Fig. 236.—Figure showing periprocts of A, Cidaris; B, Echinus, × 1. amb, Ambulacral plate; g.o, genital opening; g.p, genital plate; inter, interambulacral plate; m.p, madreporite; oc, ocular pore.
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 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.
Fig. 237.—Goniocidaris canaliculata. × 2. (From Wyville Thomson.)
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.
Fig. 238.—Oral view of Asthenosoma hystrix. × ⅔. (From Wyville Thomson.)
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 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.
