Chapter XXII: Appendix: To Ctenophora (5)

PERISTOME.—In many genera this is pentagonal and central, and in these cases the interradii commence at the peristome with a single plate, which is often covered with a thick crowd of small spines and is termed a "bourrelet." The oral ends of the radii also often consist of a crowded series of narrow plates, looking something like a "petal," and termed a "phyllode." The five bourrelets and five phyllodes constitute a flower-like figure termed a "floscelle."

AMBULACRA.—In _Echinoneus_ all five are alike and are provided with similar tube-feet, which are respiratory but possess suckers. The ambulacra are not grooved, and the petaloid arrangement of the pores is hardly marked; but in _Cassidulus_, _Pourtalesia_, and many other genera the five petals are well marked, though they are all similar to one another.

FASCIOLES.—These structures are often entirely absent; the sub-anal one alone is present in _Spatangus_. In _Eupatagus_ a peripetalous one is added. This surrounds all the "petals," and has obviously the function of sweeping fresh water over the respiratory tube-feet. In _Echinocardium_, as we have seen, there is an "internal fasciole" between the two anterior petals which has a similar function. In addition, this genus possesses an anal fasciole which surrounds the anus and sweeps away the faeces.

The CLASSIFICATION of the _Spatangoidea_ is based mainly on the degree of development of the petals, that is to say, on the extent to which the burrowing habit has been developed. But weight is also laid on the shape of the peristome, the pentagonal form being more primitive. Seven families are recognised, which are as follows:—

FAM. 1. ECHINONIDAE.—"Petals" hardly marked at all; peristome in the centre of the lower surface and pentagonal. Floscelle not developed.

One genus, _Echinoneus_ (Fig. 248).

{554}FAM. 2. NUCLEOLIDAE.—"Petals" distinct; peristome as in the foregoing family. No floscelle. _Nucleolites_, with the anus in a furrow. _Anochanus_, with a concave apical system serving as brood-pouch.

FAM. 3. CASSIDULIDAE.—"Petals" usually distinct; peristome eccentric, but provided with a well-marked floscelle.

_Echinolampas_, with the anus on the under surface.

_Neolampas_, with the anus on a projecting papilla. One specimen of this genus has been dredged in the British area.

The three foregoing families probably use their tube-feet to walk with, and bury themselves only to a slight extent. They are often united as a sub-order, the ASTERNATA, and distinguished from all the rest which possess an eccentric mouth and well-marked plastron. These families are then grouped together as STERNATA. They are as follows:—

FAM. 4. ANANCHYTIDAE.[503]—Spatangoidea with elongated apical system, ambulacra all similar and not grooved. Petals feebly marked. _Pourtalesia_, with bottle-shaped posterior prolongation of the test. _Platybrissus_, with flattened test.

FAM. 5. PALAEOSTOMATIDAE.—An aberrant family consisting of one genus, _Palaeostoma_. Petals grooved, with a peripetalous fasciole, but peristome central and pentagonal.

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FAM. 6. SPATANGIDAE.—Spatangoidea of more or less flattened shape, with well-marked petals and a sub-anal plastron as well as the ventral one. One fascicle at least, but a peripetalous one never present. The anterior ambulacrum grooved and different from the rest. This family is represented in British waters by two genera, _Spatangus_ and _Echinocardium_. The former possesses only a sub-anal fasciole, and has specially long curved spines on the ventral plastron. It is represented by two species, _S. purpureus_ and _S. raschi_, the latter being distinguished by a pointed lower lip. It is a deep-water species, found in 100 fathoms and over on the west coast. _S. purpureus_ is fairly common in rather shallow water. From observations made on specimens kept in confinement it appears to burrow only so far as to leave the petals uncovered; hence there is no need of a peripetalous fasciole. _Echinocardium_ is devoid of the thicker spines on the plastron, and has an internal fasciole and a perianal one as well as the sub-anal. As already mentioned, it is a deep burrower. It is represented by three species, _E. cordatum_, _E. pennatifidum_, and _E. flavescens_. The first, described as the type of the Spatangoidea, has a deeply grooved anterior ambulacrum. In the remaining two species this ambulacrum is not grooved. _E. flavescens_ has only six or seven pairs of pores in the posterior {556}petals, _E. pennatifidum_ twelve to fourteen. Both come from deeper water than _E. cordatum_.

FAM. 7. BRISSIDAE.—Allied to the Spatangidae, but distinguished by sunken petals and a peripetalous fasciole.

Two genera are recorded from the British area, _Schizaster_ and _Brissopsis_, but the first has only been found once in deep water; the second is common. _Schizaster_ has the front petals three times as long as the hind ones, and no sub-anal fasciole. _Brissopsis_ has the front and hind petals of about the same length, and a sub-anal fasciole. The only British species is called _B. lyrifera_, on account of the fiddle-shaped outline of the peripetalous fasciole.

_Hemiaster_ (Fig. 250) in general resembles _Schizaster_, but the petals are equal in length, and the two posterior serve as brood-pouches for the young. This genus is mainly Antarctic.

FOSSIL ECHINOIDEA.—Echinoidea are well represented in the geological record, and form a characteristic element in many fossil faunas. They appear in the Ordovician formation, but the first representatives of an existing family (Cidaridae) only appear in the Permian.

Space will only permit us to treat of the extinct members of the group very briefly. Leaving out of sight the representatives of families still living, the fossil Echinoidea may be divided into two great groups, viz.:—

(_a_) Palaeozoic forms, which in some points serve to connect the Endocyclica with the primitive Asteroidea.

(_b_) Mesozoic forms, which serve to connect the Clypeastroidea and Spatangoidea with the Endocyclica.

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The Palaeozoic forms are often called Palaeoechinoidea, and they are above all distinguished by the fact that the number of vertical bands of plates composing the corona is variable, in a word, that the corona has not yet acquired a fixed definite constitution. One genus (_Echinocystites_) has the anus outside the apical system. It has four rows of pore-plates in each radius, and numerous rows of plates each with a single spine in the interradii. Another (_Palaeodiscus_) has been shown by Sollas[504] to be in many respects the missing link between Asteroidea and Echinoidea. Inside the plates of the corona there is a series of ambulacral plates like those of Asteroidea. The tube-feet in the oral portion of the radii seem to have issued between the (outer) ambulacral plates. No anus has been detected. All the rest are Endocyclic. The oldest known form, _Bothriocidaris_ (Fig. 252, A), from the Ordovician, has only one row of interambulacral plates and two of ambulacral; no peristome is distinguishable from the corona. The ARCHAEOCIDARIDAE appear in the Devonian. They have narrow ambulacra of two rows of pore-plates as in the Cidaridae, but the interambulacra consist of many rows, the members of which overlap, and therefore were probably slightly movable, as in the Echinothuriidae; the primary tubercles are large, and there is only one on each plate. The MELONITIDAE (Fig. 252, B) appear in the Carboniferous. Each interambulacral plate, of which there may be five rows in each interradius, bears numerous small tubercles, and there may be four or more vertical rows of pore-plates, though in the genus figured, _Palaeoechinus_, there are only two. The TIARECHINIDAE are represented by one genus, _Tiarechinus_, with an enormous apical system, from the Triassic of the Tyrol. The interambulacra consist of one plate bordering the mouth, three, {558}side by side, forming the interradial area of the corona, and one large genital plate; the ambulacra, of two rows of pore-plates. This family consists of dwarfed forms which probably inhabited the land-locked seas and salt lagoons of the Triassic epoch.

When we recollect that some of the oldest Asteroidea known to us had very narrow arms and interradial areas edged by large square marginals, it does not require a very great effort to imagine how these marginals could be converted into the vertical rows of the interambulacra, and the pointed narrow arms becoming recurved, could have formed the ambulacra. The physiological advantage of this will be discussed in the chapter on development.

True Cidaridae occur in the Permian, and are abundant in all the younger formations. One Cretaceous genus, _Tetracidaris_, has four rows of interambulacral plates near the mouth, diminishing to two at the apex. This circumstance renders it probable that the Cidaridae are the direct descendants of the Archaeocidaridae. The Saleniidae, Echinothuriidae, and Diadematidae appear in the Jurassic, the Echinidae in the Cretaceous, and the Arbaciidae only in the Tertiary epoch.

Turning now to the Mesozoic forms with an excentric anus, there were a number of forms which have been grouped together as HOLECTYPOIDEA which had auricles and teeth and gills, although these were only feebly developed, and in which the pore-plates remained separate. The periproct was a comparatively large area, and in _Pygaster_, as in the surviving form _Pygastrides_, it was in contact with the apical system, although outside it. Many of the genera were of considerable height in proportion to their length. In _Conoclypeus_ and _Discoidea_ the jaws and auricles were very weak. The ECHINOCONIDAE have only vestigial auricles, and on this account are often definitely grouped with the Spatangoidea, but they are closely allied to the Holectypoidea. They are Cretaceous forms of high conical shape (_Galerites_). In _Hyboclypus_ (Fig. 253) all trace of the teeth has disappeared, {559}but the periproct is large and in contact with the apical system; these forms appeared in the Jurassic. The COLLYRITIDAE, also Jurassic, had a marginal anus. The apical system was so much elongated that two of the ocular plates are widely separated from the other three, two opposite interambulacra meeting between them. Unmistakable Spatangoidea (Spatangidae and Ananchytidae) appear in the Cretaceous, true Clypeastroidea (_Fibularites_) in the Cretaceous, the other families in the Tertiary.

Reviewing these facts, we see that from the Holectypoidea we can pass by insensible steps on the one hand into true Clypeastroidea, and on the other hand into true Spatangoidea. The Holectypoidea differed from Endocyclica only in the position of the anus, and the initial step in the backward shift of this organ is seen in _Pygaster_. One result follows from this conclusion, that the modification of the dorsal tube-feet into breathing organs, and the consequent appearance of petals which accompany the taking on of burrowing habits, were independently developed in the Clypeastroidea and Spatangoidea, since these features were absent in the more primitive members of both groups.

{560}CHAPTER XIX

ECHINODERMATA (_CONTINUED_): HOLOTHUROIDEA = SEA-CUCUMBERS

CLASS IV. HOLOTHUROIDEA

This class of the Eleutherozoa comprises those sausage-shaped, leathery Echinodermata familiarly known as Sea-cucumbers. They are named Holothuroidea from ὁλοθούριον, an animal described by Aristotle, and believed to belong to this class.

The Holothuroidea resemble Echinoidea in the fact that the radial canals of the water-vascular system run backwards and upwards from the ring-canal over the surface of the body, terminating in small papillae near the anus, which, as in the Echinoidea Endocyclica, is situated at the upper pole of the body. There are, of course, no arms; and a further resemblance to Echinoidea is shown by the fact that the ambulacral grooves are represented by closed epineural canals, and that the ectoderm consists of long, slender, flagellated cells interspersed with gland-cells, underneath which is a plexus consisting of nerve-fibres and small bi-polar ganglion cells. There are, however, no spines or pedicellariae; and Holothuroidea differ not only from Echinoidea, but from all other Echinodermata, in the vestigial character of their skeleton, which consists merely of isolated nodules of calcium carbonate embedded in the skin. The body-wall is provided with transverse muscles running across the interradii, and also with powerful longitudinal muscles, running along the radii, by means of which worm-like contractions are carried out. Similar muscles, though much less developed, occur in the Echinothuriidae, and must have been present in many extinct Echinoidea in which the plates of the corona overlapped; and hence it is exceedingly probable that from some of these {561}early forms, as, for instance, _Bothriocidaris_, Holothuroidea may have been evolved. The MUSCULAR BODY-WALL has indeed been as important a factor in the evolution and differentiation of the Holothuroidea as the MUSCULAR ARM in that of Ophiuroidea, or the MOVABLE SPINE in the case of Echinoidea.

There are about 520 species of living Holothuroidea, and of these about twenty-one have been recorded from British waters. One of the best-known of the British species is _Holothuria nigra_ (Fig. 254), commonly known as the "Cotton-spinner"; and this we shall take as a type for special description. The animal may attain a length of a foot when fully extended, and has a diameter of from 3 to 4 inches. It is of a very dark brown colour on one side, which in crawling it keeps uppermost, whilst on the lower side it is of a tawny yellow hue. Three of the radii (often termed the "trivium") are situated on the lower surface; two (termed the "bivium") on the upper surface. The podia are scattered fairly evenly over the whole surface without reference to the radii; below they are regular tube-feet provided with suckers, whilst on the upper surface they are pointed tentacles, employed only for sensory purposes.

If the animal be observed alive and in its natural surroundings, a ring of twenty large tentacles can be seen surrounding the mouth. These buccal tentacles are in every respect comparable with the buccal tube-feet of Ophiuroidea and Spatangoidea, and, like them, are employed in shovelling the muddy substratum on which the animal lies into the mouth.

Ludwig employs the term "feeler" for these buccal tentacles, {562}in order to distinguish them from the pointed podia scattered over the bivium. This procedure will be adopted here. In the Cotton-spinner the feelers, when extended, show a short smooth stem, from the apex of which springs a circle of short branches, which are in turn beset with a double row of branchlets, themselves branched. Such feelers are said to be shield-shaped.

A transverse section through the RADIUS of a Sea-cucumber is, in general, like one through the radius of a Sea-urchin; the points of difference to be noted are: (_a_) In the Sea-cucumber, beneath the ectoderm, is a thick dermis with small plates scattered in it, instead of the whole dermis being calcified, as is the case in the Sea-urchin; (_b_) the ampulla of each podium is connected with the peripheral portion by one canal, not two, as in the case of the Sea-urchin; (_c_) there is a development of coelomic nervous tissue from the outer side of the perihaemal canal; (_d_) internal to the radial water-vascular canal are to be seen cross-sections of two great bands of longitudinal muscles, by the contraction of which the body is shortened. Lengthening is brought about by the contraction of transverse muscles, which are found on the inner side of the body-wall in each interradius; the five sets taken together act like circular muscles, or a rubber band, on the incompressible fluid in the body-cavity.

When the Sea-cucumber is opened by a cut along the left dorsal interradius, the spacious coelom is laid open, and lying in it is seen the ALIMENTARY CANAL. This tube is bent on itself, so that it has a form like ~ (Fig. 255, B) running backwards to the posterior end of the body, then running forwards to near the anterior end, before it finally turns to run backwards to the anus. By taking cross-sections of the body at different levels, it can be shown that the alimentary canal makes a half-turn round the longitudinal axis (Fig. 255, A). It is suspended by bands of membrane, termed "mesenteries," to the body-wall, and of these there are three, the first of which (_i.e._ the one nearest the mouth) is attached to the mid-dorsal interradius (Fig. 255, A, M^1), the next to the left dorsal interradius (M^2), and the last to the right ventral interradius (M^3).

The alimentary tube shows four regions, which are distinguished as follows:—(1) A short oesophagus with strongly-marked longitudinal folds in its walls; this is separated by a constriction from (2) the stomach, a very short region, {563}characterised by its strong musculature. Next follows (3) the intestine, a thin-walled tube comprising the middle limb and most of the descending and ascending limbs. This finally passes into (4) the wide terminal "rectum," or "cloaca," which is connected to the body-wall by muscular bands which traverse the coelom (Fig. 256, 10).

The cells lining the oesophagus resemble ectodermal cells; those lining the stomach are nearly all gland-cells, and obviously secrete the digestive juice. The powerful muscles of this portion of the gut produce a strong peristalsis which thoroughly mixes the juice with the food, and in the thin-walled intestine absorption of the digested material takes place. The extreme thinness of the intestinal wall is common to many animals (e.g. _Sipunculus_, Vol. II. p. 412) which swallow mud and sand for the sake of the organic matter which they contain.

The rectum, or cloaca, is one of the most characteristic features in this and most other Sea-cucumbers. In addition to the passing of faeces, it is used to pump water in and out, and it thus serves as a breathing organ. This pumping is effected by alternate contractions of the radiating muscles attaching the cloaca to the body-wall, and of the circular muscles which immediately surround it. Two long branched tubes termed RESPIRATORY TREES (Fig. 256, 11) open into the cloaca, and into these the inspired water penetrates. The finer branches of these gills end in rounded thin-walled swellings termed "ampullae"; and when water is forced into these they become tense, and a considerable quantity diffuses through their walls, carrying {564}oxygen into the fluid which fills the coelom. If a Sea-cucumber be left in a limited quantity of water, it will sometimes direct the posterior end upwards until it reaches the surface of the liquid, and will pump air into the trees. Besides the trees, other much shorter tubes open into the cloaca, termed the CUVIERIAN ORGANS. These tubes are really the modified basal branches of the trees. They are unbranched, and their peritoneum consists of cells which secrete a slime which swells up enormously on the addition of sea water. When the Cotton-spinner is strongly irritated, it contracts all the muscles of the body-wall, and these, acting on the incompressible fluid in the body-cavity, transmit the pressure to the thin rectum, which tears, and allows a portion of the viscera to be forced out. The first parts to be rejected are the Cuvierian organs, and the cells covering these absorb water, and their contained mucus splits up into a tangle of white threads, in which an enemy may be completely ensnared. A large lobster has been seen so enveloped with this "cotton" as to be completely incapable of motion. The origin of the name "Cotton-spinner" requires no further elucidation. Such self-mutilation, even when it involves not only the Cuvierian organs, but the trees and the whole of the intestine, is not necessarily fatal. If the animal be left alone, it can regenerate the whole of these organs.

The WATER-VASCULAR SYSTEM in its general features resembles that of the Echinoidea. We notice as its first striking peculiarity the modification of the stone-canal. This is often multiplied, as in the species (_H. tubulosa_) represented in Fig. 256, where there are five; but whether there is one or many, they do not reach the body-wall, but end each in a swelling projecting into and bathed by the coelomic fluid. These swellings are termed "internal madreporites." They are pierced by numerous fine ciliated canals, which lead into a space from which the stone-canal takes its origin. Both stone-canal and madreporite (especially the latter) are stiffened by the deposition of carbonate of lime. In the young Holothurian there is a single ciliated pore-canal opening to the exterior and leading into a thin-walled axial sinus, which, as Bury[505] has shown, is later converted into the internal madreporite; the pore-canal, which represents the external madreporite of other Echinoderms, disappearing at the same time.

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{566}This extraordinary modification is the consequence of the habit of forcing water into the respiratory trees. The body-cavity is by this means kept tensely filled with fluid, and the stone-canal is enabled to draw on it for the supply to the water-vascular system, thus rendering the external madreporite supererogatory. A large-stalked sac—the Polian vesicle (Fig. 256, 4)—multiplied in many species, hangs down from the water-vascular ring and serves as a reservoir of fluid.

All the podia, including the feelers, have ampullae. In the feelers a semicircular valve is situated just where the external part passes into its long ampulla. When this valve is expanded, the feeler is moved about by the contraction of its muscles, but when it is contracted, the contents of the feeler can flow back into the ampulla, so that the feeler is reduced to an insignificant papilla (as in Fig. 254). The interior of the feeler is ciliated, and a current seems to flow up one side and down the other, so that this organ, like the dorsal tube-foot of a Cake-urchin or Heart-urchin, seems to assist in respiration.

The NERVOUS SYSTEM differs from that of Echinoidea in the absence of the pigment spot (or so-called eye) on the terminal podium of the radial water-vascular canal. Each podium receives a so-called nerve—really an extension of the radial nerve-cord with its ganglion-cells—and this ends in a plate of sensory epithelium in the sucker of the tube-foot or tip of the tentacle, or of each of its branches in the case of the feeler.

There is a coelomic nervous system developed from the radial perihaemal canals. The PERIHAEMAL RING is represented in Echinoidea by the lantern coelom, in Holothuroidea in all probability by the "buccal sinus," a space intervening between the water-vascular ring and the oesophagus. In the outer wall of this are developed ossicles, which constitute the CALCAREOUS RING found in all[506] Sea-cucumbers (Fig. 257, A and B). In this ring (Fig. 257, B) are to be distinguished radial and interradial pieces. The former are notched at their upper ends, and in all probability represent the auriculae of Echinoidea, as the radial nerve-cords pass out over the notches, whilst the interradial pieces probably represent a coalesced pair of jaws and their included tooth, since these ossicles develop from a single rudiment in the larval Echinoid.

{567}The so-called BLOOD SYSTEM is in its main features similar to that of Echinoidea. It consists of a blood-ring surrounding the oesophagus inside the water-vascular ring, and sending branches along the stone-canal, and of dorsal and ventral strands accompanying the gut in its course. These are best marked in the region of the intestine, where absorption principally takes place; in the wall of the stomach they are represented by a delicate plexus which can hardly be traced into connexion with the blood-ring. The dorsal "vessel" is situated in a fold of peritoneum projecting from the intestinal wall; it gives off branches to the intestine, which unite on its surface to form a plexus. In the middle limb of the intestine these branches are grouped into tufts, and the fold of peritoneum between successive tufts becomes absorbed; through the holes so formed branches of the respiratory tree penetrate, so that the trees cannot be separated from the intestine without tearing the dorsal vessel (Fig. 256, 13).

The GENITAL ORGANS consist of a single group of branched tubes situated on the left side of the dorsal mesentery, which converge to open into a short genital duct, which leads to a pore situated in the mid-dorsal line, a short distance behind the feelers. From the common point of origin of the tubes, the "genital base," as it is called, a worm-shaped genital stolon[507] extends back along the genital duct towards the body-wall. There is no genital rachis.

CLASSIFICATION OF HOLOTHUROIDEA.

The class is in many points of structure exceedingly variable, but many striking variations in important organs occur in allied {568}species, and even in the same species, and hence are probably not of physiological importance. We shall therefore confine our attention mainly to those differences in structure which are correlated with differences in habits, and therefore of systematic importance. We shall consider in order (1) the feelers; (2) the method of protecting these; (3) the rest of the water-vascular system; (4) the gills; and (5) the skeleton.

FEELERS.—These organs have been made the basis of the division of the Holothuroidea into orders, and as they are the means by which food is obtained, and are thus of first-class physiological importance, this procedure is fully justified. In three orders they have the shield-shaped ends described in the case of _Holothuria nigra_, but in another large order (Dendrochirota) they are much branched, and end in a mass of delicate twigs. In another order (Synaptida) they are feather-shaped, with two rows only of branches, whilst finally in Molpadiida they are simple finger-shaped processes with one or two lateral branches. The number of the feelers varies from ten to thirty.

In the Dendrochirota the entire anterior portion of the body can be introverted into the interior, so that in this way the crown of feelers can be effectively protected. The retractor muscles are modified portions of the longitudinal muscles of the body-wall, which traverse the body-cavity, and are inserted into the radial pieces of the calcareous ring. Similar muscles are found in the genus _Molpadia_ and in many of the Synaptida. In Aspidochirota and _Pelagothuria_ they are totally wanting, and here the feelers possess long ampullae which allow of the tentacles being individually contracted to very small dimensions. These ampullae seem to be present in nearly all cases in Molpadiida, and in Synaptida, although in the last-named order they are very feebly developed, and must be looked on as vestigial. In Dendrochirota, owing to the strongly developed retractors, they would be useless, and so are absent.

WATER-VASCULAR SYSTEM.—In Synaptida the radial canals are totally absent in the adult, and the only podia are the feelers, which spring directly from the ring-canal. The radial canals are present in _Pelagothuria_, but the feelers are still the only podia; in the Molpadiida there are only five small terminal tentacles round the anus in addition to the feelers. In the Elasipoda all the podia have pointed ends, but the dorsal podia {569}are few, long, and stiff, and often coalescent in places to form grotesque or remarkable appendages. In the remaining forms the podia of the trivium have always suckers, whilst those of the bivium may or may not be pointed. In _Psolus_ the two dorsal radial canals and their podia are totally absent.

RESPIRATORY TREES.—These are present in Aspidochirota, Dendrochirota, and Molpadiida, totally absent in the Synaptida and _Pelagothuria_, and doubtfully represented in a few Elasipoda by a single unbranched outgrowth of the gut.

SKELETON.—This consists, as explained above, of the scattered deposits in the skin and of the calcareous ring. As regards the first, their shape varies immensely, and yet one or two principal types characteristic of each of the main divisions can be defined. Thus the Synaptida are characterised by wheels, with spokes ending in a hub, and by anchors attached to a plate. The Elasipoda have simple St. Andrew's crosses, whilst the Aspidochirota are mainly characterised by "stools" (Fig. 257, C) and buckles (Fig. 257, D). The Dendrochirota have a bewildering variety of forms; the most characteristic, however, are a right-angled cross and a grating, very similar to the buckles of the Aspidochirota, except that in the former there are usually four holes placed cross-wise, whilst the buckle has generally two parallel rows of three holes. Since these ossicles are the only records we possess of the existence of fossil Holothuroidea, they have been studied with great care. The calcareous ring varies very much. The radials are always five (except in individuals where there are more than five radii), but the interradials are increased in the Synaptida, and in the other orders are in some cases diminished or occasionally suppressed altogether. The last is the case in nearly all Elasipoda; here the radials consist of a central horizontal piece with two diverging arms at each side. These arms, which can branch repeatedly, traverse the adjacent interradii, meeting those of the next radii, so that interradials are in most cases entirely absent. The Aspidochirota have usually a ring consisting of small squarish ossicles (Fig. 257, B). In the Molpadiida and Dendrochirota the radials are prolonged backwards into forked tails, which in some Dendrochirota are broken into a number of small pieces (Fig. 257, A), the lower parts of the interradialia being similarly divided.

The CLASSIFICATION of the Holothuroidea is comparatively easy. {570}All authors recognise six divisions, and the only dispute is as to whether they are to be regarded as families or orders. Ludwig[508] divides the group into two orders, PARACTINOPODA and ACTINOPODA, but the first includes only those forms which have lost the radial canals, and this is only one step farther in a degeneration, intermediate stages of which can be traced in the other divisions. There is really no ground for placing the Paractinopoda in contrast to all the other divisions, and the only alternative is to regard the six main divisions as orders, since a class must be divided into orders. In the case of only one, however, is a further division into families practicable, and therefore each of the others will contain a single family.

ORDER I. ASPIDOCHIROTA.

Holothuroidea with shield-shaped feelers provided with ampullae; with radial canals and numerous podia and with respiratory trees. Retractor muscles absent. Nearly a third (158) of the species of Holothuroidea belong to this order, but there are only six genera, and of these _Holothuria_ includes no less than 109 species. The Aspidochirota seem for the most part to live on somewhat firm ground, the surface of which they are continually sweeping with their shield-shaped feelers, which brush the adherent organisms into the capacious mouth. Four species of _Holothuria_—viz. _H. intestinalis_, _H. tremula_, _H. aspera_, and _H. nigra_ are recorded from British waters. The first-named is a northern form, distinguished by the fact that all its podia have suckers; it is found in the north of Scotland. _H. tremula_ is intermediate in structure between _H. intestinalis_ and _H. nigra_, and is found in deep water off our western coasts. _H. aspera_, remarkable for the radiating spines growing out from its ossicles, has been recorded only once from deep water. Of the other genera it is only necessary to mention _Stichopus_, remarkable for the square outline of its transverse section, and for the restriction of the ventral tube-feet to the radii; there is also a well-marked tapering of the anterior end, so that this genus may be said to have a neck. _Stichopus_ is almost entirely confined to tropical waters, and some of its species, as also species of the ubiquitous genus _Holothuria_, as well as many other undetermined species, constitute the valuable "Trepang," which is a delicacy much prized {571}by the Chinese. The Trepang are caught in various parts of the Malay Archipelago. They are cooked in sea water to preserve them, dried in the sun, and boiled in fresh water repeatedly, till all the salt is extracted. They are then dried and sent to market, where they are used in making soup.

ORDER II. ELASIPODA.

Holothuroidea with shield-shaped feelers, destitute of retractor muscles; all the podia have more or less pointed ends,[509] but there is a marked contrast between dorsal and ventral podia, and the ventral surface is flattened so as to constitute a creeping sole. No respiratory trees, at most a simple diverticulum of the intestine; frequently the primitive external madreporite is retained, and contains several pores.

A number of spherical sacs containing little spherical calcifications (otocysts) are attached to the nerve-ring in some genera. Can these be metamorphosed sphaeridia of Echinoid ancestors?

The first member of this remarkable order to be discovered was _Elpidia_, which was dredged in 1875 by the Swedish Arctic Expedition, and described by Théel.[510] The majority of the known members of the order were discovered by the dredging expedition of H.M.S. "Challenger." The species composing it are, with one exception, inhabitants of what may be termed the abysmal depths of the sea. The exception alluded to (_Ilyodaemon maculatus_) is confined to the belt between 100 and 150 fathoms in depth. The well-marked sole and the absence of suckers point to a life consisting of constant peregrinations over {572}the soft ooze forming the ocean floor. The ooze forms their food, and as their weight must to a certain extent immerse them in it, we can understand why the stiff, long dorsal podia have been specialised as respiratory organs, since there are no respiratory trees. These respiratory podia sometimes undergo extraordinary development; thus in _Peniagone_ several very long ones cohere to form a huge vertical sail, whilst in _Psychropotes_ one or two cohere to form a backwardly projecting tail. On the other hand, in _Ilyodaemon_ (Fig. 258) the dorsal podia are numerous and slender.

ORDER III. PELAGOTHURIIDA.

Holothuroidea with shield-shaped feelers provided with long ampullae which project outwards, pushing the skin before them so as to form external appendages, connected at the base by a web. Calcifications absent. No retractor muscles. No respiratory trees. The external madreporite is retained, but all podia other than the feelers have disappeared, although the radial canals have been retained.

This order contains one species, _Pelagothuria natans_, which is the only free-swimming Holothuroid known, the muscular web connecting the freely projecting ampullae being the organ of locomotion.

ORDER IV. DENDROCHIROTA.

Holothuroidea with long repeatedly branched feelers terminating in fine pointed twigs. No feeler-ampullae; but retractor muscles are present, which can introvert the anterior end of the body. Respiratory trees well developed. This order includes twelve genera and over 180 species, and, like the Aspidochirota, is of worldwide distribution. So far as can be safely generalised from the few species whose habits have been closely observed, it seems that this order is adapted to catch swimming prey—it is an order of fishers. The long branched tentacles are extended like the lines of an angler. Their surface is coated with adhesive slime, and before long becomes covered with small organisms which have come in contact with it. When a feeler has captured in this way a large enough haul, it is turned round and pushed into the mouth, which is closed on it. It is then forcibly pulled out, during which process the prey is, so to speak, stripped off it. {573}Four genera (_Cucumaria_, _Thyone_, _Phyllophorus_, and _Psolus_) and sixteen species have been recorded from British waters.

_Cucumaria_ is remarkable for being the only genus of Holothuroidea in which the body is pentagonal in cross-section. In the majority of its species the tube-feet are confined to two rows along each radius, but in a few there are some scattered tube-feet in addition. There are only ten buccal tentacles. The species figured (_C. crocea_) is an Antarctic one which carries the young on the back. _Thyone_ differs in being circular in cross-section and in having the tube-feet scattered evenly over the whole surface. In _Phyllophorus_ (Fig. 260) the tentacles are more than fifteen, and are disposed in two circles, an inner of smaller and an outer of larger tentacles. The other podia are, as in _Thyone_, scattered.

{574}

_Psolus_ is a most extraordinary genus. There is a well-marked sole, to which the tube-feet are confined, whilst the dorsal radial canals, and consequently all the dorsal tube-feet, are absent. The dorsal ossicles are enlarged to form a complete mail of plates, recalling the corona of a Sea-urchin. The two British species are small, and found in comparatively deep water, but a fine large species is found in the Gulf of St. Lawrence, and {575}extends into brackish water up the estuary. The species figured (_P. ephippifer_) is an Antarctic one, which carries the eggs until development is complete in a dorsal brood-pouch.

ORDER V. MOLPADIIDA.

Holothuroidea with simple, finger-shaped feelers, provided with ampullae; retractor muscles occasionally present; respiratory trees present. Besides the feelers, the only podia are five minute papillae terminating the radial canals in the neighbourhood of the anus.

This order includes six genera and about thirty species. Its peculiarities seem to be due to the fact that its members are burrowers, leading a life like an earthworm. Hence the absence of the tube-feet, and the small, almost vestigial character of the feelers. _Trochostoma_ (Fig. 262) and _Caudina_ are remarkable for the presence of a tail. This appendage is in reality only the narrow posterior end of the body, and is especially long in _Caudina_; and observations on a species found off the coast of Maine, U.S.A.,[511] have shown that the tail, like the siphon of a Mollusc, projects up from the burrow to the surface in order to maintain the respiratory current of water.

ORDER VI. SYNAPTIDA.

Holothuroidea with short bipinnate (_i.e._ feather-shaped) feelers, provided with only vestigial ampullae, and with well-developed retractor muscles. No other podia; radial canals absent in the {576}adult. Respiratory trees absent, and transverse muscles of adjacent interradii continuous, so as to form circular muscles. Otocysts attached to the nerve-ring as in Elasipoda.

The members of this remarkable order, like those of the preceding one, are burrowers; but though their feelers are larger, the rest of their anatomy has undergone much more profound modification than that experienced by the Molpadiida. The loss of the radial canals, which must be practically functionless in Molpadiida, is not a great step, but the change in the mode of respiration is a greater modification. Respiration appears to be effected by diffusion through the body-wall, which is always comparatively thin. The circulation of the body-cavity fluid is assisted by a number of stalked, ciliated cups placed on the mesenteries near the line of their insertion on the body-wall. In dealing with Asteroidea it was pointed out that the ends of the tube-feet are the only places where numerous sense-hairs are to be found, and which, therefore, can be called sense-organs. This is true generally throughout Echinodermata. Now in Synaptida, where the tube-feet are lost, the surface of the body has scattered over it little sense-organs consisting of hillocks of ectoderm with an aggregation of sense-cells. These may be regarded as representing the discs of the missing tube-feet. One is involuntarily reminded by the ciliated cups and scattered sense-organs of the ciliated urns and sense-organs of the Sipunculidae, which lead a similar life; and taking into consideration the general superficial likeness in {577}appearance of the two groups, the epigram is almost justified that "if the Synaptida were not extremely careful they would become Gephyrea."

This order is represented in British waters by three species of the genus _Synapta_, which is remarkable for possessing, as ossicles, only the peculiar anchors attached to anchor plates. The present author has dug up the commonest species (_S. inhaerens_) from its burrows in the sand at low water in the Clyde. These animals seem to seek their food at the surface; the feather-shaped feelers are used to seize small algae and zoophytes, of which the food apparently consists. If seized, _S. inhaerens_ readily amputates the posterior part of the body, whilst the head with its feelers immediately buries itself. The other genera of the order (except _Anapta_) are characterised by the possession of wheels with spokes as their characteristic ossicle, as the names _Trochodota_, _Trochoderma_, _Acanthotrochus_ bear witness.

The only fossil remains of Holothuroidea consist of isolated ossicles—wheels, gratings, anchors, etc.—which first make their appearance in the Carboniferous limestone and tell us practically nothing of the evolution of the group. From a comparison with one another of the living families, certain conclusions can be drawn. The Aspidochirote feeler and the method of using it recall forcibly the shape and function of the buccal tube-feet of Spatangoidea. It is probably safe to assume that it is the primitive form from which the other forms of feeler have been derived. Secondly, the anal respiration and the curious internal madreporite have been developed in correlation with one another, and are like nothing found elsewhere among the Eleutherozoa. Hence we may with high probability assume a Protoholothuroid stock with shield-shaped feelers but devoid of respiratory trees, and with an external madreporite. From this stock the Elasipoda developed by migrating into deeper water, whilst the Pelagothuriida sprang from the same root by taking to swimming; the Aspidochirota constituting the main line. The Dendrochirota were developed from a stock with respiratory trees and internal madreporite—in a word, from Aspidochirota. From them the Synaptida and the Molpadiida have developed as offshoots at different periods through taking to a burrowing life. These relationships are shown by the following diagram:—

Protoholothuroidea {578}
/ | \
/ | \
Elasipoda | Pelagothuriida
|
Protaspidochirota
/ \
Aspidochirota Protodendrochirota
|\
| Synaptida
|\
| Molpadiida
|
Dendrochirota

{579}CHAPTER XX

ECHINODERMATA (_CONTINUED_): PELMATOZOA—CRINOIDEA = SEA-LILIES—THECOIDEA—CARPOIDEA—CYSTOIDEA—BLASTOIDEA

SUB-PHYLUM II. PELMATOZOA

The PELMATOZOA differ from the ELEUTHEROZOA in several important respects. They are fixed (at any rate in the young stage) by the centre of the aboral surface, and this portion of the body usually takes on the form of a stem supported by a definite series of ossicles, so that we can discriminate a "calyx"—the main part of the body—from the "stem." Further, the podia and the ambulacral grooves seem to be always covered with powerful cilia, which are employed in producing a current which sweeps small organisms to the mouth. The podia are never locomotor in function; their use is similar to that of the tentacles on the lophophore of Polyzoa and Brachiopoda.

The living Pelmatozoa are very few in number compared with the extinct forms. It may with justice be said that the group is nearly extinct; indeed, out of its five classes one alone, and that the most highly specialised class, survives till the present day. Now we have already seen that, in the case of the Eleutherozoa, if the annectant fossil types were taken into consideration, the definition of the classes would be difficult, so that it is not to be wondered at if the classes of the Pelmatozoa are also somewhat difficult to define; and it must be added that this difficulty is not only due to the fact that intermediate types occasionally occur, but also to our ignorance of the functions of many structures found in fossil types, speculations regarding which are to be received with caution. Bearing in mind, then, the provisional nature {580}of the classification, we may give the diagnoses of the principal divisions as follows:—

Class I. CRINOIDEA.—Pelmatozoa provided typically with a well-marked stem; calyx consisting of an aboral "patina" of two or three circles of plates, and a flexible "tegmen" or oral surface with small plates or none; radial canals supported by long branched arms, which are developed as direct prolongations of the uppermost circle of plates in the patina.

Class II. THECOIDEA (Jaekel) = EDRIOASTEROIDEA (Bather).—Pelmatozoa without a stalk, fixed to the substratum by the whole aboral surface. The radial canals run out over the oral surface in grooves, which are closed by specially modified plates; but there are no arms of any kind.

Class III. CARPOIDEA (Jaekel).—Pelmatozoa with a well-developed stalk. The radial canals and their branches are devoid of a skeleton, and either produce no modifications at all on the skeleton of the calyx, or at most are supported by short horn-like outgrowths of some of its plates.

Class IV. CYSTOIDEA.—Pelmatozoa which typically possess a well-developed stalk, a sac-like calyx contracted at the mouth and covered with plates, some of which are pierced with pores or slits; the radial canals, though they may for part of their course run over the surface in grooves, have their terminal portions supported by free unbranched arms ("fingers").

Class V. BLASTOIDEA.—Pelmatozoa provided with a well-developed stalk and ovoid bud-like calyx. From the mouth the radial canals run backwards over the calyx, as in Echinoidea, but they give rise to numerous lateral branches, which are supported by free unbranched arms ("fingers"). Special respiratory organs occur on the interradial areas in the form of parallel folds called "hydrospires."

CLASS I. CRINOIDEA

This is the only class which has living representatives. There are twelve recent genera, of which eight retain the stalk throughout life; the remaining four lose it when adult, retaining only a stump, termed the "centro-dorsal," covered with fixing organs ("cirri"). The stalked forms are confined to considerable depths, and can only be obtained by deep dredging, whereas many of the {581}stalkless forms are comparatively common. We shall select as type for special description the common Feather-star, _Antedon rosacea_ (_bifida_), which can be dredged in depths of ten fathoms off the south-west coast of England.

The animal consists of a small flattened calyx, from which radiate out ten long delicate arms, each fringed with a double series of short branches called "pinnules." In the centre of the aboral surface can be seen the centre-dorsal plate (Fig. 265, _c_), a knob-like stump of the broken-off stem, covered with small whip-like outgrowths called "cirri," by means of which the animal is anchored to the substratum (Fig. 265, _cir_). When _Antedon_ is disturbed it relaxes its hold, and swims by graceful muscular movements of the arms. These are arranged in five pairs, and the corresponding members (right and left) of all the pairs are bent and relaxed together. On coming to rest the animal reattaches itself by means of the cirri. These are composed of cylindrical ossicles joined to one another by muscles, and they can thus act as efficient grasping organs. In the centre of the oral surface, which is termed the "tegmen," and is soft, flexible, and without visible calcifications, is situated the mouth, surrounded by five short triangular flaps called "oral valves." In the intervals between these valves, grooves radiate from the mouth which bifurcate at the points of origin of each pair of arms, and are continued over their surfaces. These grooves correspond to the ambulacral grooves of Asteroidea, and to the epineural canals of the other classes of Eleutherozoa. At each side of each groove {582}are to be found a series of podia in the form of delicate finger-like processes, which serve only for respiration and for producing a current of water, their surfaces, like that of the grooves between them, being covered with powerful cilia. The anus is at the extremity of a little knob called the anal papilla, situated in one of the interradii (Fig. 264, _an_).

As in Ophiuroidea, the ectoderm cells have disappeared over the whole surface of the body, except the grooves and the podia, the only trace of their former existence being a cuticle with adherent nuclei. Pedicellariae are unknown in all Pelmatozoa; and spines have only been described from one fossil species of Crinoid. Beneath the cuticle is the dermis, having the composition described in the case of _Asterias rubens_; this on the aboral side of the calyx gives rise to the "patina," consisting of plates, in part movable on one another, in part immovably fused together. Those visible from the outside are (1) the centro-dorsal ossicle, from which the cirri spring; (2) five columns of ossicles termed radials (Fig. 266, _R^1_, _R^2_, _R^3_); each column consists of three radials, extending from the centro-dorsal to the origin of a pair of arms. The uppermost radial in each column bears two facets for the articulation of these arms. Each arm is supported by a series of "brachial ossicles" (_Br_).

It is evident, both from the number of ambulacral grooves and of the columns of radials, that _Antedon_ has only five radii, and each pair of arms must be regarded as having arisen by the bifurcation of a primitive arm. This is proved to be true by {583}a study of the development, and it can further be shown that the arms fork repeatedly; but in these further bifurcations one fork remains short, and forms a pinnule, whilst the other continues the arm. Thus the arm, instead of being a single axis, is really a series of axes—in a word, it is a "sympodium."

If in the case of any bifurcation the two forks were to develop equally, the number of arms in that ray would be doubled, and this actually happens in the case of other species of _Antedon_.

DIGESTIVE SYSTEM.—The mouth leads through a short vertical oesophagus into an enlarged stomach, which lies horizontally curved around the axis of the calyx. The stomach is succeeded by a short intestine, which leads into the anal papilla. Both oesophagus and stomach are ciliated, and the food consists of minute organisms, swept into the mouth by the current produced by the cilia covering the ambulacral grooves and podia; the ten arms may indeed be compared to a net spread out in the water to catch swimming prey.

The WATER-VASCULAR SYSTEM consists of a ring closely surrounding the mouth, from which radial canals are given off which underlie the ambulacral grooves and bifurcate with them. The podia have no ampullae, but muscular strands traverse the cavities of the radial canals, and that of the ring-canal, and by their action water can be forced into the podia, which are thus extended. Numerous stone-canals hang down from the ring-canal, and open freely into the coelom; they do not, as in Holothuroidea (where the same arrangement occurs), end in sieve-like madreporites. The tegmen, _i.e._ the ventral surface of the calyx, is pierced by a number of isolated pores lined by ciliated cells, which suck in water. In the oldest Pelmatozoa there seems to have been a regular madreporite. In the larva of _Antedon_ there is but one pore-canal, which, as in most Eleutherozoa, leads into a special section of the coelom, the "axial sinus," embedded in the body-wall, with which also the single stone-canal communicates; but later the division between the axial sinus and the rest of the coelom breaks down, and then the pore-canals and stone-canals become multiplied independently of each other (Fig. 266, _m.p_, _p.c_, and _st.c_).

NERVOUS SYSTEM.—In the young stalked form the nervous system, as in other Echinoderms, consists of a ring round the {584}mouth, from which radial cords are given off which run under the ambulacral grooves (Fig. 266, _nerv.rad.v_).

The fibres of this nervous system are, as in Asteroidea, immediately beneath the bases of the ectoderm cells. A large band of fibres is given off to each podium, which is covered with minute elevations, each with pointed sense-hairs in the centre. As the animal grows, another nervous system makes its appearance, which is developed from the coelomic wall, the cells of certain tracts of which multiply and bud off ganglion cells from which the fibres grow out.

This "aboral nervous system," as it is called, has its centre in the "chambered organ" (Fig. 266, _chamb_), which is embedded in the centro-dorsal ossicle, and is roofed over by a plate called the "rosette." This represents the five coalesced "basals," a ring of plates which in other forms alternate with the lowest radials, and it intervenes between these and the centro-dorsal. The chambered organ consists of a ring of five vesicles, which have originated as pouches of the aboral coelom (Fig. 266, _chamb_). The walls of these vesicles develop nervous matter; from them radiate out five great cords, deeply embedded in the {585}plates of the patina. These cords rapidly fork, and one division of each of two adjacent cords enters the lowest radial. In the third radial all the cords are connected by a commissure which runs completely round the calyx. Each of the cords in the third radial forks again, and one branch of each cord enters each of the two arms connected with it, and the two branches entering an arm coalesce to form a single cord. In the arms, as in the calyx, the cords are deeply embedded in the ossicles, but branches extend to the ventral surface of the arms and here unite to form two longitudinal cords, one on each side of the groove. In the tegmen these cords are connected by an outer nerve-ring, branches from which join the ectodermal nerve-ring already described.

The researches first of W. B. Carpenter[512] and then of Marshall[513] have proved that it is the aboral nervous system which really controls the movements of the animal. If the chambered organ is destroyed by cautery, the whole movements of the animal are paralysed; but it will carry out its characteristic swimming movements just as well if the whole tegmen with the ambulacral nerve-ring and the whole of the alimentary canal are torn away. The commissure in the third radials co-ordinates the movements of the arms. If it is cut they move independently of one another. The position of the radial cords inside the ossicles is gradually acquired. At first they are gutter-like evaginations of the coelom; by upgrowth of their sides the gutters become canals, and are then surrounded by calcified tissue. The cirri have each a cord traversing them which originates from the chambered organ.

COELOM.—In the young stalked form the coelom consists of the water-vascular system ("hydrocoel"), and underlying it an oral coelom, separated from an aboral coelom by a horizontal mesentery. As the animal grows, this horizontal mesentery becomes largely absorbed, and the coelom becomes everywhere traversed by cellular cords (trabeculae), which are later calcified.