Chapter XX: Appendix: To Ctenophora (3)

Preyer[462] endeavoured to test the "intelligence" of Ophiuroids by observing how they would adapt themselves to circumstances which it might be fairly assumed they had never encountered {489}in their ordinary experience. To this end he passed over the arm of a specimen a piece of indiarubber tubing, which clung to it tightly. He found that the animal first tried walking off, pressing the encumbered arm against the ground, so that the piece of tubing was rubbed off. It was then replaced more tightly than before; the animal, having tried the first method without result, waved the arm to and fro in the water till the rubber floated off. In a third experiment the animal held the rubber against the ground by a neighbouring arm, and drew the encumbered arm out. When the rubber was replaced a fourth time, the animal kicked it off by alternately pressing neighbouring arms against it. Finally, when the rubber was put on so firmly that all the above-mentioned methods failed, the arm was broken off. Preyer concludes from this that Ophiuroids have a high degree of intelligence; but this may be doubted, and the reader is referred to the account of Uexküll's experiments given in the next chapter. There is, however, no doubt at all that Ophiuroidea are by far the most active of all Echinoderms, and one would naturally correlate this with higher psychic development.

The radial nerve ends in a terminal tentacle sheltered by a median plate at the end of the arm; but eyes, such as are found in Asteroids, are wanting, and the animal does not appear to be sensitive to light.

The REPRODUCTIVE SYSTEM in Ophiuroids consists of a genital stolon giving rise at its distal end to a genital rachis, which extends in a circular course round the disc, ensheathed in an "aboral sinus" (Fig. 213, _ab_) and swelling out so as to form the gonads (testes or ovaries), where it passes over the inner side of the genital bursae. The genital stolon (Fig. 213, _gen.st_) is a compact ovoid organ, often termed on account of its shape the "ovoid gland." It is situated close to the stone-canal, and, as in Starfish, it indents the outer wall of the axial sinus; but, unlike the stolon of the Asteroid, it is separated from the general coelom by a space, of which it forms the inner wall, but whose outer wall is formed by a sheet of membrane. This cavity must be carefully distinguished from the axial sinus of Asteroidea, to which it was supposed at one time to correspond; it is really formed by a pocket-like ingrowth of the general coelom into the septum dividing it from the axial sinus. The cells forming the inner side of this pocket form the primitive germ-cells, which {490}constitute the main mass of the ovoid gland; those of the outer side remain thin. The cavity of the ingrowth is shut off from the general coelom, but persists throughout life. In Asteroids a similar ingrowth takes place, but both walls thicken and become converted into germ cells, and the cavity disappears, and, as in Asteroidea, a considerable number of the germ-cells in the stolon degenerate.

The genital rachis (Fig. 213, _gen.r_) is an outgrowth of the distal end of the genital stolon, which extends in a complete circle round the disc. The rachis does not, however, lie everywhere in the same plane, but by its undulating course bears witness to the distortion which the disc has undergone. In the radii it is, as in the Asteroid, dorsal; but in the interradii it is ventral, this ventral portion having, like stone-canal and axial sinus, been carried down by the preponderant growth of the dorsal parts of the disc. It is everywhere ensheathed by the aboral sinus, which, as in Asteroids, is an outgrowth of the coelom. The rachis is embedded in a strand of modified connective tissue, to which we may (as in the case of _Asterias_) apply the name "aboral blood-ring." Both on the central and peripheral sides of this sinus are vertical muscles connecting the genital and the radial plates, which bring about the respiratory movements already referred to. Just above the madreporite, at the end of the genital stolon, is a small, completely closed space, which by its position corresponds with the madreporic vesicle of Asteroids and represents the right hydrocoel (Fig. 213, _mv_). As the rachis passes over the genital bursa it gives off branches, which swell up to form the genital organs. In _Ophiothrix_ there is {491}one such organ on each side of each bursa, but in other genera (cf. _Ophiarachna_) a large number of small ones. The genital products are shed into the water through the bursae.

CLASSIFICATION OF OPHIUROIDEA.

Before proceeding to study the classification of Brittle Stars, it is necessary to give some account of the range of structure met with in the group.

NUMBER OF RADII.—The number of arms is rarely increased, and hardly ever exceeds six; a few species (each an isolated one in its genus) have six arms, and in one case (_Ophiactis virens_), at any rate, this is associated with the power of transverse fission. In many Cladophiurae the arms fork repeatedly, so that although there are only five radii, there is quite a crowd of terminal branches.

VERTEBRAE.—The vertebrae differ in the manner in which they articulate with one another. In _Ophiothrix fragilis_ taken as the type, which in this respect resembles the vast majority of species (Zygophiurae), the knobs and pits on the faces of the vertebrae prevent the arms from being coiled in the vertical plane. In _Ophioteresis_ (Fig. 210, A) and some allied genera (Streptophiurae) the knobs are almost obsolete, and the arms are free to coil in the vertical plane; whilst in _Gorgonocephalus_ and _Astrophyton_ (Cladophiurae) the arms are repeatedly branched and the vertebrae have saddle-shaped articulating surfaces, so that they have quite a snake-like capacity for coiling themselves round external objects. In _Ophiohelus_ (Fig. 216) each vertebra consists of two rod-like plates placed parallel with the long axis of the arm and fused at both ends, but divergent in the middle, leaving a hole between them.

COVERING PLATES OF THE ARMS.—The upper arm-plates are the most variable. They may be surrounded by small supplementary plates (_Ophiopholis_) or double (_Ophioteresis_). In all (?) Cladophiurae and most Streptophiurae they are absent, being replaced by minute calcareous granules. Under arm-plates are absent in _Ophioteresis_ and in the distal portion of the arms in many Cladophiurae. Side arm-plates are constantly present, and in most Cladophiurae meet in the middle line below.

ARM-SPINES.—The spines borne by the lateral covering plates of the arms vary greatly in character. In _Ophiura_ and its {492}allies they are short and smooth, and are borne by the hinder edge of the arm and directed backwards; but in the larger number of genera they are borne nearer the centre of the plate, and are directed outwards at right angles to the arm. They may be covered by small asperities, as in _Ophiothrix_ (Fig. 215, C), when they are said to be rough; or these asperities may become secondary spines, as in _Ophiacantha_ (Fig. 215, B), when they are said to be thorny. In _Ophiopteron_ all the spines borne by a single plate are united by a web of skin so as to constitute a swimming organ. The small plates guarding the ends of the tentacles (tentacle-scales) may be absent, or more rarely double. In Cladophiurae there is a regular transition from tentacle-scale to arm-spine; the tentacle-scale being merely the smallest of the series of lateral spines.

True PEDICELLARIAE are unknown amongst Ophiuroidea, since there is no longer a soft ectoderm to protect, but in some cases, as for instance in _Ophiohelus_, small hooks movable on a basal piece attached to the arms are found which may represent the vestiges of such organs (Fig. 216). Similar hooks are found in the young _Ophiothrix fragilis_ just after metamorphosis and in all Cladophiurae, replacing in the latter case the arm-spines in the distal portion of the arm.

MOUTH-FRAME.—In its broad outlines there is practically no variation in this organ throughout the group, but in respect of {493}the spines, which are borne on the flanks of the jaws (mouth-papillae) and on their apices (teeth and tooth-papillae) there is very great variation. Teeth are always present. Mouth-papillae are very frequently present, tooth-papillae are rarer, and it is only in a restricted number of genera (_Ophiocoma_ and its allies) that both mouth-papillae and tooth-papillae are present at the same time.

SKELETON OF THE DISC.—This is typically composed of a mosaic of plates of different sizes, but in some cases (_Ophiomyxa_, most Streptophiurae, and Cladophiurae) these, with the exception of the radials and genitals, are entirely absent, and the disc is then quite soft and covered with a columnar epithelium, the persistent ectoderm. Even the scuta buccalia may disappear. Radial shields are absent in _Ophiohelus_. In many cases (_Ophiothrix_ and _Ophiocoma_) all the dorsal plates except the radials are concealed from view by a covering of small spines. In some genera (_Ophiopyrgus_) there are five large plates in the centre of the upper part of the disc, which have been termed "calycinals" from a mistaken comparison with the plates forming the cup or calyx of the Pelmatozoa, but there is no connexion between the two sets of structures.

The MADREPORITE is usually quite rudimentary, but in Cladophiurae there may be five madreporites, each with about 200 pores, and, of course, five stone-canals.

{494}The number of GENITAL ORGANS varies very much. In the small _Amphiura squamata_ there are two gonads, an ovary and a testis, attached to each bursa, but in the larger species there may be very many more.

We follow Bell's classification,[463] according to which the Ophiuroidea are divided, according to the manner in which the vertebrae move on one another (cf. Fig. 210), into three main orders, since these movements are of prime importance in their lives.

(1) STREPTOPHIURAE, in which the faces of the vertebrae have rudimentary knobs and corresponding depressions, so that the arms can be coiled in the vertical plane. These are regarded as the most primitive of Ophiuroidea.

(2) ZYGOPHIURAE, in which the vertebral faces have knobs and pits which prevent their coiling in a vertical plane.

(3) CLADOPHIURAE, in which the arms can be coiled as in (1) and are in most cases forked. No teeth; the arm-spines are papillae, the covering plates of the arms are reduced to granules.

ORDER I. STREPTOPHIURAE.

This is not a very well defined order; it includes a few genera intermediate in character between the Cladophiurae and the Zygophiurae, and believed to be the most primitive Ophiuroids living. It is not divided into families. The vertebrae have rudimentary articulating surfaces, there being two low bosses and corresponding hollows on each side, and so they are capable of being moved in a vertical plane, as in the Cladophiurae; the arms never branch, and further, they always bear arm-spines and lateral arm-plates at least. No species of this order are found on the British coast, but _Ophiomyxa pentagona_, in which the dorsal part of the disc is represented only by soft skin, is common in the Mediterranean.

_Ophioteresis_ is devoid of ventral plates on the arms, and appears to possess an open ambulacral groove, though this point has not been tested in sections. _Ophiohelus_ and _Ophiogeron_ have vertebrae in which traces of the double origin persist (see p. 491).

{495}ORDER II. ZYGOPHIURAE.

This group includes all the common and better-known British forms. They are divided into five families, all of which are represented in British waters.

FAM. 1. OPHIOLEPIDIDAE.[464]—Arm inserted in a definite cleft {496}in the disc, or (expressing the same fact in another way) the interradial lobes out of which the disc is composed are not completely united. Radial shields and dorsal plates naked. Arm-spines smooth and inserted on the posterior border of the lateral arm-plates.

This family includes all the Brittle Stars of smooth porcelanous aspect and provided with only short spines. Forbes[465] called them Sand-stars, since their short spines render these animals incapable of burrowing or of climbing well, and hence they appear to move comparatively rapidly over firm ground, sand, gravel, or muddy sand, and they are active enough to be able to capture small worms and Crustacea. The prey is seized by coiling one of the arms around it.

One genus, _Ophiura_, is fairly common round the British coast, {497}being represented by _O. ciliaris_ and _O. albida_; the former is the commoner. An allied species dredged by H.M.S. "Challenger" is represented in Figs. 217 and 218.

_Ophiomusium_ (Fig. 219) is a very peculiar genus. The mouth-papillae on each side of each mouth-angle are confluent, forming a razor-like projection on each side of each mouth-angle (Fig. 220). The arms are short, and the podia are only developed at the bases of the arms. _Ophiopyrgus_ has the dorsal surface raised into a conical elevation protected by a central plate surrounded by five large plates.

In the remaining four families the arms are inserted on the under surface of the disc; in other words, the interradial lobes which make up the disc have completely coalesced dorsally; and the spines stand out at right angles to the arm.

FAM. 2. AMPHIURIDAE.—Mouth-papillae present, but no tooth-papillae; radial shields naked; small scuta buccalia.

The most interesting Brittle Star belonging to this family is _Amphiura squamata_ (_elegans_), a small form, with a disc about ¼ inch in diameter covered with naked plates. It is hermaphrodite and viviparous, the young completing their development inside the bursae of the mother. Occasionally the whole disc, with the exception of the mouth-frame, is thrown off and regenerated. This appears to be a device to enable the young to escape. Three other species of _Amphiura_ are found in British waters.

{498}

_Ophiactis_ is another genus belonging to this family, distinguished from _Amphiura_ by its shorter arms and smoother arm-spines. It lives in the interstices of hard gravel. The British species, _O. balli_, presents no special features of interest, but the Neapolitan _O. virens_ is an extraordinary form. It has six arms, three of which are usually larger than the other three, for it is always undergoing a process of transverse division, each half regenerating the missing part. It has from 1 to 5 stone-canals, the number increasing with age; numerous long-stalked Polian-vesicles in each interradius, and in addition a number of long tubular canals which spring from the ring-canal, and entwine themselves amongst the viscera.[466] All the canals of the water-vascular system, except the stone-canals, contain non-nucleated {499}corpuscles, carrying haemoglobin,[467] the respiratory value of which compensates for the loss of the genital bursae, which have entirely disappeared.

_Ophiopholis_ is distinguished from the foregoing genera by the granular covering of its dorsal plates; whilst in _Ophiacantha_ these granules develop into prominent spinelets, and the arm-spines are also thorny. _Ophiopholis aculeata_ occurs in swarms in the branches of the Firth of Clyde, and presents a most remarkable series of variations in colour. _Ophiopsila_ is a closely allied form, distinguished by its large peristomial plates.

FAM. 3. OPHIOCOMIDAE.—Both mouth-papillae and tooth-papillae are present;[468] the arm-spines are smooth, and the disc is covered with granules.

_Ophiocoma nigra_ is the only common British representative of this family. In this species the plates of the dorsal surface are completely hidden from view by a covering of granules. _Ophiarachna_.

FAM. 4. OPHIOTHRICIDAE.—Tooth-papillae alone present, mouth-papillae absent; arm-spines roughened or thorny.

This family is represented only by _Ophiothrix fragilis_, which is perhaps the most abundant of all British Ophiuroids, and has been selected as the type for special description.

The back is covered with spinules, having, however, the triangular radial plates bare. This produces a contrast-effect, which suggested the name _pentaphyllum_, formerly used by some naturalists for the species. It occurs in swarms, and presents variations in colour nearly as marked as those of _Ophiopholis_. {500}_Ophiopteron_ is probably a swimming Ophiuroid, as the lateral spines of each segment of the arm are connected by a web of skin.

ORDER III. CLADOPHIURAE.

These, like the Streptophiurae, have the power of rolling the arms in a vertical plane, but the articulating surfaces of the vertebrae are well-developed and saddle-shaped. The dorsal surface of the disc and arms is covered with a thick skin with minute calcifications. Upper-arm plates wanting. Radial plates always present, though occasionally represented by lines of scales. {501}The order is divided into three families, two of which are represented in British waters.

FAM. 1. ASTROSCHEMIDAE.—Arms unbranched. _Astronyx_ is comparatively common in the sea-lochs of Scotland. There are a series of pad-like ridges on the arms, representing the side-plates and bearing the spines. _Astroschema_.

FAM. 2. TRICHASTERIDAE.—Arms forked only at the distal ends. _Trichaster_, _Astrocnida_.

FAM. 3. EURYALIDAE.—Arms forked to their bases. _Gorgonocephalus_ is occasionally taken in deep water off the north coast of Scotland. In it the arms repeatedly fork, so that a regular crown of interlacing arms is formed. The animal obviously clings to external objects with these, for it is often taken in fishermen's nets with its arms coiled around the meshes. The genital bursae are said to be represented by slits which open directly into the coelom. (Lyman describes the coelom as divided into ten compartments by radiating septa; it is possible—even probable—that these are really the bursae.) An allied species is common in the Bay of Fundy, being found in comparatively shallow water. _Astrophyton_ (Fig. 222) is closely allied to _Gorgonocephalus_, differing only in trifling points. It is doubtful whether the separation of these two genera is justified.

FOSSIL OPHIUROIDEA.—The Ophiuroidea are rather sparsely represented among fossils, but in the Silurian and Devonian a series of very interesting forms occur which are intermediate in character between Starfish and Brittle Stars, and which were therefore in all probability closely allied to the common ancestors of modern Ophiuroids and Asteroids. Jaekel[469] has recently added largely to our knowledge of these primitive forms, and has described a number of new genera. Thus _Eophiura_ from the Lower Silurian has an open ambulacral groove, and the vertebrae are represented by an alternating series of quadrate ossicles, each deeply grooved on its under surface for the reception of the tentacle, which was not yet (as in modern forms) enclosed in the vertebra. The lateral or adambulacral plates extended horizontally outwards, and each bore a series of spines at its outer edge.

A remarkable fact is that where the halves of the vertebrae (_i.e._ the ambulacral ossicles) diverge in order to form the {502}mouth-angles, no less than five or six vertebrae are thus affected, instead of only two as in modern forms. The actual "jaw," however, seems, as in modern forms, to consist only of the first adambulacral fused to the second ambulacral, so that instead of concluding with Jaekel that the "jaws" of modern forms result from the fusion of five or six vertebrae, a conclusion which would require that a number of tentacles had disappeared, we may suppose that the gaping "angles" of these old forms have, so to speak, healed up, except at their innermost portions.

In _Bohemura_, which belongs to a somewhat younger stratum, the structure is much the same, but the groove in the ambulacral ossicle for the tentacle has become converted into a canal, and the ambulacral groove itself has begun to be closed at the tip of the arm by the meeting of the adambulacrals.

In _Sympterura_, a Devonian form described by Bather,[470] the two ambulacral plates of each pair have thoroughly coalesced to form a vertebra, but there is still an open ventral groove, and no ventral plates.

In the Trias occurs the remarkable form _Aspidura_, which had short triangular arms, in which the tentacle pores were enormous and the ventral plates very small. The radial plates formed a continuous ring round the edge of the disc. _Geocoma_ from the Jurassic is a still more typical Ophiuroid; it has long whip-like arms, and the dorsal skeleton of the disc is made of fifteen plates, ten radials, and five interradials. In the Jurassic the living genus _Ophioglypha_, appears.

The Cladophiurae are represented already in the Upper Silurian by _Eucladia_, in which, however, the arms branch not dichotomously, as they do in modern forms, but monopodially. There is a large single madreporite.

_Onychaster_, with unbranched arms, which occurs in the Carboniferous, is a representative of the Streptophiurae.

It will therefore be seen that the evolution of Ophiuroidea must have begun in the Lower Silurian epoch. The Streptophiurae are a few slightly modified survivors of the first Ophiuroids. By the time the Devonian period had commenced, the division of the group into Zygophiurae and Cladophiurae had been accomplished.

{503}CHAPTER XVIII

ECHINODERMATA (_CONTINUED_): ECHINOIDEA = SEA-URCHINS

CLASS III. ECHINOIDEA

The Sea-urchins or Echinoidea (Gr. ἐχῖνος, Hedgehog or Sea-urchin), which constitute the third class of the Eleutherozoa, have derived both their popular and scientific names from the covering of long spines with which they are provided. At first sight but little resemblance is to be discerned between them and the Starfish and Brittle Stars. They are devoid of any outgrowths that could be called arms; their outline is generally either circular or that of an equilateral pentagon, but as their height is almost always smaller than their diameter, they are never quite spherical; sometimes it is so small that the animals have the form of flattened discs.

All doubt as to the relationship of the Echinoidea to the Starfish is at once dispelled in the mind of any one who sees one of the common species alive. The surface is beset with delicate translucent tube-feet, terminated by suckers resembling those of Starfish, although capable of much more extension. The animal throws out these organs, which attach themselves by their suckers to the substratum and so pull the body along, whilst the spines are used to steady it and prevent it from overturning under the unbalanced pull of the tube-feet. When moving quickly the animal walks on its spines, the tube-feet being little used. The tube-feet are distributed over five bands, which run like meridians from one pole of the animal to the other. These bands are termed "radii," and they extend from the mouth, which is situated in the centre of the lower surface, up to the neighbourhood of the aboral pole. The radii must be compared to the ambulacral grooves on the {504}oral surface of the arms of Starfish, and hence in Urchins the aboral surfaces of the arms have, so to speak, been absorbed into the disc, so that the oral surfaces have become bent in the form of a semicircle. The radii are separated from one another by meridional bands called "interradii," which correspond to the interradial angles of the disc of a Starfish and to the sides of its arms. The small area enclosed between the upper terminations of the radii is called the "periproct," and this corresponds to the entire dorsal surface of the Starfish, including that of the arms.

One of the commonest species of British Sea-urchin is _Echinus esculentus_. In sheltered inlets, such as the Clyde, it is often left exposed by the receding tide, whilst everywhere on the coast in suitable localities it may be obtained by dredging at moderate depths on suitable ground. In the Clyde it is easy to observe the habits of the animal through the clear still water. It is then seen to frequent chiefly rocky ground, and to exhibit a liking for hiding itself in crevices. Often specimens will be seen clinging to the rock by some of their tube-feet, and, as it were, pawing the under surface of the water with the others. In the Clyde it feeds chiefly on the brown fronds of _Laminaria_, with which the rocks are covered. In more exposed situations, such as Plymouth Sound, it does not occur in shallower water than 18 to 20 fathoms. At this depth it occurs on a rocky ridge; but in 1899, after a south-west gale, all the specimens had disappeared from this ridge, showing at what a depth wave disturbance is felt.

A full-grown specimen is as large as a very large orange; its under surface is flattened, and it tapers somewhat towards the aboral pole. The outline is that of a pentagon with rounded angles. The spines in _Echinus esculentus_ are short in comparison to the diameter of the body, and this is one of the characteristics of the species.

The animal is provided with a well-developed skeleton, consisting of a mail of plates fitting closely edge to edge, and carrying the spines. This cuirass bears the name "corona" (Fig. 227). It has two openings, an upper and a lower, which are both covered with flexible skin. The upper area is known as the "periproct" (Fig. 227, 2); it has in it small isolated plates, and the anus, situated at the end of a {505}small papilla, projects from it on one side of the centre. The lower area of flexible skin surrounds the mouth, and is called the "peristome" (Fig. 229), though it corresponds to considerably more than the peristome of Asteroidea. In the mouth the tips of the five white chisel-like teeth can be seen.

The plates forming the corona are, like all the elements of the skeleton of Echinodermata, products of the connective tissue which underlies the ectoderm, which in Echinoidea remains in a fully developed condition covering the plates, and does not, as in Ophiuroidea, dry up so as to form a mere cuticle. The ectoderm consists of the same elements as that of Asteroidea, viz. delicate tapering sense-cells with short sense-hairs, somewhat stouter supporting cells and glandular cells. It is everywhere underlaid by a plexus of nerve fibrils, which, in part, are to be regarded as the basal outgrowths of the sense-cells and partly as the outgrowths of a number of small bipolar ganglion-cells, found intermixed with the fibres.

Just as the MUSCULAR ARM has been the determining factor in the structure of the Ophiuroidea, so the MOVABLE SPINE has been {506}the leading factor in the evolution of Echinoidea. The SPINES have cup-shaped basal ends, which are inserted on special projections of the plates of the skeleton called tubercles. The tubercle is much larger than the cup, and hence the spine has a great range of possible motion. The spines differ from those of Starfish and Brittle Stars in being connected with their tubercles by means of cylindrical sheaths of muscle fibres, by the contraction of which they can be moved in any direction. The muscles composing the sheath consist of an outer translucent and an inner white layer. The former are easily stimulated and soon relax; they cause the movements of the spines. The latter require stronger stimulation, but when aroused respond with a prolonged tetanus-like contraction, which causes the spines to stand up stiffly in one position; these muscles can be torn across sooner than forced to relax. Uexküll[471] has appropriately named them "block musculature." These sheaths, like everything else, are covered with ectoderm, which is, however, specially nervous, so that we may say that the muscular ring is covered by a nerve-ring from which stimuli are given off to the muscles.

The spines are, speaking generally, of two sizes, the larger being known as "primary spines" and the smaller as "secondary." In many Echinoidea these two varieties are very sharply contrasted, but in _Echinus esculentus_ there is not such a great difference in length, and intermediate kinds occur. The forest of spines has an undergrowth of PEDICELLARIAE. All Echinoidea possess pedicellariae, which are much more highly developed than those of any Asteroid. With few exceptions all the pedicellariae of Echinoidea possess three jaws and a basal piece. This latter is, however, drawn out so as to form a slender rod, which articulates with a minute boss on a plate of the skeleton.

Of these pedicellariae there are in _E. esculentus_ four varieties, viz. (1) "tridactyle" (Fig. 225, C; Fig. 226, B): large conspicuous pedicellariae with three pointed jaws, each armed with two rows of teeth on the edges. There is a flexible stalk, the basal rod reaching only half way up. These are scattered over the whole surface of the animal.

(2) "Gemmiform" (Fig. 225, A, B; Fig. 226, A), so called from the translucent, almost globular head. The appearance of {507}the head is due to the fact that there is on the outer surface of each jaw a sac-like gland developed as a pouch of the ectoderm. From it are given off two ducts which cross to the inner side of the blades and, uniting into one, run in a groove to near the tip. The gland secretes a poisonous fluid. The basal rod reaches up to the jaws, so that this form of pedicellaria has a stiff stalk. On the inner side of each blade, near the base, there is a slight elevation (Fig. 225, B, _s_), consisting of cells bearing long cilia; this is a sense-organ for perceiving mechanical stimuli. The gemmiform pedicellariae are particularly abundant on the upper surface of the animal.

{508}(3) "Trifoliate" (Fig. 225, E; Fig. 226, D): these are very small pedicellariae, in which the jaws are shaped like leaves with the broad end projecting outwards. They are scattered over the whole surface of the body.

(4) "Ophicephalous" (Fig. 225, D; Fig. 226, C): pedicellariae in which the jaws have broad rounded distal ends fringed with teeth; these ends bear a resemblance to a snake's head, whence the name. The bases are also broad and thin, with a strong median rib and a peculiar semicircular hoop beneath the spot where they articulate with one another. The three hoops of the three jaws work inside each other in such a way as to cause the jaws to have a strong grip and to be very difficult to dislocate from their mutual articulation.

The ophicephalous pedicellariae are in _Echinus_ the most abundant of all; and they alone extend on to the peristome, where a special small variety of them is found.

A thorough investigation of the functions and reactions of the pedicellariae has quite recently been made by von Uexküll.[472] He showed, first of all, that there is a nervous centre in the stalk of each pedicellaria (see below), which causes the organ to incline towards a weak stimulus, but to bend away from a stronger stimulus. In the head there is an independent nervous centre, which regulates the opening and closing of the valves, and causes these to open on slight stimulus and close when a stronger one is applied. The amount of stimulus necessary to cause the pedicellariae to retreat varies with the kind of pedicellariae, being least with the tridactyle and most with the gemmiform, so that when a chemical stimulus, such as a drop of dilute ammonia, is applied to the skin, the tridactyle pedicellariae may be seen to flee from and the gemmiform to approach the point of stimulation. In a living Sea-urchin, if the attempt is made to seize the tridactyle pedicellariae they will evade the forceps, but the ophiocephalous are easy to catch.

The tridactyle pedicellariae open with the very slightest mechanical stimulus and close with rather greater mechanical stimuli or with exceedingly slight chemical ones. Uexküll calls them "Snap-pedicellariae," and their function is to seize and destroy the minute swimming larvae of various sessile parasitic {509}animals, which would otherwise settle on the delicate exposed ectoderm of the Sea-urchin.

The gemmiform pedicellariae are brought into action when a more serious danger threatens the Sea-urchin, such as an attack of a Starfish. The corrosive chemical influence, which it can be proved exudes not only from the stomach but even from the tube-feet of the Starfish, causes the gemmiform pedicellariae to approach and open widely. When the foe approaches so closely as to touch the sense-organs (Fig. 225, B, _s_) situated on the inner side of the valves of these pedicellariae, the blades close violently, wounding the aggressor and causing its juice to exude, thus producing a renewed and severe chemical stimulation which irritates the poison glands and causes the poison to exude. The virulence of the poison may be gauged from the fact that the bite of a single gemmiform pedicellaria caused a frog's heart to stop beating.

Prouho[473] has described a combat between a Sea-urchin and a Starfish. When the latter approached, the spines of the {510}Sea-urchin diverged widely (strong form of reaction to chemical stimulus), exposing the gemmiform pedicellariae. These at once seized the tube-feet of the enemy and the Starfish retreated, wrenching off the heads of these pedicellariae; then the Starfish returned to the attack and the same result followed, and this was repeated till all the pedicellariae were wrenched off, when the Starfish enwrapped its helpless victim with its stomach.

The minute trifoliate pedicellariae are brought into play by any prolonged general irritation of the skin, such as bright light or a rain of particles of grit or mud. They have the peculiarity that not all the blades close at once, so that an object may be held by two blades and smashed by the third. They may be seen in action if a shower of powdered chalk is poured on the animal, when they seize the particles and by breaking up any incipient lumps reduce the whole to an impalpable powder, which the cilia covering the skin speedily remove. In thus assisting in the removal of mechanical "dirt" they earn the name which Uexküll has bestowed on them, of "cleaning pedicellariae."

The ophicephalous pedicellariae, with their powerful bull-dog grip, assist in holding small animals, such as Crustacea, till the tube-feet can reach them and convey them to the mouth.

{511}The number and variety of the pedicellariae, then, is an eloquent testimony to the dangers to which the soft sensitive skins of the Sea-urchin and other Echinodermata are exposed, and afford confirmatory evidence in support of the view expressed above, that the method adopted to defend the skin was one of the great determining features which led to the division of the Asteroidea into different races.

The CORONA consists of five radial or "ambulacral" bands of plates and five interradial, or as they are usually termed, "interambulacral" bands of plates—ten in all. Each of the ten consists of two vertical rows of plates throughout most of its extent, and each plate is studded with large bosses, or "primary tubercles" for the primary spines, smaller bosses called "secondary tubercles" for the secondary spines, and finally, minute elevations called "miliary tubercles" for the pedicellariae.

{512}

Even in the dried skeleton, however, the ambulacral plates can be discriminated from the interambulacral by the presence of pores to permit the passage of the tube-feet. These pores are arranged in pairs, and each pair corresponds to a single tube-foot, since the canal connecting the ampulla with the external portion of the tube-foot is double in the Echinoidea. In _Echinus esculentus_ there are three pairs of such pores in each plate, in _Strongylocentrotus droëbachiensis_ four pairs. The ambulacral plate is really made up of a series of "pore-plates," each carrying a single pair of pores, and these become united in threes in _Echinus_ and fours in _Strongylocentrotus_, while in primitive forms like the Cidaridae they remain separate. Each ambulacral and interambulacral area ends at the edge of the periproct with a single plate. The plate terminating the ambulacral band is pierced by a single pore for the exit of the median tentacle, which, as in Asteroids, terminates the radial water-vascular canal. Thus the aboral end of the radius in an Echinoid corresponds to the tip of the arm in an Asteroid. The plate is termed "ocular," because the terminal tentacle has a mass of pigmented cells at its base; but no eye-cups can be seen, and there is no evidence that this spot is specially sensitive to light. Species which show special sensitiveness to light have often a large number of what we may perhaps term secondary eyes. The plate terminating the interambulacral {513}series is termed the "genital plate," because it is pierced by the duct of one of the five genital organs. One of the genital plates is also pierced by the madreporic pores. Some zoologists have separated the ocular and the genital plates under the name of "calyx" from the rest of the corona, under a mistaken idea that they are homologous with the plates of the body or calyx of a Crinoid.

The PERIPROCT (Fig. 228, 4) is covered with small plates and bears a few pedicellariae. The PERISTOME (Fig. 229) is covered by flexible skin with abundant pedicellariae; it terminates in a thick lip surrounding the mouth, from which the tips of five white teeth are just seen projecting. There are ten short tube-feet projecting from the peristome—one pair in each radius—and each tube-foot terminates in an oval disc and is capable of little extension, and each has around its base a little plate. The presence of these tube-feet shows that in _Echinus_ the {514}peristome extends outwards beyond the water-vascular ring, whereas in Asteroidea it is contained entirely within the ring. In the primitive Cidaridae (Fig. 235) the whole peristome down to the lip surrounding the mouth is covered with a series of ambulacral and interambulacral plates similar to those forming the corona, though smaller and not immovably united, and the series of tube-feet is continued on to it. It is thus evident that the peristome is merely part of the corona, which has become movable so as to permit of the extension of the teeth. In _Echinus_ the peristome is continued in each interradius into two branched outgrowths called gills, the relation of which to the respiratory function will be described later. These gills (Fig. 229, 2) are situated in indentations of the edge of the corona called "gill-clefts" (Fig. 230, _g_).

The most conspicuous plates in the peristome are those {515}surrounding the buccal tube-feet; besides these, however, there are in _Echinus esculentus_, and probably in most species, a large number of thinner irregularly-scattered plates (Fig. 230).

The term ambulacral plate, applied to the plate pierced by the pores for the tube-feet, conveys a misleading comparison with the ambulacral plate of an Asteroid. In Echinoids the ambulacral groove has become converted into a canal called the "epineural canal," and the ambulacral plates form the floor, not the roof, of this canal; they may perhaps correspond with the adambulacral plates of the Starfish, which one may imagine to have become continually approximated as the groove became narrower until they met.

{516}The internal organs of the Urchin can best be examined by making a horizontal incision about one-third the distance from the mouth and pulling the two parts gently asunder. A large amount of fluid escapes from the exceedingly spacious coelomic cavity, the alimentary canal being comparatively narrow.

The ALIMENTARY CANAL commences with a short vertical tube which has been shown to be a stomodaeum; this is surrounded by the upper ends of the teeth and their supporting ossicles, which are collectively termed "Aristotle's lantern." The oesophagus leads into a baggy, flattened tube, the stomach, which runs horizontally round the animal, supported by strings of tissue from the coelomic wall, so that it hangs down in a series of festoons. Having encircled the animal, it bends directly back on itself and immediately opens into the intestine, which is also a flattened tube, which runs round the circumference of the animal, but in the opposite direction, the festoons of the second circle alternating with those of the first. The intestine opens into a short rectum which ascends vertically to open by the anus. The stomach is accompanied by a small cylindrical tube called the "siphon" (Fig. 231, _siph_), which opens into it at both ends; this represents merely a gutter which has been completely grooved off from the main intestine; it is lined by cilia, and its function is believed to be that of keeping a stream of fresh water flowing through the gut, so as to subserve respiration.

_Echinus esculentus_ seems to feed chiefly on the brown fronds of _Laminaria_ and the small animals found thereon, which it chews up with its teeth, but it may regale itself on the same diet as Brittle Stars, as Allen[474] has shown to be the case in Plymouth Sound. Dohrn[475] has described the Neapolitan _Sphaerechinus granularis_ attacking and capturing Crustacea such as _Squilla_.

The WATER-VASCULAR SYSTEM presents several features of great interest. The ring-canal is situated at a considerable distance above the nerve-ring, and is separated from it by the whole of the jaws and teeth. It has five small interradial pouches on it, which apparently correspond to Tiedemann's bodies in an Asteroid. The stone-canal (Fig. 231) opens as {517}usual into the ring-canal, and is accompanied by the axial sinus and genital stolon. The name "stone-canal" is very unsuitable in this order, for there are no calcifications in its walls; it is a simple membranous tube of circular section. On reaching the upper wall of the test it expands into an ampulla, into which the numerous ciliated pore-canals traversing the madreporite open. The radial canals, starting from the ring-canal, pursue a downward course till they come into contact with the radial nerve-cords, and they then bend upwards and run along the centre of the ambulacral region, finally terminating in the small terminal tentacles. In the just metamorphosed Echinoid these are well-developed tube-feet, each with a well-developed sucker, in the centre of which is a conical sensory prominence, but as development proceeds they become enclosed in a circular outgrowth of the test, so that only the tip projects in the adult.

The long extensible tube-feet are connected by transverse canals with the radial canal. Instead of the pair of valves which in Asteroids prevent the reflux of liquid into the canal, there is a perforated diaphragm[476] with circular muscles, which by contraction close the opening in the diaphragm, while when they are relaxed fluid can return from the tube-foot. The ampulla is flattened, and is contracted by muscular fibres called "trabeculae" stretching across its cavity. These muscular strands are developed by the cells lining the ampulla. The external portion of the tube-foot, as in Asteroids, is provided with powerful longitudinal muscles, and there is the same alternate filling and emptying of the ampulla as the tube-foot is contracted and expanded. The tube-foot is connected by a double canal with the ampulla, the object of which is to assist in respiration. The cells lining it are ciliated, and produce a current up one side of the tube-foot and down the other, and the double canal leading to the ampulla separates these two currents and prevents them interfering with one another. Thus water is continually transported from the ampulla to the tube-foot, through the thin walls of which it absorbs oxygen, and it is then carried back to the ampulla, and transfers its oxygen to the fluid of the general body-cavity through the walls of the ampulla. The disc of the tube-foot is supported by a calcareous plate {518}(Fig. 232, _oss_), a circumstance which enabled Johannes Müller to recognise the Echinoid larva when the form of the adult was as yet unrecognisable. Below the edge of the disc there is a well-marked nerve-ring, from which two bundles of nerve-fibres go to the disc itself, in the edge of which there is an abundance of sense-cells.

The buccal tube-feet (Fig. 229, 4) are much shorter than the rest, and are provided with oval discs which are highly sensory. These feet are not used for seizing, but for tasting food; when a piece of food is placed near them they are thrown into the most violent agitation.

The NERVOUS SYSTEM has the same form as in an Asteroid, viz. that of a ring surrounding the mouth and giving off radial nerve-cords (Fig. 232, _n.r_), one of which accompanies each water-vascular canal to the terminal tentacle, where it forms a nervous cushion in which pigmented cells are embedded.

A large band-like nerve is given off from the radial nerve-cord to each tube-foot. This pedal nerve, as it is called, contains bipolar neurons, and is really an extension of the nerve-cord itself. Beneath the sucker it branches out to form a sensory ring. From the base of the pedal nerve, branches are given off {519}which run to the ectoderm and enter into connexion with the plexus there. Romanes[477] scraped away the radial cords and found that the spines still converged when a point on the ectoderm was stimulated, but that, on the other hand, if definite locomotor movements were to be carried out, the presence of these cords was a necessity; hence he concluded that the superficial plexus sufficed for ordinary reflexes, but that for purposeful movements the central nervous system was necessary.

Von Uexküll[478] has made an exhaustive study of the physiology of the nervous system in the Echinoidea. He points out that all the organs controlled by the nervous system, spines, pedicellariae, tube-feet, and (see below) Aristotle's lantern, give two opposite reactions in response to the same stimulus according as it is strong or weak, bending away from the point of stimulation when it is strong and towards it when it is weak. This reversal of reaction can only be due to the action of the neuron in altering the effect of the stimulus on the muscles, and this Uexküll regards as its fundamental property. Thus in Preyer's[479] experiments with Starfish the strong form of stimulation is obtained by directly applying the stimulus to the radial cord or to the tube-feet, the weak form by stimulating the back, when of course the stimulus has to traverse a longer path before affecting the tube-feet, and is consequently weakened. Von Uexküll also introduces the conception of "tone" with regard to the nervous system. This term has been used to denote the amount of chronic contraction in a muscle, and it is to be distinguished from the fleeting contractions which cause movement. The more tone there is in a muscle the less responsive it is to stimuli tending to bring about movement. As applied to the nervous system "tone" denotes a condition when it is not receptive to small stimuli, but when it is maintaining a condition of tone in a muscle by which of course its own tone is measured. Tone in a neuron can therefore be measured by the produced tone in the muscle, and the one is to be discriminated from the other only by using stimulants, such as caffeine, which have no direct action on muscle. Tone can also be measured by the amount of stimulus necessary to irritate the neuron. When {520}muscles are stretched the tone is lowered, and this loss of tone extends to the neuron controlling the muscle, and _vice versa_. When the spines on being gently stimulated bend towards the point of stimulation, this is due to the contraction of the muscles on the side towards the point of stimulus, for if the superficial plexus of nerve-fibres be cut through so that the stimulus has to pursue a round-about course the spine will bend towards the direction from which the stimulus comes. The bending of the spines away from the stronger stimulus is likewise due to the muscles on the side towards the stimulus. It is caused by a sudden fall of tone in these muscles, which causes them to yield to the tone of the muscles on the opposite side, and this fall of tone is due to a fall of tone in the neurons, for it can be produced by chemicals, and the direct action of all chemicals applied to muscle is to raise tone.

In _Arbacia_ this form of reaction cannot be produced; the spines respond to stimuli of all degrees of intensity by convergence towards the point of stimulation.

When a general skin-irritant like dilute acetic acid, or even strong light, is applied to the skin of a Sea-urchin the spines bend alternately to all points of the compass, or, in a word, rotate. This is due to the fact that the weight of the inclined spine stretches the muscles of one side and so renders them more open to the general stimulus; these muscles in consequence, contract, and so move the spine to a new position in which other muscles are stretched, and a similar result follows. A continuation of this process brings about rotation.

When a piece of glass rod or other light object is laid on the spines of a Sea-urchin, it naturally, by its weight, presses asunder the spines and stretches their muscles on one side, thus lowering the tone. If now the skin be stimulated at any point the piece of rod will be rolled by the spines towards the point of stimulation. This is caused by the fact that the muscles of the spines holding the rod are made more receptive by being stretched, and therefore they contract more than do the others in response to the stimulation, and so the rod is rolled onwards on to the next spines, which then act in the same manner. This passage of stimulus is entirely independent of direct nervous connexion between the bases of the spines, for it will traverse at right angles a crack going clean through the shell; it is merely the {521}result of the mechanical weight of the object and of the juxtaposition of the spines.

If the stimulation be too violent the first spines affected diverge wildly and strike their neighbours with vehemence, so arousing into activity the block musculature of these. This causes them to stand rigidly up, and so the path of the stimulus is barred.

Now the escape movements of the animal under strong stimulation which Romanes[480] alludes to are just an example of this handing on of stimulation from spine to spine, not by nervous connexion but by mechanical touch only; the object in this case is the substratum on which the animal lies, which is, so to speak, rolled towards the point of stimulation, or putting it otherwise, the animal is rolled away from it. Righting when upset is another example of the same phenomenon; the aboral spines are stretched by the weight of the animal, and the animal acts as if it were stimulated in the region of the periproct. When a Sea-urchin is in its normal position and is stimulated in the periproct (as for instance by a strong light), it would, according to this rule, tend to move downwards, which is of course impossible; but as the stimulus never affects all sides quite alike the result is that the Urchin rotates, turning itself ever away from the point of strongest stimulation. In the case of _Strongylocentrotus lividus_ when living on limestone, as on the west coast of Ireland, this results in the animal excavating for itself holes in the rock, where it is safe from the action of the breakers.[481]

But it may be objected that no account is taken in the above description of the action of the "central nervous system," _i.e._ of the ring and the radial cords, and yet Romanes found that when they were removed the escape movements could not be carried out. The answer is that the central nervous system is a store-house of _tone_, not, as in higher animals, a controlling centre for co-ordinating the movements of the spines. When it is removed at first the escape movements can be carried out, but in a day or two all tone in the spine-muscles is lost, and then, since the tone of all is equally low, there is no tendency in those that are stretched to be more responsive than others, and hence the escape movements cannot be carried out. Sea-urchins kept in the tanks {522}of an aquarium are apt to lose the tone of their spines owing to the poisoning of the nervous system.

The central nervous system is, however, the system which controls the movements of the tube-feet. As we have seen, extensions of the radial nerves run to the tip of each podium. Tube-feet are chiefly used in ordinary progression; when this is quickened the spines come into play exclusively. The extent to which these two organs of locomotion are used varies from genus to genus. Thus _Centrostephanus_ uses its spines a good deal, _Echinus_ and _Strongylocentrotus_ very little. The last-named genus sometimes walks on its tube-feet entirely without touching the ground with its spines.