Chapter XXIII: Appendix: To Ctenophora (6)

Both oral and aboral coelom become, like the hydrocoel, bent into hoops, and along the axis of the aboral coelom a cord of germ-cells is developed, which constitutes the "genital stolon." The chambered organ is developed from the aboral coelom, and {586}in the centre of its five chambers a median pocket grows down into the centro-dorsal, along the side of which is an extension of the genital stolon. In the arms the mesentery separating the extensions of the oral and aboral coelom persists; the oral extension consists of two parallel canals called "subtentacular" (Fig. 267, _s.c_), whilst the aboral space is termed the "coeliac" canal (Fig. 267, _c.c_). In the tip of the pinnule, that is to say at the extremity of a ramification of the arm, the coeliac and subtentacular canals communicate. As portions of the lining of both canals are ciliated, a circulation of the coelomic fluid is thus kept up. The GENITAL STOLON gives rise at the level of the remnant of the horizontal mesentery in the disc to a circular genital rachis, whence cords pass down the arms in the tissue separating subtentacular and coeliac canals (Fig. 267, _g.r_). Each cord is contained in a special tube, the "genital canal," which is probably developed in the same way as the aboral sinus of the Eleutherozoa, _i.e._ as a special sheltering outgrowth of the coelom (Fig. 267, _g.c_). In the pinnule the rachis swells out into a genital organ, from which a short duct is developed when the organ is mature. The eggs are large (3 mm. in diameter), and adhere for a considerable period of their development to the pinnules.

{587}The MUSCLES of _Antedon_ are of two kinds. Those of the water-vascular system are, as in Eleutherozoa, basal outgrowths of the cells forming the walls of the system. The muscles moving the joints of the arms appear to be modifications of connective-tissue cells. When the brachials are isolated their terminal faces, strikingly long, recall those of Ophiuroid vertebrae. There is a ventral groove for the coelomic canal. Above this groove the face is divided by ridges into four areas for attachment of the muscles. Dorsal to this is the pit for the strong ligament which binds the ossicles together; then comes the canal for the aboral nerve-cord, whilst dorsal to this is the pit for what is called the "dorsal elastic ligament." The theory underlying this name is that the muscles bend the arms ventrally, and the ligament by its elasticity restores them to their places; but there seems reason to believe that the "ligament" is really a dorsal muscle. It is particularly to be noted that similar muscles occur between the first and second radials, proving that the primary arm really begins with the first radial. The second and third radials, as also the first two ossicles and certain others of each arm, are closely united by calcified fibres, and this kind of union is called a "syzygy" (Fig. 265, _syz_). The cirri have all their ossicles united by muscular attachment, and can move rapidly.

The BLOOD SYSTEM (see pp. 449-451) forms a ring consisting of a network of strings round the oesophagus. This is termed the "labial plexus." From this cords can be traced to the wall of the stomach and to the surface of the genital stolon. The assertion that radial strands intervene between the ectodermic nerve-cord and the radial water-vascular canal, though usually made, does not appear to be justified, since what is termed the vessel appears to be a crevice formed by shrinkage in preservation.

The process of respiration is doubtless largely carried out by the podia, but it must be assisted by the constant instreaming of fresh sea-water through the pore-canals. The process of EXCRETION has not been directly observed in _Antedon_, but structures called "sacculi" may be connected with this function. These are spherical masses of amoebocytes embedded in the tegmen. During life they are colourless, but after death they become coloured, showing that they secrete a peculiar compound. These sacculi abound in the disc, and a row of them is to be found at each side of the ambulacral groove in the arms. When, as in {588}tropical species, the groove is supported by side-plates, these are notched for the reception of the sacculi.

Turning now to survey the group Crinoidea as a whole, lack of space forces us to confine our attention mainly to the living forms. These differ amongst themselves chiefly in the following points: (1) the condition of the stem; (2) the structure of the calyx; (3) what is intimately connected with this, the method of branching of the arms; and (4) the length of the alimentary canal.

CONDITION OF THE STEM.—This is represented by a centro-dorsal stump in _Antedon_ and most of its allies, but in _Actinometra_ it becomes a flat plate, and in some species in old age all the cirri drop off. In _Uintacrinus_ and _Marsupites_ (fossil genera) there is no trace of cirri. In Pentacrinidae there is a long stem, pentagonal in cross-section, in which alternate ossicles carry whorls of cirri; in Rhizocrinidae the stem consists of compressed ossicles, elliptical in section, bearing cirri only at the rooting tip, whilst in _Hyocrinus_ the stem is made up of cylindrical ossicles, cirri being apparently absent. Finally, in _Holopus_ the stem is represented by an uncalcified leathery outgrowth from the calyx.

SKELETON OF CALYX AND ARM.—In living Crinoidea, with the doubtful exceptions of _Holopus_ and _Hyocrinus_, the calyx is supposed to be built up originally of four whorls of plates, viz. "infra-basals," "basals," "radials," and "orals," the last named forming the skeleton of the oral valves round the mouth. In the two exceptions named there is no certain evidence of the existence of infra-basals. In living forms the infra-basals coalesce with the uppermost joint of the stem; the basals remain large and conspicuous, though they are fused into a ring in Rhizocrinidae, _Atelecrinus_, and _Thaumatocrinus_, whilst in Pentacrinidae this ring is nearly, and in _Antedon_ and its allies completely, hidden when the calyx is viewed from the outside. In _Hyocrinus_ the basals are represented by three ossicles. The lowest radials are an important element in the patina in every case, but the upper radials, the incipient portions of the arms, may be incorporated in the calyx (_Pentacrinus_, _Antedon_) or may be free (Rhizocrinidae and _Hyocrinus_); in _Metacrinus_ there are five to eight radials in each column, all incorporated.

The oral plates are very large in _Hyocrinus_, _Holopus_, and {589}_Thaumatocrinus_, small in _Rhizocrinus_, vestigial or absent in _Bathycrinus_, and completely absorbed in _Antedon_. In addition to these main elements, in many species small accessory plates are developed (_a_) at the sides of the ambulacral groove, over which they can close down (many species of _Antedon_, _Hyocrinus_, _Holopus_, Rhizocrinidae, some species of Pentacrinidae); these are "covering plates," and correspond in function to the adambulacrals of Asteroidea; (_b_) supporting the sides of the groove and corresponding to the ambulacrals of Asteroidea;[514] these are "side-plates," and the covering plates articulate with them; (_c_) on the surface of the tegmen; these are the interradial plates, which in _Thaumatocrinus_ alone among recent forms, but in many fossil forms, are continued into the patina, where they separate the radial plates.

MODE OF BRANCHING OF THE ARMS.—All modern Crinoids have pinnules, and this, as has already been explained, is due to a suppressed dichotomy. The extent of the suppression determines the number of arms, which varies within the same genus.

ALIMENTARY CANAL.—In _Hyocrinus_ there is no dilatation which could be called a stomach; in _Actinometra_ the mouth is excentric, and the anal papilla occupies the centre of the tegmen. The intestine is elongated, and describes several turns round the papilla before ending in the anus.

The CLASSIFICATION of Crinoidea cannot properly be considered without taking account of fossil forms, but to do so at all adequately is impossible on account of limitations of space. Less regret may be felt because the three specialists in this branch, viz. Bather in England, Springer in America, and Jaekel in Germany, come to fundamentally different conclusions on the subject. If we confine our attention to living forms we may, with P. H. Carpenter,[515] select the stem as the basis of classification. As the method of gaining food is the same in all cases, the Crinoidea have probably split on the method of attachment to the substratum. These families—it is impossible, in view of the greater range of variety in fossils, to dignify them with the name of orders—are as follow:—

{590}FAM. 1. HYOCRINIDAE.—Stem long and persistent; cirri absent; stem ossicles cylindrical—ligaments uniting them not specialised. Arms (five) short, but with extremely long pinnules. Patina composed of long exposed basals and a ring of five spade-shaped radials. Five large persistent orals. Interradials and covering plates present. One species (_Hyocrinus bethellianus_, Figs. 268, 269) dredged up in the Southern Pacific Ocean.

FAM. 2. RHIZOCRINIDAE.—Stem long and persistent; cirri confined to a few near the root or replaced by rooting branches of the stem. Stem-ossicles thin and pentagonal at the summit, but lower down compressed, elliptical in section, and united by two ligaments separated by a transverse ridge for articulation. Patina composed of exposed basals and a ring of five short radials. Orals large {591}or vestigial. Covering plates and interradials present. Two genera, both from great depths in the Atlantic—_Rhizocrinus_ (Fig. 270), with five arms and well developed orals (attachment by branching root-cirri); and _Bathycrinus_, with ten arms and vestigial orals; attachment by root-like branches of stem (this is essentially the same as root-cirri).

FAM. 3. PENTACRINIDAE.—Stem consisting of ossicles which are pentagonal in section, united in pairs by syzygy, the upper one of each pair bearing a whorl of cirri and united by five bundles of fibres of petal-like section with the lower one of the pair above it. No rooting processes. Patina consists almost entirely of columns of radials, the basals being almost or completely hidden. Orals absent, but side-plates in the ambulacral grooves. Two recent genera, _Pentacrinus_ (_Isocrinus_), with three radials in each column; _Metacrinus_, with five to eight radials in a column, but the third {592}radial bears a pinnule. _Pentacrinus_ is found in both the Caribbean Sea and the Pacific Ocean; _Metacrinus_ in the Pacific. It appears that the Pentacrinidae when young are attached by a foot-plate at the apex of the stem; but when adult, the stem is broken in two and the animals, like _Antedon_, swim by movements of the arms, dragging a large part of the stem after them, by which they effect temporary attachment. As in other stalked forms, the cavities of the chambered organ are prolonged into canals which traverse the stalk; but in this family there is the peculiarity that a repetition of the chambered organ is found opposite every whorl of cirri.

FAM. 4. HOLOPODIDAE.—Stem represented by a leathery noncalcified outgrowth from the base of the calyx; one circle of radials indistinguishably fused with the basals and with each other to form the walls of the calyx. Large oral plates, ten short arms. One genus, _Holopus_, in shallow water in the Caribbean Sea.

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FAM. 5. COMATULIDAE.—Stem in the adult broken off, leaving only a stump, the centro-dorsal, covered with cirri. Six genera. _Antedon_ (= _Comatula_) has already been described; many tropical species have numerous arms and often side-plates and covering plates. _Actinometra_ is distinguished by its excentric mouth, and by the fact that the centro-dorsal is flat and has cirri only round its edges; _Atelecrinus_ has an acorn-shaped centro-dorsal, and the basals are externally visible; _Eudiocrinus_ differs from _Antedon_ only in having five arms; _Promachocrinus_ is a remarkable form, having ten radii (this is a unique feature in Crinoidea); finally, _Thaumatocrinus_ has basals externally visible, large persistent orals and interradial plates, and in addition a short free appendage of several plates on the anal interradius. _Antedon_ and _Actinometra_ are almost world-wide. Six species of the first have been recorded from British waters, of which the commonest is _Antedon rosacea_; four others are distinguished by having longer cirri, and do not seem to be well defined; but _A. eschrichtii_, a northern form, is larger, and is distinguished by having long proximal pinnules. The other genera are rare, and occur in deep water.

When we turn to survey FOSSIL CRINOIDEA, we are met with a bewildering variety of forms ranging from the Lower Cambrian to the present day. As already mentioned, there is no agreement amongst experts as to how they should be classified. Bather makes the fundamental cleavage depend on the possession of two whorls of plates in the base (Fig. 274), or of only one whorl. These two divisions he calls DICYCLICA and MONOCYCLICA respectively. He admits that in many forms allied to Dicyclica the infra-basals have disappeared; these he terms "pseudomonocyclic" forms, and believes that he is able to discriminate them from true Monocyclica.

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The present author is utterly unable to believe that the Crinoidea diverged into two groups on what is a trifling point of meristic variation comparable to the varying number of rows of plates in the interradial areas of the older Echinoidea; and he is equally sceptical as to the validity of Jaekel's division of the group into CLADOCRINOIDEA and PENTACRINOIDEA, leading to the view that organs like pinnules represent totally different structures in different groups. Wachsmuth and Springer adopt as bases of classification the extent to which the arms and their branches are incorporated in the disc, and they recognise three main divisions: INADUNATA, in which the arms are completely free from the calyx; ARTICULATA, in which the arms are partly incorporated but the tegmen remains flexible; and finally CAMERATA, in which the arms and their first branches are largely incorporated in the cup; the tegmen is converted into a rigid dome and the ambulacral grooves on it become closed, as does the mouth, by the meeting of overarching folds; the grooves remaining, of course, open in the distal portions of the arms (Fig. 273). This classification, founded as it is on physiological factors, seems to the present author more satisfactory. Speaking generally, the points in which fossil Crinoids may differ from living genera are: (1) the total absence or irregular nature of the branching in the arms, so that pinnules may be said to be absent; (2) the closure of the ambulacral grooves and mouth already alluded to, and (3) the adhesion of the arms in the same ray to produce net-like structures (_Crotalocrinus_, Fig. 274), or a fan-shaped structure (_Petalocrinus_); (4) the frequent presence of two rows of brachials in one arm (biserial structure); (5) the {596}development of an enormous anal tube, so large that in extreme cases (_Eucalyptocrinus_) the arms may be lodged in grooves of it.

CLASS II. THECOIDEA (EDRIOASTEROIDEA, BATHER)

These remarkable Pelmatozoa are the most primitive known. They have sac-like or sometimes cushion-shaped or even disc-shaped bodies, covered with numerous irregular plates without any symmetry in their arrangement. There is no stem, but when they are fixed this is effected by an adhesion of the aboral pole. There are no arms, but on the upper surface is to be seen the impression of five ambulacral grooves radiating from a central mouth. These grooves are bordered by covering plates, which in the earliest form (_Stromacystis_) are seen to be slight modifications of the plates covering the upper surface of the body, but in the later genera (Fig. 275, _Thecocystis_) become specialised. The anus is situated on the side, as is also the madreporite. It has been suggested that Eleutherozoa were derived from this group; that individuals were occasionally overturned by the waves or currents, and in this way compelled to use their podia for locomotion. When Eleutherozoa, however, have a fixed stage in their development, they are fixed by the oral, not the aboral, surface, and hence can have no close affinity to Thecoidea. Thecoidea begin in the Middle Cambrian, but according to Jaekel impressions in the Lower Cambrian, referred to Medusae, may be casts of this group.

CLASS III. CARPOIDEA

Pelmatozoa with a well-developed stem; body bilaterally compressed; only two rays apparently developed. These are indicated only by grooves radiating from the mouth; but in {597}some cases slight horn-like outgrowths of some of the plates of the calyx may support prolongations of the grooves.

This group, which, like the foregoing, commences in the Cambrian, is perhaps more primitive than the Thecoidea in showing less influence of the water-vascular system on the skeleton; but in the presence of a differentiated stem and the development of only two rays, it is more differentiated. The anus is on one of the flat sides, covered with a flat plate acting as a valve. The members of this group were formerly confounded with Cystoidea, from which they differ in the absence of the characteristic pores. _Trochocystis_, the genus figured, is devoid of any horn-like outgrowths of the calyx.

CLASS IV. CYSTOIDEA

Pelmatozoa with respiratory organs in the form of "diplopores" or "pore-rhombs." In a great many cases there is a stalk, but in other cases this is atrophied, and the animal is attached by the base of the calyx. The radial canals run for a shorter or longer distance over the calyx, but the plates of the calyx themselves are not modified for them. Either they run in simple grooves, or they are protected by a special series of plates lying above the plates of the calyx. The terminal portions of the radial canals are in all cases free, supported by unbranched arms consisting usually of a double row of ossicles. These arms are termed "fingers."

It will be gathered from the description just given that the fingers and the respiratory organs distinguish Cystoidea from {598}the two foregoing classes. Formerly this class was a lumber-room in which were placed all primitive irregular Pelmatozoa. The labours of Jaekel[516] have, however, dispelled the mist which enveloped this group, and in his monograph all that can be extracted both from superficial examination and dissection of these fossils is contained. It seems possible to the present author that the class may eventually require to be divided into two, corresponding to the two main divisions which Jaekel recognises, viz. DICHOPORITA, with pectinated rhombs, and DIPLOPORITA, with diplopores.

The pore-rhombs of the Dichoporita (indicated in Fig. 277 by the small parallel lines crossing the boundaries of the plates) were, according to Jaekel, nothing but a series of folds of thin integument projecting into the interior, the outer opening of which in most cases adhered in the middle, leaving two pores connected by a groove. The inner boundaries of the folds are sometimes preserved, but in many cases they were entirely devoid of calcification, and so were lost. The radial vessels either branched a great deal, giving rise to a multitude of fingers, or, as in _Echinosphaerites_ (Fig. 277), there were a few long fingers supporting a reduced number of radial canals. In some cases the calyx can be analysed into a regular series of cycles of plates, consisting of basals, orals, and three intervening whorls, thus including one more ring than the calyx of Crinoidea. Jaekel regards this as a primitive arrangement, believing that the irregularity seen in _Echinosphaerites_ secondary. This is a doubtful hypothesis.

{599}The diplopores of the Diploporita appear to consist of two canals traversing the body-wall, opening close together into a common pit externally, but diverging internally. Since in some cases, as in _Aristocystis_ (Fig. 278), this common pit is proved to have been closed externally by a very delicate layer of calcification, it is probable that the pores represent in other cases the points of origin of finger-like gills similar to those of Asteroidea. Where they were closed by calcification this was so thin and porous that the diffusion through it sufficed for respiration. Jaekel regards the Diploporita as a group derived from Dichoporita, but this seems to be extremely doubtful.

CLASS V. BLASTOIDEA

Pelmatozoa with respiratory organs in the form of longitudinal calcified folds, termed "hydrospires," radiating from the mouth. Stem well developed; calyx regular, consisting of a whorl of basals surmounted by a whorl of forked radiais, in the clefts of which lay the recumbent radial water-vascular vessels, supported each on a special plate ("lancet plate"), and giving off two rows of branches supported by short fingers (Fig. 279). Side-plates and covering plates were also developed; five orals ("deltoids") completed the calyx. The anus was at the side, just beneath one of the orals.

The hydrospires, which are the great characteristic of the class, are seen in section in Fig. 279, B (_hyd_). They consist of a varying number of parallel folds on each side of each "pseudambulacrum," as the lancet plate with its adhering side-plates and covering plates has been termed. In the most primitive genus, _Codaster_, they appear to have opened directly to the exterior, and to have been placed at right angles to the lines of union of the radial and oral plates, just like the grooves of a pectinated rhomb. In more modified forms, such as _Pentremites_ and _Granatocrinus_ (Fig. 279), the outer openings were overarched {600}by the extension of the side-plates of the radial vessel, and the whole group of folds has a common opening near the mouth; indeed, in the highest form there is one common "spiracle" for the two groups of folds in an interradius, which in one interradius is confluent with the anus. The hydrospires, when they reach this form, irresistibly recall the genital bursae of Ophiuroidea (Fig. 214, p. 490), and very possibly served the same purpose.

Reviewing the whole group of the Pelmatozoa, we see that in the Cambrian they begin with the extremely primitive Thecoidea and Carpoidea, together with some obscure forms which, combining a stem with pentamerous symmetry in the calyx, are supposed to be the forerunners of the Crinoidea. In the Lower Silurian or Ordovician the two groups of the Cystoidea make their appearance, possibly independently developed from either Carpoidea or primitive Crinoidea, which in this period are present in unmistakable form. In the Upper Silurian the Blastoidea appear, distinguishable from the most regular Cystoidea only by their hydrospires. It seems practically certain that they were developed from Cystoidea, and we follow Jaekel in believing that they arose from Dichoporita. The Carpoidea do not extend beyond the Ordovician, and by the end of the Carboniferous period Cystoidea and Blastoidea die out, leaving only the Crinoidea, which at that period were at their maximum development. From the Carboniferous to the present day the Crinoidea have continually decreased, leaving in recent seas, as sole representatives of the Pelmatozoa, only the few forms described at the beginning of this chapter.

{601}CHAPTER XXI

ECHINODERMATA (_CONTINUED_): DEVELOPMENT AND PHYLOGENY

In Chapter XVI. it was stated that whilst a more or less perfectly developed radial symmetry was one of the characteristic features of the phylum Echinodermata when in the adult condition, yet in the immature or larval condition the members of the group have a strongly marked bilateral symmetry. In this feature larval Echinodermata resemble the other Phyla of the animal kingdom which have a well-developed coelom, such as Annelida, Mollusca, Vertebrata, etc. Since, then, the peculiar radial symmetry is gradually acquired during the growth of the Echinoderm, we may possibly discover by a close scrutiny of the life-history what is the nature and meaning of this departure from the ordinary type of structure among coelomate animals.

There are two kinds of development met with amongst Echinodermata, which may be roughly characterised as the "embryonic" and the "larval" type respectively, although neither description is exact. In developmental histories of the first type so much reserve material is laid up in the egg in the form of food-yolk that the young animal whilst in the bilateral stage requires little or no food. In some cases, however, as in _Amphiura squamata_, the mother pours out a nourishing exudation; but whether this is so or not, the parent in nearly every case carries the young about with her until they have reached the adult condition. In some Asteroidea, as for instance in the Antarctic species _Asterias spirabilis_ (Fig. 280), the young become fixed to the everted lips of the mother; in _Amphiura squamata_, and some other Ophiuroidea the eggs remain in the genital bursae, which serve as nurseries; in some Spatangoidea, {602}as for instance in _Hemiaster philippi_ (Figs. 250, 281), the eggs are carried in some of the deeply grooved petaloid ambulacra; whilst in Holothuroidea they may develop in the body-cavity (_Phyllophorus urna_), or they may adhere to the back of the mother (_Cucumaria crocea_, Fig. 259, p. 573), or they may be protected in special brood-pouches either on the ventral side of the parent (_Cucumaria laevigata_) or on the dorsal surface (_Psolus ephippifer_, Fig. 261).

The majority of these cases of embryonic development have been recorded from Arctic or Antarctic waters; it appears as if conditions there were not favourable to the larval type of development. In Pelmatozoa the development of _Antedon rosacea_ alone is known, and that is of the embryonic type.

So far, however, as their mode of propagation is known, it may confidently be affirmed that the development of the majority of the species of Eleutherozoa is of the second or larval type. In this type there is little food-yolk in the egg, and the young animal or larva is forced from a very early period of development to seek its own living, and hence it is usually a considerable time (from a fortnight to two months) before the adult form is attained. When the _embryos_ of different groups of Eleutherozoa are compared, there is no obvious agreement in structure between them; but the _larvae_ of the four classes of Eleutherozoa exhibit with differences in detail a most remarkable fundamental similarity in type, and we are accordingly justified in regarding the larval development as primitive, and the embryonic type as derived from it and differently modified in each case.

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In the typical larval development the eggs are fertilised after being laid, and they then undergo segmentation into a number of equal, or nearly equal, segments or "blastomeres." These arrange themselves in the form of a hollow sphere or "blastula," the cavity of which is called the "blastocoel" and afterwards becomes the primary body-cavity of the larva. This cavity contains an albuminous fluid, at the expense of which development appears to be carried on (Fig. 282, B). The cells forming the blastula acquire cilia, and the embryo begins to rotate within the egg-membrane, which it soon bursts, and, rising to the surface of the sea, begins its larval life. The blastula is therefore the first well-marked larval stage, and it is found in a more or less recognisable form in life-histories of members of every large group in the animal kingdom. Only in the case of Echinodermata and of forms still lower in the scale, however, does it appear as a larval stage. The free-swimming blastula stage is reached in from twelve to twenty-four hours. Soon the spherical form of the blastula is lost; one side becomes flattened and thickened, owing to a multiplication of cells, so that they become taller and narrower in shape. Shortly afterwards this thickened plate becomes buckled inwards, encroaching on the cavity of the blastocoel. The larva has now reached the second stage of its development; it has become a "gastrula" (Fig. 282, C). The plate of thickened cells has become {604}converted into a tube called the "archenteron" (Fig. 282, C, _arch_), which is the rudiment of both the alimentary canal and the coelom of the adult. This tube communicates with the exterior, in virtue of its mode of formation, by a single opening which is called the "blastopore," which becomes the anus of the later larva and adult. Whilst the gastrula stage is being acquired, the blastocoel or primary body-cavity is invaded by wandering cells budded from the wall of the archenteron (Fig. 282, A, B, C, _mes_). These cells, which are called "mesenchyme," are the formative cells of the skeleton, connective tissue, and wandering cells of the adult. When the larva has a skeleton they are formed very early, arising in the young blastula stage (Ophiuroidea) or in the stage immediately before the formation of the archenteron (Echinoidea, Fig. 282, A, B) and secreting the skeleton. When the larva is devoid of a skeleton (Asteroidea and Holothuroidea), the mesenchyme usually does not appear till the gastrula is fully formed.

The gastrula stage is reached in twenty to thirty-six hours. Then one side of the larva becomes concave, and the cilia become restricted to a thick band surrounding this area. In this way is formed the rudiment of the _longitudinal band of cilia_, which is the organ of locomotion throughout the larval life. At the apex of the archenteron a thin-walled vesicle is formed, which soon becomes divided off from the rest. This vesicle, which almost immediately divides into two sacs, right and left, is the {605}rudiment of the "coelom" or secondary body-cavity of the larva; the remainder of the archenteron forms the definitive gut, and becomes divided by constriction into an oesophagus, a stomach, and an intestine, and at the same time bent into a shallower or deeper V-shape, the concavity of which is towards the concave side of the body. Within this area of the surface a new funnel-shaped depression makes its appearance. This is the "stomodaeum," the rudiment of the mouth of the larva, and it soon joins the apex of the larval oesophagus; the conjoined tubes henceforth bearing the name oesophagus since the ectodermal and endodermal parts become indistinguishably fused. Along the sides and floor of the oesophagus is formed a V-shaped ridge bearing strong cilia; this is the "adoral band of cilia" which sweeps the food (consisting of Diatoms, Infusoria, etc.) into the mouth. The larva is now known as a Dipleurula and appears in four modifications, each characteristic of a Class of Eleutherozoa. These differ from one another principally in the following points:—(_a_) The folding of the ciliated band; (_b_) the divisions of the coelomic sacs; (_c_) the development and fate of the praeoral lobe (_i.e._ the part of the body in front of the mouth); (_d_) the fate of the larval mouth. The types of Dipleurula are as follows:—

(1) The _Bipinnaria_, the larva of Asteroidea. In this type there is a very long prae-oral lobe. The ciliated band runs along its edges, and is produced into a backwardly directed loop on its under surface. This loop soon becomes separated from the rest of the band as a distinct prae-oral loop, the rest forming a post-oral loop. Both loops are drawn {606}out into short tag-like processes, in which we may distinguish (following Mortensen's[517] notation) in the prae-oral loop an anterior median process (Fig. 283, _a.c.o.b_), and a pair of prae-oral processes (_a.v.a_). In the post-oral loop there is a median dorsal process (_p.c.o.b_) and paired anterior dorsal (_a.d.a_), posterior dorsal (_p.d.a_), posterior lateral (_p.l.a_), and post-oral (_p.v.a_) processes. At the apex of the prae-oral lobe between prae-oral and post-oral ciliated rings there is an ectodermic thickening, recalling the so-called apical plate of Annelid larvae.

(2) The _Ophiopluteus_, the larva of the Ophiuroidea. In this type the prae-oral lobe remains small, and the primitive ciliated band is undivided. The processes into which it is drawn out are very long, and are supported by calcareous rods. Of these processes we may distinguish prae-oral, postero-dorsal, postero-lateral, and post-oral. The postero-lateral are always much longer than the rest, so that the larva when swimming appears to the naked eye as a tiny V. In the case of _Ophiothrix fragilis_ (Fig. 284, A) the postero-lateral processes are many times longer than the rest of the body. The Ophiopluteus was the first Echinoderm larva to be recognised. It was discovered by Johannes Müller,[518] who also discovered the other three types of {607}Dipleurula. He named this one _Pluteus_ (easel), from a fancied resemblance, when turned upside down, to a painter's easel. The same name was bestowed on the next type, to which it presents a superficial resemblance, and hence the distinguishing prefix "Ophio-" was added to the original name by Mortensen.

(3) The _Echinopluteus_, the larva of the Echinoidea. This type is strikingly like the preceding one in possessing a very small prae-oral lobe and in having the processes of the ciliated ring supported by calcareous rods, but a close inspection of these shows that they do not exactly correspond to those of the Ophiopluteus. Thus we have prae-oral, postero-dorsal, and post-oral processes (Fig. 285), but usually no postero-lateral process, and when it does occur it remains short. On the other hand, an antero-lateral process unrepresented in the Ophiopluteus is constantly present, and in its later stage the Echinopluteus develops, out of parts of the ciliated ring, horizontally-placed crescentic ridges of cilia, which are termed ciliated epaulettes (Fig. 285, _a.cil.ep_). There may even be, as in the larva of _Echinus esculentus_, a second posterior set of these (Fig. 285, _p.cil.ep_). In the older larva at the apex of the prae-oral lobe there is an ectodermic thickening, at the base of which are developed nerve-cells and nerve fibres constituting a larval brain (Fig. 285, _ap_).

(4) The _Auricularia_, the larva of the Holothuroidea. This type strikingly resembles the Bipinnaria in its external features. The prae-oral lobe is well developed, and has on its under surface a backwardly projecting loop of the ciliated band, which is not, {608}however, as in the Bipinnaria, separated from the rest of the band. The processes of the band are much more faintly marked than in the Bipinnaria, the anterior median, prae-oral, and median dorsal processes being absent; but a pair of intermediate dorsal processes are developed in the interspace between anterior and posterior dorsal.

In the Bipinnaria, Ophiopluteus, and Echinopluteus the coelomic vesicle, after separation from the archenteron, divides into right and left halves. The left then sends out a short dorsal process, which, fusing with the ectoderm, acquires an opening to the exterior. This opening is the primary madreporic pore, and the process of the left coelomic sac, which is ciliated, is the pore-canal. In the Auricularia the pore and pore-canal are formed _before_ the division of the coelom. In the Bipinnaria the right and left sacs subsequently fuse in the front part of the prae-oral lobe. In the first three types of larva the coelomic sac on each side then undergoes a segmentation into anterior and posterior portions. At the hinder end of the anterior sac on each side a swelling occurs. That on the left side is the "hydrocoel," or rudiment of the water-vascular system (Fig. 287, A^3, _l.hy_); it {609}quickly assumes a crescentic form, and gives off five blunt outgrowths, which are the rudiments of the radial canals, and the terminal tentacles. It remains in connexion with the anterior coelom by a narrow neck, which later becomes the stone-canal. That on the right side separates completely from the right anterior coelom; it remains small, and forms the madreporic vesicle (Fig. 287, A^3, _r.hy_) of the adult. In the Ophiopluteus and in the larva of _Asterina gibbosa_ (_v. infra_) it occasionally takes on a form similar to that of the hydrocoel; from which circumstance, as well as from the similarity in its mode of origin, it is here regarded as a right hydrocoel, _i.e._ a rudimentary fellow of the organ which develops into the water-vascular system.

In Auricularia (Fig. 287, D) the coelomic vesicle, after the pore-canal is formed, divides into an anterior and a posterior half. The posterior part then divides into right and left halves, {610}whilst the anterior sac divides into dorsal and ventral halves, connected by a narrow neck. The ventral half soon assumes the familiar feature of the hydrocoel (Fig. 287, D^3, _l.hy_), whilst the dorsal half forms an insignificant swelling on the course of the conjoined stone- and pore-canals, which represents the left anterior coelom of the other types; neither right anterior coelom nor right hydrocoel being developed. The neck of communication between dorsal and ventral halves is, of course, the stone-canal.

The Dipleurula larva leads a free-swimming life for a period varying from two weeks to two months, and then undergoes METAMORPHOSIS into the adult form. The details of this process have been worked out in comparatively few cases; and the species in which they are most thoroughly known is the Asteroid _Asterina gibbosa_. The development of this species is intermediate in character between the embryonic and larval types. The eggs are larger than is usual among Asteroidea, and are filled with a bright orange yolk. The larva differs from the Bipinnaria in the absence of the characteristic ciliated bands and in the very early occlusion of the anus. There is, however, a band of cilia round the edge of the prae-oral lobe, which corresponds to portions of the prae-oral and post-oral bands combined of the Bipinnaria.[519]

The larva has a form which may be described as boot-shaped (Figs. 288, 289). The sole of the boot is the great prae-oral lobe, behind which is the mouth. The larva takes little or no food, and completes its metamorphosis in ten to twelve days. It does not swim at the surface, but creeps slowly over the bottom by the aid of the ciliated band mentioned above, while it can also attach itself, using the edges of the prae-oral lobe as a sucker.

After leading an existence of this kind for seven or eight days it fixes itself permanently by a disc-like prominence, which appears on the anterior surface of the prae-oral lobe within the area surrounded by the thickened rim which, as explained above, forms a margin to the prae-oral lobe. The larva then becomes divided by a constriction into a disc and a stalk, and the former is gradually converted into the body of the young Starfish, whilst {611}the latter continually diminishes in size, and eventually entirely disappears, when the young Starfish commences to walk about on its podia. The disc becomes bent downwards and to the left, so as to make nearly a right angle with the stalk, and the last vestige of the latter springs from the peristome of the Starfish inside the water-vascular ring (Figs. 289, B, C).

The form of the Starfish is attained principally by the preponderant growth of the left hydrocoel and of the left posterior coelom. Both these sacs take on the form of hoops, which, by the meeting of their ends, are converted into rings. The hydrocoel has already grown out into five lobes, which are the rudiments of the radial water-vascular canals, and the tips of which become the terminal sensory tentacles (Figs. 288, 289, 1-5); but now the left posterior coelom grows out into five lobes also, forming a parallel but outer ring. These lobes (Figs. 288, 289, I.-V.) are the rudiments of the arms, which are at first quite independent of those of the radial canals, but gradually, when the larva has attained the age of nine days (Fig. 289, B, C), they become applied to the outgrowths of the hydrocoel. These by this time have developed each two pairs of branches, the rudiments of the first two pairs of tube-feet in each radius. The larval mouth vanishes, and a new mouth is formed on the left side in the centre of the hydrocoel ring, when the metamorphosis is complete. The adult anus is {612}formed about the same time. The primary pore-canal in _Asterina_ as in all Dipleurulae opens into the anterior coelom; the stone-canal is formed from a ciliated groove running along the neck of communication between this and the hydrocoel. The constriction dividing the body into disc and stalk divides the anterior coelom (single in _Asterina_ as in the older Bipinnaria) into two parts; the portion included in the disc forms the axial sinus of the adult. The lower end of the axial sinus expands and surrounds the adult mouth, forming the inner perihaemal ring; the outer perihaemal ring is formed by the juxtaposition of four wedge-shaped outgrowths from the left posterior coelom and one from the anterior coelom. From these the radial perihaemal canals subsequently grow out into the arms. The metamorphosis of Bipinnaria has been well worked out by Goto,[520] and it agrees in essential features with that of _Asterina gibbosa_; in fact, the differences which Goto maintains between the two types may be reasonably explained on the supposition of some stages having escaped the notice of this observer. The larva develops on the apex of the prae-oral lobe three papillae for occasional attachment,[521] and in the centre of these a cup-shaped disc for permanent fixation when the prae-oral lobe is converted into a stalk. When these papillae (Fig. 290, _fix_) have been developed the larva is known as a Brachiolaria.

{613}

The metamorphoses of the other types of Dipleurula contain no fixed stage. They are what might be called "cataclysmal metamorphoses." That is to say, the outer form and habits of the larva are preserved till the last moment, whilst the organs of the adult are being gradually perfected; then in an hour or two all trace of larval structures disappears. The Ophiopluteus preserves the larval mouth, round which the hydrocoel grows; the long lateral ciliated processes are preserved till the animal has attained all the adult characters. Before this, however, it passes through what may be called an "Asteroid" stage in development, in which the ambulacral grooves are open. The Echinopluteus loses both larval mouth and anus. It develops the adult organs on the floor of a sac-like invagination of the ectoderm, situated on the left side within a loop of the ciliated band (Fig. 291, B, C). This invagination becomes completely closed. It is termed the "amniotic cavity," and its roof is termed the "amnion." On its floor are developed the primary tentacles, terminating the radial canals, as well as a number of spines. After taking on a creeping life and losing its larval appendages, the young Sea-urchin passes through an "Asteroid" condition, in which the arched dorsal surface, the future periproct, is greater in extent than the ventral, and the radial canals run horizontally out from the water-vascular ring and terminate in free movable podia (Fig. 291, C and D, _pod_), ending in suckers, in the centre of which are pointed sense-organs. These {614}podia become later enclosed in grooves in the corona, and are reduced to vestiges in the adult.

The Auricularia is the only type of Dipleurula in which larval mouth and anus are retained. For this reason it has been supposed that its median plane of symmetry remains the median plane of the adult. The researches of Bury[522] have shown that this is not so. As in other types of Dipleurula (with the possible exception of the Ophiopluteus) the adult position of the {615}mouth is on the left side of the larva, and in the commencement of the metamorphosis the mouth migrates into this position (Fig. 286, C). Then the rudimentary prae-oral lobe is rapidly absorbed, so that the mouth again acquires a terminal position. The hydrocoel (Fig. 286, A, _hy_) has by this time completely encircled the oesophagus, and from it grow out the radial canals which bud off the feelers[523] (buccal tentacles) into the larval stomodaeum. This, although it later flattens out to form the adult peristome, forms in these stages an almost closed sac, reminding us of the amniotic cavity in the Echinopluteus. The ciliated band breaks up into a number of pieces, which rearrange themselves so as to form a series of transverse rings of cilia; so that the free-swimming life can be carried on somewhat longer. The animal in this stage is called a "pupa" (Fig. 292); it eventually loses the rings, drops to the bottom, and develops tube-feet. From specimens which the author has seen, he has little doubt that in some cases the young animal passes through an "Echinoid" stage, for it possesses, besides the feelers, only median tube-feet, terminating the radial canals, and it is covered by a cuirass of plates, which recalls the Echinoid corona.[524]

Reviewing the development of the Eleutherozoa in the light of the facts so far presented, and using the same method of reasoning which is employed in the case of other groups of animals, we seem to be justified in concluding that the Echinodermata are descended from a simple free-swimming ancestor possessing the fundamental characters of the Dipleurula. These would include a longitudinal folded band of cilia as the principal organ of locomotion; a thickened plate of nervous epithelium at {616}the anterior end serving as combined sense-organ and brain; a V-shaped band of cilia projecting into the oesophagus as the organ of nutrition; a wide, shallow stomodaeum and an alimentary canal consisting of three well-marked divisions, viz. oesophagus, stomach, and intestine; and finally a secondary body-cavity or coelom, consisting of three divisions on each side, though possibly the most anterior pair were confluent in the prae-oral lobe. On the left side the anterior coelom opened to the exterior by a short ciliated canal. To the hypothetical group so defined which were certainly not Echinodermata the name PROTOCOELOMATA may be given.

Now amongst the lowest types of animal in which traces of Vertebrate structure can be detected, there is one group, the HEMICHORDATA (Vol. VII. p. 3), in which there is a larva which strikingly recalls the Dipleurula. This larval form belongs to Balanoglossus and is called the Tornaria. It possesses a well-marked prae-oral lobe and a folded longitudinal ciliated band, which resembles that of Auricularia. Its peculiarity is that in addition there is a posterior ring of cilia (Fig. 293, _p_). The coelom is in five divisions:—a median anterior sac (_a.c_) opening to the exterior by a short ciliated canal on the left side; and paired middle divisions (_m.c_) and posterior divisions (_p.c_). At the apex of the prae-oral lobe there is a plate consisting of sensory epithelium, with nerve-fibres at its base, which acts as a brain. Tornaria undergoes metamorphosis, assumes a worm-like form, and takes on a burrowing life. The five divisions of {617}the coelom are retained, and it can be proved that the pore-canal, like the madreporite of Echinodermata, is used for taking in water. Further, there are two aberrant sessile members of the group (_Cephalodiscus_ and _Rhabdopleura_), in which the middle divisions of the coelom which would correspond to the hydrocoels are produced into long arms, each with a double row of ciliated tentacles, which strikingly recall the radial canals and podia of the Pelmatozoa. Taking all these facts into consideration, it seems probable that Vertebrata and Echinodermata both arose from PROTOCOELOMATA.

When we turn to the developmental history of Echinodermata for light on the question as to how the bilaterally symmetrical ancestor became converted into the radially symmetrical Echinoderm, it seems probable that only in the development of the Asteroidea can we hope to find the solution of the problem. The abrupt changes of habits shown in the metamorphoses of the other types are clearly secondary phenomena. No species of animal could suddenly change its habits from swimming by means of cilia to walking with tube-feet. In the development, however, of _Asterina gibbosa_ we get a hint of the way in which a free-swimming life could alternate with periods of temporary fixation, gradually passing into a condition in which the fixation was permanent. This period in the history of the race when ancestral Echinodermata were sessile would mark the point at which Eleutherozoa diverged from Pelmatozoa, and the former existence of a fixed ancestor explains the tendency first to asymmetry and later to radial symmetry. Bilateral symmetry is characteristic of most free-swimming animals which have to pursue a straight course through the water, but in fixed forms no disadvantage arises from want of symmetry. A radial disposition of organs is, however, valuable to them, since food must be sought and danger avoided from all points of the compass; and hence we can understand, when fixation became permanent, how one hydrocoel could grow larger than the other, and finally assume the form of a ring.

The last question which arises is the vexed one of the mutual relationships of the various Classes constituting the Phylum. Before attempting to seek for light on this problem from development, it will be necessary to sketch the life-history of _Antedon rosacea_, the only Pelmatozoon whose development is known.

{618}

The eggs are comparatively large and full of food-yolk, and they adhere for a considerable period to the pinnules. They pass through a large portion of the development within the egg-membrane. The blastula and gastrula are formed in the usual way, but the formation of the coelom is most remarkable (Fig. 287, E^1, E^2). The archenteron divides into anterior and posterior divisions. The posterior divides into right and left, posterior coelomic sacs, but before the division is complete a dorsal and a ventral tongue grow out from the anterior division and unite posteriorly, encircling the band of connexion between right and left posterior coelomic sacs like a ring. This band of connexion becomes solid and is absorbed, and _pari passu_ the ring becomes converted, by the disappearance of its central opening, into a sac, which is the definitive gut (Fig. 287, E). The rest of the {619}anterior division divides into a thick-walled sac, the hydrocoel, on the left, and a median thin-walled anterior coelom, which sends a long extension into the anterior portion of the larva, which we may compare to the prae-oral lobe of the Bipinnaria. The anterior coelom communicates with the exterior by a short pore-canal, and later forms a connexion, the stone-canal, with the hydrocoel. At the apex of the prae-oral lobe there is formed a thickened patch of ectoderm, bearing stiff sensory hairs, and having at their bases nerve-fibres and ganglion cells. This larval brain corresponds to that of the Tornaria and Echinopluteus. Behind the brain there is a glandular pit, which is used for fixation, and recalls the similar organ in the Bipinnaria. A series of ciliated rings is then formed, and between the second and third of them an oval depression appears. This is the stomodaeum; but as the larva takes no food it does not communicate with the gut (Fig. 295, A, _stom_).

The larva next escapes from the egg-membrane and swims freely for a day or two, and then, like the Bipinnaria, fixes itself by the apex of the prae-oral lobe, which is converted into a stalk. The larval stomodaeum closes, and the oesophagus of the adult appears as a solid peg of cells abutting against it; round this peg the hydrocoel grows like a ring.

The closed stomodaeum and the underlying hydrocoel are now rotated backwards until they come to be at the end of the animal opposite the stalk (Fig. 295, C). The left posterior coelom, which has also, as in the Asteroid larva, assumed a hoop-like form, is carried along with them; but the right posterior coelom becomes shifted forwards and sends out five outgrowths into the stalk, which form the rudiments of the chambered organ, and a central one as a continuation of the genital stolon (Fig. 295, D, _gen.st_), the extension of the anterior coelom (Fig. 294, B) having disappeared.

Then the outer wall of the stomodaeum splits into five valves—the future oral valves. The radial canals appear as freely projecting tentacles, which issue in the intervals of these valves and soon acquire two pairs of lateral branches. The skeleton consists of five oral plates in the oral valves, of a ring of five basals, of three small under-basals, and of a series of "columnals," _i.e._ stem-ossicles, as rings embracing the stalk. The area of attachment is supported by a "foot-plate." The radial {620}plates next appear as a ring of small ossicles between the orals and basals, and simultaneously the arms make their appearance as five outgrowths supported by the first radials, and by the other radials when these appear. The free radial canals now become adherent to the arms, but these canals soon give off paired branches of unlimited growth, which are supported by bifurcations of the primitive arms, and in this way the ten arms of the adult are established. So far, then, as the water-vascular system is concerned, the apparent forking is not a true dichotomy, but results from the production of two opposite branches, whilst the main axis ceases to grow. The appearance of cirri marks the fusion of the uppermost stem-ossicles to form a centro-dorsal, and shortly afterwards the young _Antedon_ snaps off its stem and swims away.