Fig. 269.—Tegmen of Hyocrinus, viewed from above after removal of the arms, × 8. (From Wyville Thomson.)
Fam. 2. Rhizocrinidae.—Stem long and persistent; cirri confined to a few near the root or replaced by rooting branches of the stem. Stem-ossicles thin and pentagonal at the summit, but lower down compressed, elliptical in section, and united by two ligaments separated by a transverse ridge for articulation. Patina composed of exposed basals and a ring of five short radials. Orals large or vestigial. Covering plates and interradials present. Two genera, both from great depths in the Atlantic—Rhizocrinus (Fig. 270), with five arms and well developed orals (attachment by branching root-cirri); and Bathycrinus, with ten arms and vestigial orals; attachment by root-like branches of stem (this is essentially the same as root-cirri).
Fig. 270.—Rhizocrinus lofotensis. × 1½. (From Wyville Thomson.)
Fam. 3. Pentacrinidae.—Stem consisting of ossicles which are pentagonal in section, united in pairs by syzygy, the upper one of each pair bearing a whorl of cirri and united by five bundles of fibres of petal-like section with the lower one of the pair above it. No rooting processes. Patina consists almost entirely of columns of radials, the basals being almost or completely hidden. Orals absent, but side-plates in the ambulacral grooves. Two recent genera, Pentacrinus (Isocrinus), with three radials in each column; Metacrinus, with five to eight radials in a column, but the third radial bears a pinnule. Pentacrinus is found in both the Caribbean Sea and the Pacific Ocean; Metacrinus in the Pacific. It appears that the Pentacrinidae when young are attached by a foot-plate at the apex of the stem; but when adult, the stem is broken in two and the animals, like Antedon, swim by movements of the arms, dragging a large part of the stem after them, by which they effect temporary attachment. As in other stalked forms, the cavities of the chambered organ are prolonged into canals which traverse the stalk; but in this family there is the peculiarity that a repetition of the chambered organ is found opposite every whorl of cirri.
Fig. 271.—Pentacrinus asteria. × ¼. (From Wyville Thomson.)
Fam. 4. Holopodidae.—Stem represented by a leathery noncalcified outgrowth from the base of the calyx; one circle of radials indistinguishably fused with the basals and with each other to form the walls of the calyx. Large oral plates, ten short arms. One genus, Holopus, in shallow water in the Caribbean Sea.
Fig. 272.—Arms and portion of stem of Pentacrinus maclearanus, slightly enlarged. In this species the basals can be seen. (From Wyville Thomson.)