Fig. 216.—A portion of an arm of Ophiohelus umbella, near the distal extremity, treated with potash to show the skeleton, × 55. The vertebrae are seen to consist of two curved rods united at their ends. The triangular side-plates bear a row of movable hooks which articulate with basal outgrowths of the plate. (After Lyman.)

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.

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.