Class II. Thecoidea (Jaekel) = Edrioasteroidea (Bather).—Pelmatozoa without a stalk, fixed to the substratum by the whole aboral surface. The radial canals run out over the oral surface in grooves, which are closed by specially modified plates; but there are no arms of any kind.
Class III. Carpoidea (Jaekel).—Pelmatozoa with a well-developed stalk. The radial canals and their branches are devoid of a skeleton, and either produce no modifications at all on the skeleton of the calyx, or at most are supported by short horn-like outgrowths of some of its plates.
Class IV. Cystoidea.—Pelmatozoa which typically possess a well-developed stalk, a sac-like calyx contracted at the mouth and covered with plates, some of which are pierced with pores or slits; the radial canals, though they may for part of their course run over the surface in grooves, have their terminal portions supported by free unbranched arms ("fingers").
Class V. Blastoidea.—Pelmatozoa provided with a well-developed stalk and ovoid bud-like calyx. From the mouth the radial canals run backwards over the calyx, as in Echinoidea, but they give rise to numerous lateral branches, which are supported by free unbranched arms ("fingers"). Special respiratory organs occur on the interradial areas in the form of parallel folds called "hydrospires."
CLASS I. CRINOIDEA
This is the only class which has living representatives. There are twelve recent genera, of which eight retain the stalk throughout life; the remaining four lose it when adult, retaining only a stump, termed the "centro-dorsal," covered with fixing organs ("cirri"). The stalked forms are confined to considerable depths, and can only be obtained by deep dredging, whereas many of the stalkless forms are comparatively common. We shall select as type for special description the common Feather-star, Antedon rosacea (bifida), which can be dredged in depths of ten fathoms off the south-west coast of England.
Fig. 264.—Oral view of Antedon rosacea. × 3. a, Arm; a.g, ambulacral groove; an, anus; c, calyx; m, mouth; p, pinnules.
The animal consists of a small flattened calyx, from which radiate out ten long delicate arms, each fringed with a double series of short branches called "pinnules." In the centre of the aboral surface can be seen the centre-dorsal plate (Fig. 265, c), a knob-like stump of the broken-off stem, covered with small whip-like outgrowths called "cirri," by means of which the animal is anchored to the substratum (Fig. 265, cir). When Antedon is disturbed it relaxes its hold, and swims by graceful muscular movements of the arms. These are arranged in five pairs, and the corresponding members (right and left) of all the pairs are bent and relaxed together. On coming to rest the animal reattaches itself by means of the cirri. These are composed of cylindrical ossicles joined to one another by muscles, and they can thus act as efficient grasping organs. In the centre of the oral surface, which is termed the "tegmen," and is soft, flexible, and without visible calcifications, is situated the mouth, surrounded by five short triangular flaps called "oral valves." In the intervals between these valves, grooves radiate from the mouth which bifurcate at the points of origin of each pair of arms, and are continued over their surfaces. These grooves correspond to the ambulacral grooves of Asteroidea, and to the epineural canals of the other classes of Eleutherozoa. At each side of each groove are to be found a series of podia in the form of delicate finger-like processes, which serve only for respiration and for producing a current of water, their surfaces, like that of the grooves between them, being covered with powerful cilia. The anus is at the extremity of a little knob called the anal papilla, situated in one of the interradii (Fig. 264, an).
As in Ophiuroidea, the ectoderm cells have disappeared over the whole surface of the body, except the grooves and the podia, the only trace of their former existence being a cuticle with adherent nuclei. Pedicellariae are unknown in all Pelmatozoa; and spines have only been described from one fossil species of Crinoid. Beneath the cuticle is the dermis, having the composition described in the case of Asterias rubens; this on the aboral side of the calyx gives rise to the "patina," consisting of plates, in part movable on one another, in part immovably fused together. Those visible from the outside are (1) the centro-dorsal ossicle, from which the cirri spring; (2) five columns of ossicles termed radials (Fig. 266, R1, R2, R3); each column consists of three radials, extending from the centro-dorsal to the origin of a pair of arms. The uppermost radial in each column bears two facets for the articulation of these arms. Each arm is supported by a series of "brachial ossicles" (Br).