C, d}. A pair ventral to and behind the lateral arms is then formed, constituting the posterior arms (D, e'\ and finally a pair between the lateral arms and the anterior, constituting the anterolateral arms (D,^).
The concave area between the arms forms the greater part of the ventral surface of the body. Even before the appearance of any of the arms, and before the formation of the mouth, two calcareous rods are formed, which meet behind at the apex of the post-anal lobe, and are continued as a central support into each of the arms as they are successively formed. These rods are shewn at their full development in fig. 262. The important points which distinguish a Pluteus larva from the Auricularia or Bipinnaria are the following :
(i) The presence of the postanal lobe at the hind end of the body. (2) The slight development of a prae-oral lobe. (3) The provisional calcareous skeleton in the larval arms.
Great variations are presented in the development of the arms and provisional skeleton. The presence of lateral arms is however a distinctive characteristic of the Ophiuroid Pluteus. The other arms may be quite absent, but the lateral arms never.
The formation of the permanent Ophiuroid takes place in much the same way as in the Asteroidea.
FIG. 262. OPHIUROID. after Miiller.)
PLUTEUS LARVA OF AN (From Gegenbaur ;
A. rudiment of young Ophiuroid ; (?. lateral arms; d. anterior arms; e . posterior arms.
There is formed (fig. 262) on the right and dorsal side of stomach the abactinal disc supported by calcareous plates, at first only five in number and radial in position 1 . The disc is at first not symmetrical, but becomes so at the time of the resorption of the larval arms. It grows out into five processes the five future rays. The original five radial plates remain as the terminal segments of the adult rays, and new plates are always added between the ultimate and penultimate plate (Mu'ller), though it is probable that in the later stages fresh plates are added in the disc.
The ventral surface of the permanent Ophiuroid is formed by the concave surface between the mouth and anus. Between this and the stomach is
FIG. 263. DIAGRAMMATIC FIGURES SHEWING THE EVOLUTION OF ECHINOID PLUTEI. (Copied from Miiller.) The calcareous skeleton is not represented. E. Pluteus of Spatangus.
m. mouth; an. anus; d. anterior arms; d' . point where lateral arms arise in the Ophiuroid Pluteus; e. anterointernal arms; e. posterior arms; g'. anterolateral arms; g. anteroexternal arms.
situated the water-vascular ring. It is at first not closed, but is horseshoeshaped, with five blind appendages (fig. 262). It eventually grows round the cesophagus, which, together with the larval mouth, is retained in the adult. The five blind appendages become themselves lobed in the same way as in Asterias, and grow out along the five arms of the disc and become the radial canals and tentacles. All these parts of the water-vascular system are of course covered by skin, and probably also surrounded by mesoblast cells, in which at a later period the calcareous plates which lie ventral to the radial canal are formed. The larval anus disappears. As long as the larval appendages are not absorbed the ventral and dorsal discs of the permanent Ophiuroid fit as little as in the case of the Brachiolaria, but at a certain period the appendages are absorbed. The calcareous rods of the larval arms
1 Whether interradial plates are developed as in Asterias is not clear. They seem to be found in Ophiopholis bellis, Agassiz, but have not been recognised in other forms (vide Carpenter, No. 548, p. 369).
break up, the arms and anal lobe become absorbed, and the dorsal and ventral discs, with the intervening stomach and other organs, are alone left. After this the discs fit together, and there is thus formed a complete young Ophiuroid.
The whole of the internal organs of the larva (except the anus), including the mouth, cesophagus, the body cavity, etc. are carried on directly into the adult.
The larval skeleton is, as above stated, absorbed.
The viviparous larva of Amphiura squamata does not differ very greatly from the larvae with very imperfect arms. It does not develop a distinct ciliated band, and the provisional skeleton is very imperfect. The absence of these parts, as well as of the anus, mentioned on p. 549, may probably be correlated with the viviparous habits of the larva. With reference to the passage of this larva into the adult there is practically nothing to add to what has just been stated. When the development of the adult is fairly advanced the part of the body with the provisional skeleton forms an elongated rod-like process attached to the developing disc. It becomes eventually absorbed.
Echinoidea. The Echinus larva (fig. 263} has a Pluteus form like that of the Ophiuroids, and in most points, such as the
FIG. 264. Two LARV/E OF STRONGYLOCENTRUS. (From Agassiz.) m. mouth; a. anus; o. cesophagus; d. stomach; c. intestine; '. and v. ciliated ridges; iv. water- vascular tube; r. calcareous rods.
presence of the anal lobe, the ciliated band, the provisional skeleton, etc., develops in the same manner. The chief difference between the two Pluteus forms concerns the development of the lateral arms. These, which form the most prominent arms in the Ophiuroid Pluteus, are entirely absent in the Echinoid
Pluteus, which accordingly has, as a rule, a much narrower form than the Ophiuroid Pluteus.
A pair of ciliated epaulettes on each side of and behind the ciliated ring is very characteristic of some Echinoid larvae. They are originally developed from the ciliated ring (fig. 266 A
FIG. 265. LATERAL AND VENTRAL VIEW OF A LARVA OF STRONGYLOCENTRUS.
(From Agassiz.) General references as in fig. 264.
b. dorsal opening of madreporic canal; e '. posterior arms ; e'". anterior arms; f lV . anterointernal arms.
and B, z>"). The presence of three processes from the anal lobe supported by calcareous rods is characteristic of the Spatangoid Pluteus (fig. 263 E).
The first two pairs of arms to develop, employing the same names as in Ophiuroids, are the anterior attached to the oral process (fig. 263 C, d] and the posterior pair (*?') A pair of anterolateral arms next becomes developed (j^). A fourth pair (not represented in Ophiuroids) appears on the inner side of the anterior pair forming an anterointernal pair (e}, and in the Spatangoid Pluteus a fifth pair may be added on the external side of the anterior pair forming an anteroexternal pair (g).
Each of the first-formed paired calcareous rods is composed of three processes, two of which extend into the anterior and posterior arms ; and the third and strongest passes into the anal lobe, and there meets its fellow (fig. 265). A transverse bar in front of the arms joins the rods of the two sides meeting them at the point where the three processes diverge. The process in the anterolateral arm (fig. 266 B) is at first independent of this system of rods, but eventually unites with it. Although our knowledge of
the Pluteus types in the different groups is not sufficient to generalise with great confidence, a few points seem to have been fairly determined 1 . The Plutei of Strongylocentrus (figs. 266 and 267) and Echinus have eight arms and four ciliated epaulettes. The only Cidaris-like form, the Pluteus of which is known, is Arbacia : it presents certain peculiarities. The anal lobe develops a pair of posterior (auricular) appendages, and the ciliated ring, besides growing out into the normal eight appendages, has a pair of short blunt anterior and posterior lobes. An extra pair of non-ciliated accessory mouth arms appears also to be developed. Ciliated epaulettes are not present. So far as is known the Clypeastroid larva is chiefly characterized by the round form of the anal lobe. The calcareous rods are latticed. In the Pluteus of Spatangoids there are (fig. 263) five pairs of arms around the mouth pointing forwards, and three arms developed from the anal lobe pointing backwards. One of these is unpaired, and starts from the apex of the anal lobe. All the arms have calcareous rods which, in the case of the posterior pair, the anterolateral pair, and the unpaired arm of the anal lobe, are latticed. Ciliated epaulettes are not developed.
Viviparous larvae of Echinoids have been described by Agassiz 2 .
The development of the permanent Echinus has been chiefly worked out by Agassiz and Metschnikoff.
In the Pluteus of Echinus lividus the first indication of the adult arises, when three pairs of arms are already developed, as an invagination of the skin on the left side, between the posterior and anterolateral arms, the bottom of which is placed close to the water-vascular vesicle (fig. 266 B, u/\ The base of this invagination becomes very thick, and forms the ventral disc of the future Echinus. The parts connecting this disc with the external skin become however thin, and, on the narrowing of the external aperture of invagination and the growth of the thickened disc, constitute a covering for the disc, called by Metschnikoff the amnion. The water- vascular vesicle adjoining this disc grows out into five processes, forming as many tube feet, which cause the surface of the involuted disc to be produced into the same number of processes. The external opening of the invagination of the disc never closes, and after the development of the tube feet begins to widen again, and the amnion to atrophy. Through the opening of the invagination the tube feet now project. The dorsal and right surface of the Pluteus, which extends so as to embrace the opening of the madreporic canal and the anus, forms the abactinal or dorsal surface of the future Echinus (fig. 267, a). This disc fits on to the actinal invaginated surface which arises on the left side of the Pluteus. On the right surface of the larva (dorsal of permanent Echinus) two pedicellariae appear, and at a later period spines are formed, which are at first arranged in a ring-like form round the edge of the primitively flat test. While these changes are taking place, and the two surfaces of the future Echinus are gradually moulding themselves so as to
1 Vide especially Muller, Agassiz, and Metschnikoff.
2 For viviparous Echini vide Agassiz, Proc. Amer. Acad. 1876.
form what is obviously a young Echinus, the arms of the Pluteus with their contained skeleton have been gradually undergoing atrophy. They become irregular in form, their contained skeleton breaks up into small pieces, and they are gradually absorbed.
The water-vascular ring is from the first complete, so that, as in Asterias, it is perforated through the centre by a new oesophagus. According
FIG. 266. SIDE AND DORSAL VIEW OF A LARVA OF STRONGYLOCENTRUS.
(From Agassiz.) General reference letters as in figs. 264 and 265. e" . anterolateral arms; v" '. ciliated epaulettes; ?&'. invagination to form the disc of Echinus.
to Agassiz the first five tentacles or tube feet grow into the radial canals, and form the odd terminal tentacles exactly as in Asterias 1 . Spatangus only differs in development from Echinus in the fact that the opening of the invagination to form the ventral disc becomes completely closed, and that the tube feet have eventually to force their way through the larval epidermis of the amnion, which is ruptured in the process and eventually thrown off.
Crinoidea. The larva of Antedon, while still within the egg-shell, assumes an oval form and uniform ciliation. Before it
1 Gotte (No. 549) supported by Muller's and Krohn's older, and in some points extremely erroneous observations, has enunciated the view that the radial canals in Echinoids and Holothuroids have a different nature from those in Asteroids and Ophiuroids.
becomes hatched the uniform layer of cilia is replaced by four transverse bands of cilia, and a tuft of cilia at the posterior extremity. In this condition it escapes from the egg-shell
FIG. 267. FULL-GROWN LARVA OF STRONGYLOCENTRUS. (From Agassiz.) The figure shews the largely-developed abactinal disc of the young Echinus enclosing the larval stomach. Reference letters as in previous figs.
(fig. 268 A), and becomes bilateral, owing to a flattening of the ventral surface. On the flattened surface appears a ciliated
depression corresponding in position with the now closed blastopore (vide p. 550). The third ciliated band bends forward to pass in front of this (fig. 269). Behind the last ciliated band there is present a small depression of unknown function, also
FIG. 768. THREB STAGES IN THE DEVELOPMENT OF ANTEDON (COMATULA.)
(From Lubbock; after Thomson.)
A. larva just hatched; B. larva with rudiment of the calcareous plates; C. Pentacrinoid larva.
situated on the ventral surface. The posterior extremity of the embryo elongates to form the rudiment of the future stem, and a fresh depression, marking the position of the future mouth, makes its appearance on the anterior and ventral part.
While the ciliated bands are still at their full development, the calcareous skeleton of the future calyx makes its appearance in the form of two rows, each of five plates, formed of a network of spicula (figs. 268 B and 269). The plates of the anterior ring are known as the orals, those of the posterior as the basals. The former surround the left, i.e. anterior peritoneal sack ; the latter the right, i.e. posterior peritoneal sack. The two rows of plates are at first not quite transverse, but form two oblique circles, the dorsal end being in advance of the ventral. The rows soon become transverse, while the originally somewhat ventral oral surface is carried into the centre of the area enclosed by the oral plates.
By the change in position of the original ventral surface relatively to the axis of the body, the bilateral symmetry of the larva passes into a radial symmetry. While the first skeletal elements of the calyx are being formed, the skeleton of the stem is also established. The terminal plate is first of all established, then the joints, eight at first, of the stem. The centro-dorsal plate is stated by Thomson to be formed as the uppermost joint of the stem 1 . The larva, after the completion of the above changes, is shewn in fig. 268 B, and somewhat more diagrammatically in fig. 269.
After the above elements of the skeleton have become established the ciliated bands undergo atrophy, and shortly after 1 Gotte (No. 549) on the other hand holds that the centro-dorsal plate is developed by the coalescence of a series of at first independent rods, which originate simultaneously with, and close to, the lower edges of the basals, and that it is therefore similar in its origin to the basals.
FIG. 269. LARVA OF ANTEDON WITH RUDIMENTS OF CALCAREOUS SKELETON. (From Carpenter; after Thomson.)
i. Terminal plate at the end of the stem ; 3. basals ; or. orals ; bl. position of blastopore.
wards the larva becomes attached by the terminal plate of its stem. It then passes into the Pentacrinoid stage! The larva in this stage is shewn in fig. 268 C and fig. 270. New joints are added at the upper end of the stem next the calyx, and a new element the radials makes its appearance as a ring of five small plates, placed in the space between the basals and orals, and in the intervals alternating with them (fig. 270, 4). The roof of the oral vestibule (vide fig. 253 and p. 551) has in the meantime become ruptured ; and the external opening of the mouth thus becomes established. Surrounding the mouth are five petal-like lobes, each of them supported by an oral plate (fig. 268 C). In the intervals between them five branched and highly contractile tentacles, which were previously enclosed within the vestibule, now sprout out : they mark the position of the future radial canals, and are outgrowths of the water-vascular ring. At the base of each of them a pair of additional tentacles is soon formed. Each primary tentacle corresponds to one of the radials. These latter are therefore, as their name implies, radial in position; while the basals and orals are interradial. In addition to the contractile radial tentacles ten non-contractile tentacles, also diverticula of the water- vascular ring, are soon formed, two for each interradius.
In the course of the further development the equatorial space between the FlG - 2 7<>. YOUNG PEN . TACRINOID LARVA OF AN
orals and the basals enlarges, and gives TEDON. (From Carpenter ; rise to a wide oral disc, the sides of which after w >' ville Thom s"-)
- , , . ... . i. terminal plate of stem;
are formed by the radials resting on the c d. centro-donal plate; 3 . basals; while in the centre of it are bftsals J 4- radials; or. orals. placed the five orals, each with its special lobe.
The anus, which is formed on the ventral side in the position
of the blastopore (p. 551), becomes surrounded by an anal plate, which is interradial in position, and lies on the surface of the oral disc between the orals and radials. On the oral plate in the next interradius is placed the opening of a single funnel leading into the body cavity, which Ludwig regards as equivalent to the opening of the madreporic canal (vide p. 55 1) 1 .
From the edge of the vestibule the arms grow out, carrying with them the tentacular prolongation of the water-vascular ring. Two additional rows of radials are soon added.
The stalked Pentacrinoid larva becomes converted, on the absorption of the stalk, into the adult Antedon. The stalk is functionally replaced by a number of short cirri springing from the centro-dorsal plate. The five basals coalesce into a single plate, known as the rosette, and the five orals disappear, though the lobes on which they were placed persist. In some stalked forms, e.g. Rhizocrinus Hyocrinus, the orals are permanently retained. The arms bifurcate at the end of the third radial, and the first radial becomes in Antedon rosacea (though not in all species of Antedon) concealed from the surface by the growth of the centro-dorsal plate. An immense number of funnels, leading into the body cavity, are formed in addition to the single one present in the young larva. These are regarded by Ludwig as equivalent to so many openings of the madreporic canal ; and there are developed, in correspondence with them, diverticula of the water-vascular ring.
Comparison of Echinoderm Larvce and General Conclusions.
In any comparison of the various types of Echinoderm larvae it is necessary to distinguish between the free-swimming forms, and the viviparous or fixed forms. A very superficial examination suffices to shew that the free-swimming forms agree very much more closely amongst themselves than the viviparous
1 I have made no attempt to discuss the homologies of the plates of the larval Echinodermata because the criteria for such a discussion are still in dispute. The suggestive memoirs of P. H. Carpenter (No. 548) on this subject may be consulted by the reader. Carpenter attempts to found his homologies on the relation of the plates to the primitive peritoneal vesicles, and I am inclined to believe that this method of dealing with these homologies is the right one. Ludwig (No. 559) by regarding the opening of the madreporic canal as a fixed point has arrived at very different results.
COMPARISON OF ECHINODERM LARV.-E.
forms. We are therefore justified in concluding that in the viviparous forms the development is abbreviated and modified.
All the free forms are nearly alike in their earliest stage after the formation of the archenteron. The surface between the anus and the future mouth becomes flattened, and (except in Antedon, Cucumaria, Psolinus, etc. which practically have an abbreviated development like that of the viviparous forms) a ridge of cilia becomes established in front of the mouth, and a second ridge between the mouth and the anus. This larval form, which is shewn in fig. 264 A, is the type from which the various forms of Echinoderm larvae start.
In all cases, except in Bipinnaria, the two ciliated ridges soon become united, and constitute a single longitudinal postoral ciliated ring.
The larvae in their further growth undergo various changes, and in the later stages they may be divided into two groups :
(1) The Pluteus larva of Echinoids and Ophiuroids.
(2) The Auricularia (Holothuroids) and Bipinnaria (Asteroids) type.
The first group is characterized by the growth of a number of arms more or less surrounding the mouth, and supported by calcareous rods. The ciliated band retains its primitive condition as a simple longitudinal band throughout larval life. There is a very small prae-oral lobe, while an anal lobe is very largely developed.
The Auricularia and Bi- A. B
pinnaria resemble each other in shape, in the development of a large prae-oral lobe, and in the absence of provisional calcareous rods ; but differ in the fact that the ciliated band is single in Auricularia (fig 271 A), and is double in Bipinnaria (fig. 271 B).
THUROID. B. THE LARVA OF AN ASTEa great tendency to develop RIAS.
soft arms; while in the Auri- . ' mouth; st. stomach; a. anus; I.e. , . ,_, , *_ 1-1- primitive longitudinal ciliated band; pr.c.
cularia the longitudinal ciliat- p r3 e-oral ciliated band.
THE LARVA OF A
ed band breaks up into a number of transverse ciliated bands. This condition is in .some instances reached directly, and such larvae undoubtedly approximate to the larvae of Antedon, in which the uniformly ciliated condition is succeeded by one with four transverse bands, of which one is prae-oral.
All or nearly all Echinoderm larvae are bilaterally symmetrical, and since all Echinodermata eventually attain a radial symmetry, a change necessarily takes place from the bilateral to the radial type.
In the case of the Holothurians and Antedon, and generally in the viviparous types, this change is more or less completely effected in the embryonic condition ; but in the Bipinnaria and Pluteus types a radial symmetry does not become apparent till after the absorption of the larval appendages. It is a remarkable fact, which seems to hold for the Asteroids, Ophiuroids, Echinoids, and Crinoids, that the dorsal side of the larva is not directly converted into the dorsal disc of the adult; but the dorsal and right side becomes the adult dorsal or abactinal surface, while the ventral and left becomes the actinal or ventral surface.
It is interesting to note with reference to the larvae of the Echinodermata that the various existing types of larvae must have been formed after the differentiation of the existing groups of the Echinodermata ; otherwise it would be necessary to adopt the impossible position that the different groups of Echinodermata were severally descended from the different types of larvae. The various special appendages, etc. of the different larvae have therefore a purely secondary significance; and their atrophy at the time of the passage of the larva into the adult, which is nothing else but a complicated metamorphosis, is easily explained.
Originally, no doubt, the transition from the larva to the adult was very simple, as it is at present in most Holothurians ; but as the larvae developed various provisional appendages, it became necessary that these should be absorbed in the passage to the adult state.
It would obviously be advantageous that their absorption should be as rapid as possible, since the larva in a state of transition to the adult would be in a very disadvantageous
576 COMPARISON OF ECHINODERM
position. The rapid metamorphosis, which we find in Asteroids, Ophiuroids, and Echinoids in the passage from the larval to the adult state, has no doubt arisen for this reason.
In spite of the varying provisional appendages possessed by Echinoderm larvae it is possible, as stated above (p. 574), to recognise a type of larva, of which all the existing Echinoderm larval forms are modifications. This type does not appear to me to be closely related to that of the larvae of any group described in the preceding pages. It has no doubt certain resemblances to the trochosphere larva of Chaetopoda, Mollusca, etc., but the differences between the two types are more striking than the resemblances. It firstly differs from the trochosphere larva in the character of the ciliation. Both larvae start from the uniformly ciliated condition, but while the prae-oral ring is almost invariable, and a peri-anal ring very common in the trochosphere; in the Echinoderm larva such rings are rarely found ; and even when present, i.e. the prae-oral ring of Bipinnaria and the terminal though hardly peri-anal patch of Antedon, do not resemble closely the more or less similar structures of the trochosphere. The two ciliated ridges (fig. 264 A) common to all the Echinoderm larvae, and subsequently continued into a longitudinal ring, have not yet been found in any trochosphere. The transverse ciliated rings of the Holothurian and Crinoid larvae are of no importance in the comparison between the trochosphere larvae and the larvae of Echinodermata, since such rings are frequently secondarily developed. Cf. Pneumodermon and Dentalium amongst Mollusca.
In the character of the prae-oral lobe the two types again differ. Though the prae-oral lobe is often found in Echinoderm larvae it is never the seat of an important (supra-oesophageal) ganglion and organs of special sense, as it invariably is in the trochosphere.
Nothing like the vaso-peritoneal vesicles of the Echinoderm larvae has been found in the trochosphere ; nor have the characteristic trochosphere excretory organs been found in the Echinoderm larvae.
The larva which most nearly approaches those of the Echinodermata is the larva of Balanoglossus described in the next chapter.
(542) Alex. Agassiz. Revision of the Echini. Cambridge, U.S. 1872 74.
(543) Alex. Agassiz. " North American Starfishes." Memoirs of the Museum of Comparative Anatomy and Zoology at Harvard College, Vol. v., No. i. 1877 (originally published in 1864).
(544) J. Barrois. " Embryogenie de 1'Asteriscus verruculatus " Journal dc VAnat. et Phys. 1879.
(545) A. Baur. Beitrdge zur Naturgeschichte d. Synapta digitata. Dresden, 1864.
(546) H. G. Bronn. Klassen u. Ordnungen etc. Strahlenthiere, Vol. II. 1860.
(547) W. B. Carpenter. "Researches on the structure, physiology and development of Antedon." Phil. Trans. CLVI. 1866, and Proceedings of the Roy. Soc., No. 166. 1876.
(548) P. H. Carpenter. " On the oral and apical systems of the Echinoderms." Quart. J. of Micr. Science, Vol. xvm. and xix. 18789.
(549) A. Gotte. " Vergleichende Entwicklungsgeschichte d. Comatula mediterranea." Arch, fur micr. Anat., Vol. xn. 1876.
(550) R. Greeff. "Ueber die Entwicklung des Asteracanthion rubens vom Ei bis zur Bipinnaria u. Brachiolaria." Schriften d. Gesellschaft zur Beforderung d. gesammten Natunvissenschaften zu Marburg, Bd. xn. 1876.
(551) R. Greeff. "Ueber den Bau u. die Entwicklung d. Echinodermen." Sitz. d. Gesell. z. Beforderung d. gesam. Naturwiss. zu Marburg, No. 4. 1879.
(552) T. H. Huxley. "Report upon the researches of Miiller into the anat. anddevel. of the Echinoderms." Ann. and Mag. of Nat. Hist., 2nd Ser., Vol. vin. 1851.
(553) Koren and Danielssen. "Observations sur la Bipinnaria asterigera. Ann. Scien. Nat., Ser. in., Vol. vii. 1847.
(554) Koren and Danielssen. "Observations on the development of the Starfishes." Ann. and Mag. of Nat. Hist., Vol. XX. 1857.
(555) A. Kowalevsky. " Entwicklungsgeschichte d. Holothurien. " Mhn.Ac. Petersbourg, Ser. VII., Tom. XL, No. 6.
(556) A. Krohn. "Beobacht. a. d. Entwick. d. Holothurien u. Seeigel." Miillers Archiv, 1851.
(557) A. Krohn. "Ueb. d. Entwick. d. Seesterne u. Holothurien." Miillcr's Archiv, 1853.
(558) A. Krohn. "Beobacht. lib. Echinodermenlarven." Mailer's Archiv, 1854.
(559) H. Ludwig. "Ueb. d. primar. Steinkanal d. Crinoideen, nebst vergl. anat. Bemerk. lib. d. Echinodermen." Zeit.f. wiss. ZooL, Vol. xxxiv. 1880.
(560) E. Metschnikoff. "Studien iib. d. Entwick. d. Echinodermen u. Nemertinen." Mem. Ac. Petersboiirg, Series vii., Tom. xiv., No. 8. 1869.
(561) 1 Joh. Miiller. "Ueb. d. Larven u. d. Metamorphosed. Echinodermen." Abhandlungen d. Berlin. Akad. (Five Memoirs), 1848, 49, 50, 52 (two Memoirs).
(562) Joh. Mtiller. "Allgemeiner Plan d. Entwicklung d. Echinodermen." Abhandl. d. Berlin. Akad., 1853.
1 The dates in this reference are the dates of publication. B. II. 37
(563) E. Selenka. "Zur Entwicklung d. Holothurien." Zeit. f. wiss. Zool., Bd. xxvii. 1876.
(564) E. Selenka. "Keimblatter u. Organanlage bei Echiniden." Zeit.f.-wiss. Zool., Vol. xxxin. 1879.
(565) Sir Wyville Thomson. "On the Embryology of the Echinodermata." Natural History Review, 1 864.
(566) Sir Wyville Thomson. "On the Embryogeny of Antedon rosaceus." Phil. Trans. 1865.