Chapter XXI: Appendix: To Ctenophora (4)

The faculty of vision in its simplest form may be defined as sensitiveness to light and shade. Now strong light acts on all Sea-urchins as a general skin irritant. They fly from it towards the darkest corner, and then if it continues the spines rotate. A number of little violet spines on the aboral pole of _Centrostephanus longispinosus_ are especially sensitive to light, and hence are almost constantly in rotation. This is due, according to Uexküll,[482] to a pigment of a purple colour, which can be extracted by means of alcohol and which is decomposed by light, the products of decomposition being supposed to irritate the nerves. _Centrostephanus_ when exposed to light becomes darker in colour. This is due to the migration outwards of amoebocytes, which carry a pigment which acts as a screen in order to prevent the valuable visual purple being too rapidly decomposed. Not all Sea-urchins, in fact very few of those living in northern waters, give a reaction to shadow. _C. longispinosus_ is one of the few; it reacts to a shadow by converging its spines towards it. A much larger number of species inhabiting tropical waters show this reaction. It is entirely stopped if the radial nerve-cords be removed, whereas the reaction to strong light continues. The reaction to shade is strongest after a long previous exposure to light, hence Uexküll has given the following explanation of it. The continued irritation due to light, having spread to all the spines, eventually reaches the radial cords and is there stored in the bipolar nerve-cells as _tone_. When the light-stimulus is interrupted some of {523}the stored tone spreads upwards to the spines, causing the weak form of spine reaction, and the spines converge.

It will be seen therefore that the so-called central nervous system of _Echinus_ does not act in any sense as a brain, as indeed might have been guessed from the absence of any differentiation in it. As Uexküll points out, when an animal is covered all over with similar organs, such as spines and pedicellariae, capable of acting automatically, a brain is not needed. The object of a brain is to direct organs which are in a certain place to a danger which may come from any quarter, but in the Sea-urchin any spine is as good as any other spine, and such orientation is not needed. "In a dog the animal moves its legs, in a Sea-urchin the legs move the animal." What the Sea-urchin does need is a means to prevent its pedicellariae attacking its own organs with which they may come into contact. Thus it possesses an "autodermin," a chemical contained in the ectoderm which paralyses the muscles of the pedicellariae, as may be seen by offering to them a spine of the same animal. If, however, the spine be treated with boiling water, and then offered, it is viciously seized, showing that this substance can be dissolved out.

Just as in the case of the Starfish, when the nerve-ring is {524}cut through, the tube-feet in the various radii are no longer co-ordinated with one another.

Besides the tips of the tube-feet the Urchin possesses another kind of sense-organ, the SPHAERIDIA (Fig. 233). These are minute glassy spheres of calcareous matter attached by connective tissue to equally minute bosses on the plates of the ambulacra, generally near the middle line. They are in fact diminutive spines, and like the latter are covered with a thick layer of ectoderm, beneath which is a particularly well-developed cushion of nerve-fibrils. Only the layer of muscles which connects a normal spine with its boss is wanting. Although definite experimental proof is lacking, the whole structure of the sphaeridia shows that they belong to the category of "balancing organs." As the animal sways from side to side climbing over uneven ground, the heavier head of the sphaeridia will incline more to one side or to another, and thus exercise a strain on different parts of the sheath, and in this way the animal learns its position with regard to the vertical.

Intervening between the radial nerve-cord and the radial vessel is a single RADIAL PERIHAEMAL CANAL (Fig. 232, _perih_), representing the two parallel canals found in the same position in the Asteroid. The five perihaemal canals lead downwards to a space called the LANTERN-COELOM, surrounding the oesophagus.[483] Since the skeleton of the corona is composed of plates immovably connected together, muscles corresponding to the ambulacral muscles of the Asteroids would be useless, and so the wall of the perihaemal canal remains thin and the side of it turned towards the general coelom develops no muscles, and that turned towards the nerve-cord no nerve-cells. Where, however, the radial nerve enters the nerve-ring, and on the ring itself, an inner layer of nerve-cells is developed from the lantern-coelom which represents the lower or oral portions of the radial perihaemal canals. These cells control the muscles moving the teeth. These canals are originally parts of the lantern-coelom, but in the adult they become closed off from it.

{525}

In the outer wall of this space are developed the calcareous rods forming ARISTOTLE'S LANTERN. These are first: five teeth (Fig. 234, 11), chisel-shaped ossicles of peculiarly hard and close-set calcareous matter, the upper ends (1) pushing out projections of the upper wall of the lantern-coelom. These projections are the growing points of the TEETH, whose lower ends pierce the {526}ectoderm and project into the lower end of the oesophagus. Each tooth is firmly fixed by a pair of ossicles inclined towards one another like the limbs of a V and meeting below. Each ossicle is called an "alveolus," and taken together they form a "jaw." Their upper ends are connected by a pair of ossicles called "epiphyses" (13). These two epiphyses meet in an arch above. The jaws and their contained teeth are situated interradially. Intervening between successive alveoli are radial pieces called "rotulae," which extend directly inwards towards the oesophagus. Above the rotulae are pieces termed "radii" or "compasses" (2), which are not firmly attached to the other pieces but lie loosely in the flexible roof of the lantern-coelom.

The uses of the various components of this structure can be made out from an inspection of the muscles which connect them together.

Overarching each radial perihaemal canal where it leaves the lantern is a bridge of calcareous matter called the "auricula" (Fig. 234, 8). This arises as two rods which meet each other in a pent-house over the canal. It is the only part of the skeleton which can be compared to the ambulacral ossicles of the Asteroidea, and like them it serves as the point of insertion for important muscles. Thus we find (1) protractor (Fig. 234, 7) muscles which arise from the upper ends of the alveoli and are inserted in the auricula; when these contract they tend to push the whole "lantern" outwards so as to expose the tips of the teeth. (2) The retractor muscles (Fig. 234, 6) extend from the auriculae to the lower ends of the jaws and restore the lantern when it has been extruded to its original position. (3) The comminator muscles connect adjacent jaws with one another: these on contraction approximate the pair of jaws into which they are inserted, and it will easily be seen that by the successive contraction of the five comminator muscles a rotating movement of the teeth would be produced which would cause them to exert an action something like that of an auger; by their simultaneous contraction the teeth are brought to a point. (4) The internal and external rotula muscles: these are small muscles which connect the outer side of the epiphysis with the rotula. There are two facets on the epiphysis, which permit it to rock to and fro on the rotula under the action of these muscles. This rocking action must greatly increase the cutting power of the {527}tooth. These muscles are controlled by the nerve-ring and the incipient portions of the radial nerves, which, as we have seen, have an inner layer of nerve-cells. If the nerve-ring be gently stimulated on one side the upper end of the lantern bends away from the spot, causing the lower end, _i.e._, the teeth, to move towards it; but a stronger stimulation produces the opposite effect, just as is the case with spines. But besides these masticatory muscles there are others which have nothing to do with moving the teeth. These muscles are attached to the rods called radii or compasses (Fig. 234, 2),[484] which lie in the upper wall of the lantern-coelom, and may be termed the compass muscles. There are two sets:—(1) The elevator muscles (Fig. 234, 3), which connect the inner ends of the compasses with one another. When these contract, the radii tend to bend upwards at the inner ends and thus raise the roof of the coelom. (2) The depressor muscles (Fig. 234, 4), which run downwards from the forked outer ends of the compasses to the auriculae. Uexküll[485] has shown that the function of these muscles and of the rods to which they are attached is respiratory. These muscles are also controlled by the nerve-ring. If this be stimulated by passing a pin-head into the oesophagus, the roof of the lantern cavity is raised by the contraction of the elevator muscles. This is followed by contraction of the depressor muscles lowering it; the same result may be brought about by placing the animal in water with excess of carbonic acid. The ten branched gills described on p. 514 are outgrowths of the lantern-coelom. When the roof of this cavity is depressed the fluid contents are driven out into the gills, which are thus expanded and then absorb oxygen from the surrounding sea water. When, on the other hand, the roof is raised the aerated water is sucked back into the lantern cavity, and the oxygen passes easily through the thin walls of the lantern into the fluid filling the main coelomic cavity. There are thus two independent respiratory mechanisms in the Sea-urchin, the one being the compass muscles, the other the cilia lining the interior of the tube-feet.

The function of EXCRETION is performed, as in Asteroidea, by {528}the amoebocytes floating in the general coelomic cavity. These in part escape through the thin bases of the gills. In other parts of the body they seem not to succeed in reaching the exterior at all, but to degenerate and to form masses of pigment; the colour of the animal is largely due to these excrementitious substances.

The REPRODUCTIVE SYSTEM, as in the two preceding orders, consists of a vertical pillar, the "genital stolon," and a circular "genital rachis" giving off interradial branches from which the genital organs bud. The genital stolon is developed from the wall of the general coelom near the upper end of the axial sinus; it attains a great development and ultimately completely surrounds the axial sinus, which then appears like the cavity of a glandular tube, the walls of which are constituted by the genital stolon. The compound structure consisting of stolon and axial sinus was actually described as a nephridium by the Sarasins[486] in the case of _Asthenosoma_. Its true nature, however, is shown when the upper end is examined; it is then seen to open into the stone-canal and to be in communication with the ampulla, into which the pore-canals open. Lying alongside the upper end of the axial sinus is the somewhat elongated "madreporic vesicle," or right hydrocoele, which was described by Sarasin as the accessory kidney (_Nebenniere_), since like the axial sinus it is partly enveloped by the genital stolon. Leipoldt,[487] however, showed clearly that it is a completely closed space.

The genital rachis springs from the upper end of the stolon, and as in Asteroids, it lies in the outer wall of a space called the "aboral sinus" (Fig. 234, 20) intervening between it and the test. In adult specimens it seems to degenerate. The genital organs are situated at the ends of five interradial branches of the rachis (Fig. 231, _gon_). Each is an immense tree-like structure consisting of branching tubes, which are lined by the sexual cells. So enormous do they become in the breeding season that they form an article of food among fishermen. The term _esculentus_ is derived from this circumstance. Other species are regularly sold for food as _Frutta di Mare_ (Fruit of the Sea) at Naples, and {529}as "sea eggs" in the West Indian Islands. One female _Echinus esculentus_ will produce 20,000,000 eggs in a season.

The so-called BLOOD SYSTEM is more distinctly developed in Echinoidea than in Asteroidea and Ophiuroidea. There is an oral ring of lymphoid tissue surrounding the oesophagus below the water-vascular ring. From this are given off two strands, the so-called "dorsal" (Fig. 231, _b.v_), and "ventral" vessels (Fig. 234, 16), which run along the two opposite sides of the stomach or first coil of the alimentary canal. The position of these strands suggests that like the lacteals of the human intestine they are channels along which the products of digestion exude from the stomach. The dorsal strand is situated on the same side as the genital stolon, and from it branches are given off which ramify on the surface of the stolon, on account of which this organ, as in Asteroidea, was at one time regarded as a "heart," but the distinction of the stolon from the strands is easily made out. An aboral ring enclosing the genital rachis lies embedded in the septum dividing the aboral sinus (Fig. 234, 20) from the general coelom.

CLASSIFICATION OF ECHINOIDEA.

The Echinoidea are sharply divided into three main orders, which differ from each other profoundly in their habits and structure. These are: (1) The ENDOCYCLICA or Regular Urchins, of which the species just described may be taken as the type. (2) The CLYPEASTROIDEA or Cake-urchins, which are of extremely flattened form, and in which the periproct is shifted from the apical pole so that it is no longer surrounded by the genital plates, while some of the tube-feet of the dorsal surface are flattened so as to serve as gills. (3) The SPATANGOIDEA or Heart-urchins, in which the outline is oval: the periproct is shifted, as in the Cake-urchins, and the dorsal tube-feet are similarly modified; but the Heart-urchins have totally lost Aristotle's lantern, whilst the Cake-urchins have retained it. This strongly-marked cleavage of the group was primarily due, as in all such cases, to the adoption of different habits by different members of the same group. Were we to term the three orders Rock-urchins, Sand-urchins, and Burrowing-urchins, it would not be entirely true, for secondary invasions of the other's territory on {530}the part of each order have undoubtedly taken place; but still the statement would remain roughly true, and would give a fair idea of the differences in habitat which have led to the differentiation of the group.

ORDER I. ENDOCYCLICA (_Regular Urchins_).

The principal variations concern (1) the peristome, (2) the periproct, (3) the corona, (4) Aristotle's lantern and its appendages, (5) the spines, (6) the pedicellariae, and lastly, (7) the tube-feet. We shall consider these points in order.

PERISTOME.—In the vast majority of species this region is covered only with flexible skin in which ten small plates are embedded, pierced by pores for the buccal tube-feet; besides these there are irregularly arranged thin plates. In the Cidaridae both the ambulacral and the interambulacral series of plates are continued on it; these plates differ from those of the corona in being movable on one another. In Echinothuriidae only the ambulacral series of plates is continued on to the peristome. In the case of both these families there are a considerable number of tube-feet within the region of the peristome which may be classed as buccal.

PERIPROCT.—This area, which represents the whole dorsal surface of Asteroidea, is very large in the Cidaridae, where, as in _Echinus_, it is covered with leathery skin in which small plates are embedded. In the Saleniidae it is covered with a single large sur-anal plate, in the edge of which the anus is excavated; in the Arbaciidae it is covered with four valve-like plates; whilst in the remaining species its condition is similar to that described in the case of _Echinus esculentus_.

CORONA.—In Echinothuriidae all the plates are separated by slips of membranous skin, so that the test is flexible. In all other families it is an unyielding cuirass. In the Cidaridae the pore-plates remain separate throughout life, and are therefore identical with the ambulacral plates. These are small and placed in two vertical rows, and so the ambulacra are exceedingly narrow. In Echinothuriidae there is some tendency to adhesion amongst the pore-plates; these are of different sizes, and usually one larger and one smaller adhere to one another. In all other species regular ambulacral plates are formed at least {531}in the lower part of the radii near the peristome by the adhesion of the pore-plates in groups of two, three, or more. Sometimes as many as nine pore-plates may thus adhere.

When adhesion takes place between the pore-plates it is of course preceded by crowding, and this interferes with their equal development. Some which extend so far horizontally as to meet their fellows of the opposite side of the radius are called primary plates; others which are small and wedged in between the larger ones are called demi-plates. Systems of classification have been built up (chiefly by palaeontologists) in which great stress has been laid on how the primaries and secondaries enter into the constitution of the compound plate, but it does not seem to the present author as if this were at all a satisfactory basis for classification. All the pore-plates are primarily equivalent, and the question as to which are interfered with in their growth so as to become secondary is trivial. The so-called Arbacioid type consists of one primary with a secondary on each side; the Diadematoid type of three primaries, with occasionally a secondary between the aboral and the middle primary; and finally the Triplechinoid type of two primaries, with one or more secondaries between them.

ARISTOTLE'S LANTERN.—Under this head we may consider the auriculae and gills as well as the jaws and teeth. In Cidaridae external gills appear to be absent, but from the lantern coelom large radial pouches project upwards into the general coelom cavity. These pouches are supposed to be respiratory, and are termed INTERNAL GILLS or STEWART'S ORGANS.[488] They co-exist with external gills in Echinothuriidae and in Diadematidae, though in the last family they are present only in a vestigial form, two being found in each radius. The auricular arch both in Cidaridae and in Arbaciidae is composed of two pillars which do not meet, but in the last-named family they are based, as in Echinidae, generally on the ambulacral plates, whereas in Cidaridae they arise from the interambulacral plates (the ambulacral plates being here very narrow). The epiphyses are absent in Cidaridae and Arbaciidae, and are imperfect in Diadematidae.

SPINES.—These organs are extraordinarily variable, and {532}usually differ very much in species of the same genus. In the vast majority of species there is a limited number of long spines called "primaries," amongst the bases of which a large number of much shorter "secondaries" are distributed. In Cidaridae the primaries are very long and thick and blunt at the ends, and the secondaries form small circles around their bases. The primaries in Cidaridae and the tips of the primaries in Arbaciidae and Echinothuriidae are covered with a special investment of extremely close, hard, calcareous matter very different from the loosely fenestrated material out of which the bodies of the spines of all species are composed. In _Colobocentrotus_ and _Heterocentrotus_ the primaries are very thick and triangular in section, whilst the secondaries on the aboral surface have expanded outer ends, which form a close-set pavement protecting the ectoderm from the shocks of the breakers. In Echinothuriidae the primaries are short and so delicate as to be termed silky.

PEDICELLARIAE.—In Cidaridae only gemmiform and tridactyle pedicellariae are found. In the gemmiform the glands lie inside the grooved blades instead of outside as normally, and they are covered internally by ingrowths of calcareous matter from the edges. In Echinothuriidae only tridactyle and trifoliate are found in most species, but rudimentary gemmiform are found in one species and well-developed ophicephalous in another. In some species (_Centrostephanus longispinosus_) there are found gemmiform pedicellariae which have lost the jaws but retained the glands. These are termed "globiferae." Mortensen[489] uses minute details in the structure of the pedicellariae to discriminate species and even genera, but in this the present author is not prepared to follow him.

TUBE-FEET.—The tube-feet belonging to the aboral surface are pointed and devoid of a sucker in Diadematidae, Echinothuriidae, Arbaciidae, and Cidaridae;[490] in the last-named family those belonging to the oral surface have suckers, in the centre of which a pointed (sensory) prominence is to be noted.

The CLASSIFICATION of the Endocyclica is by no means in a satisfactory condition, and different authorities have arrived at {533}widely different results. Agassiz,[491] for instance, places the genera _Echinus_ (the common British form) and _Strongylocentrotus_ (the commonest American form) in different families. Bell,[492] on the other hand, considers them to be closely allied. Bell's system, based as it is on the development of the peristome, seems to the present author the most justifiable, for the peristome is undoubtedly a differentiation of the corona, which has been brought about by the manner in which the animal breathes and masticates, two functions of prime importance. The periproct is also of importance, representing as it does the whole aboral surface of the Starfish, and so are to a less extent the arrangements of the spines and of the tube-feet. Proceeding in this way, living Endocyclica can be divided into six families, which are briefly described below.

FAM. 1. CIDARIDAE.—Endocyclica with a large peristome and a large periproct. The peristome is covered with a regular series of both ambulacral and interambulacral plates, the former pierced by tube-feet. No special buccal tube-feet and no external gills. The periproct is large, and is covered with irregular plates (Fig. 236, A). The lantern coelom is provided with large Stewart's organs.

{534}The auriculae are incomplete and consist only of pillars arising from the interambulacral plates. The ambulacral pore-plates remain disunited, and the pores are arranged in a single vertical series; hence the ambulacra are very narrow. The interambulacral plates each bear one large primary spine surrounded by several circles of secondaries. No ophicephalous or trifoliate pedicellariae are to be found, and the gland of the gemmiform pedicellaria is placed inside the concavity of the blade.

The Cidaridae are in many respects the most primitive of the six families living. They are distributed all over the world, and chiefly inhabit deep water. No two naturalists agree as to how they are to be divided into genera. Mortensen,[493] who takes the structure of the pedicellariae as his principal guide, recognises fourteen genera. Others (as for instance Bell) have been inclined to attribute nearly all the living species to one polymorphic genus, _Cidaris_, finding all attempts to divide the genera from one another frustrated by the discovery of transitional forms. _Goniocidaris_ (Fig. 237), however, distinguished by its comparatively broad poriferous zones, by bare places in the middle line of both radii and interradii, and by deep pits on the lines of suture of the plates, is by general consent distinct. This genus is confined to the Eastern Pacific, but from British waters three species of _Cidaris_ have been recorded, only one of which, _C._ (_Dorocidaris_) _papillata_, is at all common. It is found in water from 100 to 500 fathoms in depth {535}off the western coast of Ireland and Scotland. It also occurs in the Mediterranean, and has been carefully examined and described when living by Prouho.[494] From his description it appears that locomotion is effected almost entirely by spines, and that the tube-feet of the lower parts of the radii have each in the centre of the disc a pointed sense-organ like those in the centre of the first tube-feet of the just metamorphosed _Echinus_, whilst those of the aboral surface have no suckers.

FAM. 2. ECHINOTHURIIDAE.—Endocyclica with a large peristome and comparatively small periproct. The peristome has a regular series of ambulacral plates bearing pores for tube-feet, but no interambulacral plates. No specially modified buccal tube-feet, but external gills are present, and internal gills (Stewart's organs) also occur. The periproct is covered with numerous small plates. All the plates of the corona are separated by thin slips of flexible body wall. Numerous comparatively short primary spines on both ambulacral and interambulacral plates; these spines are covered on the tips with a layer of hard dense material.

{536}

This remarkable family is divided by Mortensen into ten genera, based as usual on the pedicellariae, but taking into account also the shape of the tip of hard material on the spines. Most authors refer the majority of the species to two genera, _Phormosoma_ and _Asthenosoma_ (Fig. 238), recognising also a genus _Sperosoma_ for one or two aberrant species. _Asthenosoma_ is distinguished by having wide interspaces of membrane between the plates, and by having ten longitudinal folds of the body-wall, two in each radius, in which powerful longitudinal muscles are developed projecting inwards in the radii. The organs of Stewart are very large. In _Phormosoma_, on the contrary, the interspaces of membrane are very narrow, and the longitudinal folds are thin and membranous and the organs of Stewart are vestigial. _Asthenosoma hystrix_ and _Phormosoma placenta_ have both been dredged in deep water off the Irish coast. _A. urens_, in which there are ectodermic poison-sacs at the bases of the spines, inhabits the Indian Ocean near Ceylon, and was thoroughly {537}described by the Sarasins,[495] who regarded its structure as a proof that Echinoidea were derived from Holothuroidea. Both palaeontology and embryology have, however, yielded strong evidence that Echinoidea were derived from Asteroidea, and hence there is ground for believing that Holothuroidea are descended from primitive Echinoidea and not _vice versa_. The Echinothuriidae may perhaps be regarded as showing the first steps in the change, and though possibly not closely related to the actual ancestors of the Holothuroidea, they at any rate show parallel modifications.

FAM. 3. SALENIIDAE.—Endocyclica with a large peristome and periproct. The peristome is covered with thin, scattered, irregular plates. There are five pairs of special buccal tube-feet, each supported by a special plate, and there are external gills. The periproct is excavated in the side of a large central pentagonal plate. It is covered with fifteen or twenty plates.[496] The ambulacral plates are separate as in the Cidaridae, but occasionally adhere in pairs near the peristome. The interambulacral plates also, as in Cidaridae, each bear one large primary spine surrounded by a {538}circle of secondaries. A few deep-water forms belong to this family, the type genus _Salenia_ (Fig. 240) being the best known. None occur in the British area. Superficially they resemble the Cidaridae, but in reality they are widely separated by the essentially modern character of the peristome.

FAM. 4. ARBACIIDAE.—Endocyclica with a peristome on which, as in Saleniidae, there are only ten prominent plates perforated by the buccal tube-feet, and besides these thin irregular plates; external gills are present, and the auricles consist of incomplete arches springing from the ambulacral plates. The periproct is covered by four valve-like plates. The ambulacral pore-plates are separate near the periproct, but near the peristome unite on the "Arbacioid" pattern (_v._ p. 531) to form secondary plates. The interambulacral plates each carry several spines. No representatives of this remarkable family are known in British waters, but _Arbacia_ is found both on the east coast of North America and in the Mediterranean. It is distinguished by its conical test. All the upper tube-feet are devoid of a sucker; only those on the oral surface are used for locomotion.

Uexküll has studied the Mediterranean species, and has shown that the spines converge no matter how strong the stimulus may be, and so are incapable of aiding in locomotion; also that the ectoderm is devoid of ciliation, and hence the faecal matter which falls on the surface of the animal is not, as in other genera, allowed to fall off by the divergence of the spines nor swept off by the action of the cilia. In its natural habitat the wash of the ripples on the shore cleanses the animal. In captivity it is liable to suffocate itself.

FAM. 5. DIADEMATIDAE.—Endocyclica with a peristome similar to that of the Arbaciidae and the Saleniidae. External gills present and ten buccal tube-feet. Periproct small, covered with numerous small plates. The auricles form complete arches arising from the ambulacral region. Aristotle's lantern is provided with rudimentary Stewart's organs. The ambulacral pore-plates are separated at the apex, but unite orally in {539}"Diadematoid" fashion (p. 531) to form compound plates. The interambulacral plates bear numerous primaries. The aboral tube-feet are pointed, having lost their suckers.

This family is represented (according to Agassiz) at the present day by seven genera, none of which are found in British waters, though one (_Centrostephanus_) enters the Mediterranean. _C. longispinus_[497] was investigated by Uexküll and found to be distinguished by its sensitiveness to light and shade, and by the quickness of its movements, which were mainly carried out by its long spines. The family resembles the Arbaciidae in the pointed aboral tube-feet, but in the complete auriculae it resembles the next family.

FAM. 6. ECHINIDAE.—Endocyclica with peristome and periproct as in the preceding family. External gills and buccal tube-feet present, but Stewart's organs totally absent. Ambulacral plates combined on the "Triplechinoid" plan (p. 531) to form secondary plates. Interambulacral plates with numerous tubercles. All the tube-feet have suckers.

This family contains by far the larger number of living genera. It is divided into two sub-families, viz.:—

(_a_) TEMNOPLEURINAE.—Echinidae in which the plates of the corona dovetail into each other by means of pits and knobs along the line of suture. This sub-family does not occur in British waters; almost all the species are confined to the Indian and Pacific Oceans, but on the east coast of America it is represented by several genera, which however inhabit deep water, _e.g._ _Trigonocidaris arbacina_.

(_b_) ECHININAE.—Echinidae in which the plates meet each other in straight, simple sutures.

This sub-family is represented in British waters by three genera, viz. _Echinus_, _Sphaerechinus_, and _Strongylocentrotus_. _Echinus_ is distinguished by having its pores arranged in arcs of three, owing to the fact that its pore-plates are united in threes to form secondary plates, whilst in the other two genera the ambulacral plates are composed of four or more pore-plates. Six species of _Echinus_ have been recorded from British waters, viz. _E. esculentus_, _E. acutus_, _E. miliaris_, _E. norvegicus_, _E. microstoma_, and _E. elegans_. The validity of the last three is very doubtful. {540}Mortensen[498] regards _E. norvegicus_ and _E. microstoma_ as mere variations of _E. acutus_, and this is probably correct. _E. esculentus_ has already been described; its most marked character is the forest of comparatively short, close-packed, reddish or white primary spines with which it is covered, between the bases of which the delicate secondaries are hard to detect. It is essentially a shallow-water species. _E. acutus_ is distinguished by having much fewer and longer primaries and numerous delicate secondaries. It is an inhabitant of deeper water, being abundant at 100 fathoms, though stragglers are found in shallower water. At the depths at which it lives wave-disturbance can scarcely be felt, and hence the long primaries are not irritated.

_E. elegans_ has spines intermediate in character between those of _E. esculentus_ and those of _E. acutus_. Like the latter it is an inhabitant of the deeper water. It seems to the present author not at all improbable that further research might show that _E. acutus_, _E. elegans_, and _E. esculentus_ are all members of continuous series of forms; certainly the larvae and early development of _E. acutus_ and _E. esculentus_, the extreme members of the series, are strikingly similar.

_E. miliaris_ differs somewhat widely from the other species and is closely allied to _E. microtuberculatus_ of the Mediterranean, from which it is distinguished mainly by the greater thickness of the scattered plates on the peristome of the latter species. From the other British species it differs in its much smaller size and in the greenish hue of its primary spines, which are short and thick and possess purple tips. Its larva is markedly distinct from the larva of _E. esculentus_. _E. miliaris_ is a littoral species, and is found in great numbers in some of the Scottish sea-lochs; when the tide recedes, under every stone of the gravelly beach several specimens will be found. It has a curious habit of "dressing" itself, _i.e._ of covering itself with fragments of dead shell, sea-weed, etc., which are held in position by the aboral tube-feet. This habit aids in concealing the animal, and has probably been developed on account of the dangers to which _E. miliaris_ is exposed owing to its littoral habit of life.

_Sphaerechinus_ differs from _Echinus_ in the structure of the ambulacral plates, in which it agrees with _Strongylocentrotus_, but it is distinguished from this genus by the very deep {541}gill-clefts, or indentations of the edge of the corona from which the gills are extruded. Its most marked peculiarity, however, as shown by both Mortensen and Uexküll, consists in the highly developed character of its gemmiform pedicellariae, on the stalks of which are situated glands. When the head with its poison-glands is torn off, the secretion of these stalk-glands can envelop an enemy with a glutinous secretion, which impedes its movements. The blades on a slight mechanical stimulus divaricate very widely and become locked in this position, so that the enemy's body gets in well within their reach. The muscles of the poison-glands contract, but their ducts are bent by the act of opening, so that the secretion cannot escape. The sense-organs have stiff hairs, which penetrate the surface of the enemy and cause its juices to exude and so stimulate the blades to close, and at the same time permit the poison to be expelled. It will be remembered that the gemmiform pedicellariae of _Echinus_ open in response to a chemical stimulus and close on a mechanical one being superadded; so that their responses are the direct opposite of what occurs in _Sphaerechinus_. _S. granularis_, a Mediterranean species with short red spines, just reaches the Channel Islands.

_Strongylocentrotus_ has shallow gill-clefts and gemmiform pedicellariae, like those of _Echinus_, except that they have a muscular stalk. In the British area it is represented by two species, _S. lividus_, in which the primary spines are markedly longer than the secondaries and are of a brownish purple colour, and _S. droëbachiensis_, in which the primaries are little longer than the secondaries and are of a greenish brown colour. _S. lividus_ occurs abundantly in the Mediterranean, and reaches the English Channel and the west coast of Ireland. In the last-named locality, where it is exposed to the full sweep of the Atlantic, it is said to excavate holes for itself in the limestone rocks, about ten inches in depth.[499] _S. droëbachiensis_, which has been recorded in the British area, chiefly from the west coast of Scotland, is one of the most abundant members of the fauna of the east coast of America. In the Gulf of St. Lawrence and in the branches of the Bay of Fundy it is found in thousands, and {542}is frequently left bare at low tide. It thus takes the place of _E. miliaris_ in the British fauna. An allied if not identical species, _S. purpuratus_, is found in Puget Sound on the Pacific coast.

Other interesting genera of the Echininae are _Echinometra_, _Colobocentrotus_, and _Heterocentrotus_. All possess large, thick primaries, and all are elliptical in outline. In _Echinometra_ the primaries are pointed, and the long axis of the body makes an oblique angle with the axis passing through mouth and madreporite. In _Colobocentrotus_ and _Heterocentrotus_ the axis passing through mouth and madreporite is the short axis of the ellipse, and the primary spines are very thick and triangular in section, whilst the expanded ends of the secondaries form a closely set armour between the bases of these. In _Colobocentrotus_ the test is markedly flattened on the under side, and this flattened area is fringed with a circle of primaries; but in _Heterocentrotus_ there are a few rows of primaries all over the test. These are tropical genera and are found on the outer side of coral reefs, and they require the cuirass of expanded secondaries to protect them against the waves.

ORDER II. CLYPEASTROIDEA (_Cake-urchins_).

The "Cake-urchins" have only one representative in the British area, and this is unsuitable for dissection on account of its small size. We shall therefore select as type the "Sand-dollar" _Echinarachnius parma_ (Figs. 241, 242), which occurs abundantly in shallow water on the east coast of North America. As its popular name implies, this is an extremely flattened Sea-urchin of nearly circular outline, so as to suggest a resemblance to the silver dollar of North American currency. The peristome is exceedingly small, and is placed in the centre of the lower surface (Fig. 241), whilst the periproct is placed on one edge. The outline is not quite circular, for the periproct lies in a slight indentation of the edge; and this side is broader and of a lesser degree of curvature than the opposite one, so that a secondary bilateral symmetry is superimposed on the fundamental radial symmetry common to all Echinoderms. A line drawn so as to pass through the anus and the centre of the disc will divide the animal into two similar halves; the periproct of course lies in {543}an interradius and the axis of symmetry passes through the centre of one radius. We can thus distinguish an anterior group of three radii, or "trivium," from a posterior pair or "bivium." The madreporite lies in the left anterior interradius. The five genitals and five oculars surround a dorso-central plate, which covers the spot which in Endocyclica is occupied by the periproct.

The whole test is covered with extremely short delicate spines, which form a velvety felt-work, and are all of approximately the same length; they are of a brownish purple colour. The spines on the dorsal surface are all ciliated, and these cilia cause a current of fresh sea-water to flow continually over the modified tube-feet. Pedicellariae are scattered amongst the bases of the {544}spines; they are of the tridactyle, the gemmiform, and the ophicephalous types, but they have only two jaws.

The ambulacral areas on the upper surface of the test can be distinguished only by the flattened respiratory tube-feet (Fig. 242, _pod'_), which can be seen protruding from between the spines. Below these areas are clearly marked, for in the centre of each is a well-marked groove proceeding inwards to the peristome. This groove receives lateral branches on its course which traverse the adjacent interambulacral regions. The purpose of these grooves will be explained later. The interambulacral regions do not reach the peristome, which is entirely surrounded by the ambulacral areas. The ambulacral and interambulacral areas both consist of somewhat large hexagonal plates, except in the region of the respiratory tube-feet. Here the pore-plates are not united with one another. This region in each radius is termed {545}a "petal" (Fig. 243, A, _p_), for the respiratory tube-feet are arranged in two rows which diverge from their commencement at the "calyx" and slightly converge again towards the outer margin of the disc, and thus in a dried specimen the two rows of double pores outline an area having some resemblance in shape to the petal of a flower. Besides these double pores for the larger tube-feet there are numerous small single pores for the smaller tube-feet; these are found in all the plates, ambulacral and interambulacral, of the dorsal surface, but in the neighbourhood of the grooves only on the ventral side.

The sphaeridia are only present to the number of one in each radius. Each sphaeridium is enclosed in a pit situated near the edge of the peristome.

A remarkable feature in the skeleton of _Echinarachnius_ which is characteristic in greater or lesser degree of all Clypeastroidea is the presence of vertical partitions of calcareous matter traversing the coelom and stretching from the upper to the lower surface of the test. These are found principally in the peripheral region of the animal; and there can be no doubt that they have originated as cellular bands traversing the coelom, for the formation of similar structures can be followed step by step in the Crinoidea. In the axis of these trabeculae, or folds of the coelomic wall, jelly is secreted, and into this the lime-producing {546}amoebocytes wander. In _Echinarachnius_ these partitions are arranged in groups, each group radiating from a common centre.

The main peculiarities in the structure of _Echinarachnius_ are comprehensible when the species is viewed from above in its normal environment. It is found in comparatively shallow water on a sandy bottom, and normally is nearly but not quite buried in the sand. It might thus be overturned by the force of the waves and currents, and it is protected against this fate by its flattened shape. This shape, however, necessitates some kind of support for the upper part of the test, and this is provided by the internal partitions.

In order to view the internal anatomy of the "Sand-dollar," it is necessary carefully to pick away the dorsal surface of the shell piece by piece. In this way the whole course of the alimentary canal is exposed; as in _Echinus esculentus_ it can be seen to issue from the upper surface of Aristotle's lantern. It then bends sharply to the left, and makes a complete circle round the edge of the disc; this portion is the stomach, and is considerably inflated and accompanied by a "siphon." It then bends sharply back on itself, but only goes half way round; when it reaches the posterior interradius it ends in the anus (Fig. 244).

Aristotle's lantern is greatly simplified as compared with its condition in the Regular Urchins. Both rotulae and compasses are absent; the jaws are sharply bent on themselves, and their appearance gives one the impression that they have shared in the process of compression which the test as a whole has undergone, and have thus become bent. The teeth are nearly horizontal, and they actually articulate with the auriculae, which, as in Cidaridae, consist of disconnected pillars and spring from the plates of the interradius. Each pillar is fused with the adjacent one belonging to the next radius, so that the system which in _Echinus_ consists of five radial arches here consists of five interradial pillars. Aristotle's lantern has lost its respiratory function and apparently its masticatory function as well, for the teeth are used as spades to shovel into the mouth the sand mixed with organic detritus and small organisms on which the animal lives.

The water-vascular system is highly modified. There are two sharply marked kinds of tube-feet—(_a_) the respiratory tube-feet, (_b_) the locomotor tube-feet. Both kinds are terminated by {547}suckers, but the first variety are much larger than the second; they possess a flattened lobed base, and are connected with the ampulla by a double canal. They issue only from the double pores which form the petal. The locomotor tube-feet are small and cylindrical; they are, as already mentioned, scattered over the whole upper surface of the test, penetrating both ambulacral and interambulacral plates, but all are connected by transverse canals with the radial canals of the water-vascular system. On the under surface they are confined to the neighbourhood of the ambulacral grooves, which have nothing to do with the ambulacral grooves of an Asteroid, but are due to secondary localisations of the tube-feet, which are here also connected in each radius with a single radial canal. The appearance of a living _Echinarachnius_ covered with a veritable forest of short brown tube-feet is very striking.[500]

The condition of the water-vascular system is to be explained entirely by the peculiar environment of the animal. The demand for specialised respiratory organs is brought about by the habit of living half buried in the sand. Under these circumstances the strain of supplying the needful oxygen is thrown on the {548}dorsal tube-feet, and they become modified in order to fit them for this function. The locomotor tube-feet are very small and feeble compared with those of _Echinus esculentus_, but this is comprehensible when it is recollected how little resistance the yielding sand would offer to the pull of a powerful tube-foot like that of the Regular Urchins, for in order to move the creature through the sand a multitude of feeble pulls distributed all over its surface is necessary, and the locomotor tube-feet are exactly fitted, both as to size and number, for this object.

The principal points in which Clypeastroidea vary amongst themselves are (1) the nature of the internal skeleton, (2) the shape, and (3) the spines.

INTERNAL SKELETON.—In _Echinocyamus_ and its allies this consists in each interradius of two simple partitions radiating out towards the edge of the disc; in _Laganum_ it consists of walls parallel to the edge of the disc; in _Clypeaster_, of isolated pillars.

SHAPE.—In _Echinocyamus_ the outline is oval and the test comparatively high. In _Clypeaster_ and its allies the outline is pentagonal, and the test is swollen up into a blunt elevation in the centre. In a large number of genera, however, the test is, as in _Echinarachnius_, extremely thin and flat, and the outline may be variously indented. A first indication of this process is seen in _Echinarachnius_ itself, but in _Rotula_ the edge is drawn out into finger-like processes which are all interradial. In _Mellita_ these processes unite with one another distally so as to surround spaces called "lunules," which appear as perforations of the test.

The CLASSIFICATION of the Clypeastroidea adopted by Agassiz is based chiefly on the degree of development of the internal skeleton, and as this is of great physiological importance to the animals we shall follow it here; but since it was published the remarkable discovery has been made of _Pygastrides_, a type previously known only from fossils. We must therefore recognise two sub-orders:—

SUB-ORDER I. PROTOCLYPEASTROIDEA.

Anus on dorsal surface near apical pole. One species, _Pygastrides relictus_,[501] with no "petals," from deep water in the Caribbean Sea.

{549}SUB-ORDER II. EUCLYPEASTROIDEA.

Anus on under surface.

FAM. 1. FIBULARIDAE.—The "petals" are short and imperfect, and the internal skeleton consists of two short outwardly-directed septa in each interradius. To this family the only British Clypeastroid, _Echinocyamus pusillus_, belongs. This animal never exceeds an inch in length, and has an oval outline. It inhabits shallow water, and is often found in the same ground as _Echinus miliaris_, but like all Clypeastroids it prefers a sandy bottom.

FAM. 2. ECHINANTHIDAE or CLYPEASTRIDAE.—"Petals" well marked, internal skeleton consisting of isolated pillars. The largest Cake-urchins belong to this family, which is found chiefly in tropical waters. _Clypeaster_, the great Cake-urchin, with a deeply sunken peristome, belongs to this family.

FAM. 3. LAGANIDAE.—Closely allied to the foregoing, but distinguished by the fact that the internal skeleton consists of walls parallel to the edge of the test. (_Laganum_, _Arachnoides_, _Peronella_.)

FAM. 4. SCUTELLIDAE.—This family includes about half the genera, and is sharply distinguished from all the rest by (1) the extremely flattened shape, (2) the indentation of the outline in the anal interradius and often elsewhere, (3) the branching of ambulacral furrows on the under surface. _Echinarachnius_, taken as the type in describing the anatomy of the Cake-urchins, is the best-known genus. Others are _Mellita_, with five perforations in the edge of the test; and _Rotula_, with the edge produced into a number of finger-like processes.

ORDER III. SPATANGOIDEA (_Heart-urchins_).

As the type we may select _Echinocardium cordatum_, which occurs abundantly in the Clyde and on the west coast of Ireland. The animal is found buried in sand at a depth of about 8-10 inches from the surface. At this depth it lies in a burrow, the walls of which are kept from collapsing by the somewhat broadened tips to the spines. This burrow communicates with the surface by a narrow cylindrical opening similar to the opening of the burrows made by the Clams and other bivalves. A little practice, however, enables one to distinguish the burrow of the Heart-urchin from these.

{550}The animal is about the size of a small potato, and is of light straw colour. Its outline is oval, and the test is about two-thirds as high as the shorter diameter. It is thus higher in proportion to its width than is the case with any living Cake-urchin. The highest point is behind the centre. The narrower end of the animal terminates in a vertical edge, in the upper part of which is a large periproct covered with a number of thin movable plates. The mouth is situated on the under surface, considerably nearer the front end of the test than the hinder end. It is entirely devoid of jaws or of teeth, and also of gills or of a movable peristome.

Aristotle's lantern has entirely disappeared, leaving as the only trace of its former presence a canal with membranous walls encircling the mouth, which has the form of a transverse slit, the posterior lip projecting considerably forward.

The ambulacral areas are easily distinguishable from the interambulacral areas by being comparatively bare of spines. On the upper surface they are distinctly grooved, the groove being especially deep in the case of the anterior one. On the lower surface they coalesce round the mouth, shutting out the interambulacral regions, and are here perforated by the large pores of the buccal tube-feet. Between the two posterior radii on the oral surface there is a space with specially arranged spines called the plastron or sternum. The interambulacral plates composing this region are very much lengthened, and interdigitate with one another at the sutures. To this lengthening is due the apparent forward shift of the mouth. The spines are very characteristic, and are very different from any which have as yet been described. They are the sole organs of locomotion. The primaries are long and curved, with flattened tips, admirably adapted to plough through the sand in which the animal lives. On the upper surface, mingled with the tube-feet, are a large number of small secondary spines. Between the two posterior petals there is a hoop-shaped band of very small black spines. These spines are ciliated, and draw a current of fresh sea-water over the respiratory tube-feet. Beneath the periproct there is a similar band called the "sub-anal fasciole"; this probably produces a current of water which sweeps away the material ejected from the anus.

The pedicellariae are of the trifoliate and gemmiform varieties. {551}The sphaeridia are situated in open pits, one or two in each, situated at the bases of the tube-feet nearest the mouth.

When the upper part of the test is picked away, the course of the alimentary canal is exposed (Fig. 247). It is very similar to the alimentary canal of _Echinarachnius_, except that from the first coil a large blind pouch, called the caecum, is given off.

The water-vascular system shows many characteristic features. The tube-feet are confined to two rows in each ambulacrum, the scattered smaller feet found in such abundance in _Echinarachnius_ being entirely absent. There are four distinct varieties of tube-feet in _Echinocardium_, which are as follows:—(_a_) The respiratory tube-feet of the petals. These have, as in _Echinarachnius_, broad flat bases, but they have lost the sucker. (_b_) The prehensile tube-feet of the anterior ambulacrum. These are enormously long structures, measuring when expanded several times the length of the body. They end in discs, which are frayed out into fingers, so as to look like miniature sea-anemones. These tube-feet are comparatively few in number and are confined to the apical portion of the anterior ambulacrum. (_c_) The buccal tube-feet. These are short, thick, and pointed, and covered with a multitude of club-shaped processes. They are found on all the ambulacra in the neighbourhood of the mouth. (_d_) The degenerate tube-feet found in the portions of the ambulacra between the "floscelle" (see p. 553) and the petals. These are single and pointed, few in number, and issue from single pores in the test.

This extraordinary diversity in the tube-feet is fully explained {552}when the habits of the animal are known. The function of the respiratory tube-feet requires, of course, no special elucidation, but the peculiar anterior ambulacrum was a mystery till the feeding habits of the animal were observed by the late Dr. Robertson[502] of Cumbrae. He found that the animal protruded the long prehensile tube-feet through the opening of the burrow up to the surface of the sand. With their finger-like processes they then collected the surface film of the sand, which was impregnated with Diatoms and other small organisms. When a "handful," so to speak, of this nutritive material has been collected, the long tube-foot is withdrawn down the burrow and passed over the deeply grooved part of the ambulacrum to the buccal tube-feet, to which the food is given up. These last then push it into the mouth. Only one prehensile tube-foot is extended at a time.

The stone-canal is very short and soon opens into the axial sinus; it is widely separated from the pore-canals which traverse the madreporite. Communication between the two is effected by the long axial sinus. There are only four genital organs.

Heart-urchins vary amongst themselves chiefly in the {553}following points, viz.:—(1) the shape and position of the peristome, (2) the characters of the "petals," and (3) the number and position of the fascioles.