Condition of the Stem.—This is represented by a centro-dorsal stump in Antedon and most of its allies, but in Actinometra it becomes a flat plate, and in some species in old age all the cirri drop off. In Uintacrinus and Marsupites (fossil genera) there is no trace of cirri. In Pentacrinidae there is a long stem, pentagonal in cross-section, in which alternate ossicles carry whorls of cirri; in Rhizocrinidae the stem consists of compressed ossicles, elliptical in section, bearing cirri only at the rooting tip, whilst in Hyocrinus the stem is made up of cylindrical ossicles, cirri being apparently absent. Finally, in Holopus the stem is represented by an uncalcified leathery outgrowth from the calyx.
Skeleton of Calyx and Arm.—In living Crinoidea, with the doubtful exceptions of Holopus and Hyocrinus, the calyx is supposed to be built up originally of four whorls of plates, viz. "infra-basals," "basals," "radials," and "orals," the last named forming the skeleton of the oral valves round the mouth. In the two exceptions named there is no certain evidence of the existence of infra-basals. In living forms the infra-basals coalesce with the uppermost joint of the stem; the basals remain large and conspicuous, though they are fused into a ring in Rhizocrinidae, Atelecrinus, and Thaumatocrinus, whilst in Pentacrinidae this ring is nearly, and in Antedon and its allies completely, hidden when the calyx is viewed from the outside. In Hyocrinus the basals are represented by three ossicles. The lowest radials are an important element in the patina in every case, but the upper radials, the incipient portions of the arms, may be incorporated in the calyx (Pentacrinus, Antedon) or may be free (Rhizocrinidae and Hyocrinus); in Metacrinus there are five to eight radials in each column, all incorporated.
The oral plates are very large in Hyocrinus, Holopus, and Thaumatocrinus, small in Rhizocrinus, vestigial or absent in Bathycrinus, and completely absorbed in Antedon. In addition to these main elements, in many species small accessory plates are developed (a) at the sides of the ambulacral groove, over which they can close down (many species of Antedon, Hyocrinus, Holopus, Rhizocrinidae, some species of Pentacrinidae); these are "covering plates," and correspond in function to the adambulacrals of Asteroidea; (b) supporting the sides of the groove and corresponding to the ambulacrals of Asteroidea;[[514]] these are "side-plates," and the covering plates articulate with them; (c) on the surface of the tegmen; these are the interradial plates, which in Thaumatocrinus alone among recent forms, but in many fossil forms, are continued into the patina, where they separate the radial plates.
Mode of Branching of the Arms.—All modern Crinoids have pinnules, and this, as has already been explained, is due to a suppressed dichotomy. The extent of the suppression determines the number of arms, which varies within the same genus.
Alimentary Canal.—In Hyocrinus there is no dilatation which could be called a stomach; in Actinometra the mouth is excentric, and the anal papilla occupies the centre of the tegmen. The intestine is elongated, and describes several turns round the papilla before ending in the anus.
The classification of Crinoidea cannot properly be considered without taking account of fossil forms, but to do so at all adequately is impossible on account of limitations of space. Less regret may be felt because the three specialists in this branch, viz. Bather in England, Springer in America, and Jaekel in Germany, come to fundamentally different conclusions on the subject. If we confine our attention to living forms we may, with P. H. Carpenter,[[515]] select the stem as the basis of classification. As the method of gaining food is the same in all cases, the Crinoidea have probably split on the method of attachment to the substratum. These families—it is impossible, in view of the greater range of variety in fossils, to dignify them with the name of orders—are as follow:—
Fam. 1. Hyocrinidae.—Stem long and persistent; cirri absent; stem ossicles cylindrical—ligaments uniting them not specialised. Arms (five) short, but with extremely long pinnules. Patina composed of long exposed basals and a ring of five spade-shaped radials. Five large persistent orals. Interradials and covering plates present. One species (Hyocrinus bethellianus, Figs. 268, 269) dredged up in the Southern Pacific Ocean.
Fig. 268.—Hyocrinus bethellianus. × 2. (From Wyville Thomson.)
