(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. 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 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 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 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.