Class III. Crinoidea.—Pelmatozoa in which epithecal extensions of the food-grooves, ambulacrals, superficial oral nervous system, blood-vascular and water-vascular systems, coelom and genital system are continued exothecally upon jointed outgrowths of the abactinal thecal plates (brachia), carrying with them extensions of the abactinal nerve-system. The number of these processes is primitively and normally five, but may become less by atrophy. The brachia rise from a corresponding number of thecal plates, “radials (RR).” Below these is always a circlet, or traces of a circlet, of plates alternating with the radials, i.e. interradial, and called “basals (BB).” Through all modifications, which are numerous and vastly divergent, these elements persist. A circlet of radially situate infrabasals (IBB) may also be present. Below BB or IBB there follows a stem, which, however, may be atrophied or totally lost (see fig. 1).
The classification here adopted is that of F.A. Bather (1899), which departs from that of Wachsmuth and Springer mainly in the separation of forms with infrabasals or traces thereof from those in which basals only are present. These two series also differ from each other in the relations of the abactinal nerve-system. O. Jaekel (1894) has divided the crinoids into the orders Cladocrinoidea and Pentacrinoidea, the former being the Camerata of Wachsmuth and Springer (Monocyclica Camerata, Adunata and Dicyclica Camerata of the present classification), and the latter comprising all the rest, in which the arms are either free or only loosely incorporated in the dorsal cup. In minor points there is fair agreement between the American, German and British authors. The families are extinct, except when the contrary is stated.
Sub-class I. Monocyclica.—Crinoidea in which the base consists of BB only, the aboral prolongations of the chambered organ being interradial; new columnals are introduced at the extreme proximal end of the stem.
Order 1. Monocyclica Inadunata.—Monocyclica in which the dorsal cup is confined to the patina and occasional intercalated anals; such ambulacrals or interambulacrals as enter the tegmen remain supra-tegminal and not rigidly united. Families: Hybocrinidae, Stephanocrinidae, Heterocrinidae, Calceocrinidae, Pisocrinidae, Zophocrinidae, Haplocrinidae, Allagecrinidae, Symbathocrinidae, Belemnocrinidae, Plicatocrinidae, Hyocrinidae (recent), Saccocomidae.
Order 2. Adunata.—Monocyclica with dorsal cup primitively confined to the patina and an occasional single anal; tegmen solid; portions of the proximal brachials and their ambulacrals tend to be rigidly incorporated in the theca. Arms fork once to thrice, and bear pinnules on each or on every other brachial. BB fused to 3, 2 or 1. (Eucladocrinus and Acrocrinidae offer peculiar exceptions to this diagnosis.) Families: Platycrinidae, Hexacrinidae, Acrocrinidae.
Order 3. Monocyclica Camerata.—Monocyclica in which the first, and often the succeeding, orders of brachials are incorporated by interbrachials in the dorsal cup, while the corresponding ambulacrals are either incorporated in, or pressed below, the tegmen by interambulacrals; all thecal plates united by suture, somewhat loose in the earliest forms, but speedily becoming close, and producing a rigid theca; mouth and tegminal food-grooves closed; arms pinnulate.
Sub-order i. Melocrinoidea.—RR in contact all round; first brachial usually quadrangular. Families: Glyptocrinidae, Melocrinidae, Patelliocrinidae, Clonocrinidae, Eucalyptocrinidae, Dolatocrinidae.
Sub-order ii. Batocrinoidea.—RR separated by a heptagonal anal; first brachial usually quadrangular. Families: Tanaocrinidae, Xenocrinidae, Carpocrinidae, Barrandeocrinidae, Coelocrinidae, Batocrinidae, Periechocrinidae.
Sub-order iii. Actinocrinoidea.—RR separated by a hexagonal anal; first brachial usually hexagonal. Families: Actinocrinidae, Amphoracrinidae.
Sub-class II. Dicyclica.—Crinoidea in which the base consists of BB and IBB, the latter being liable to atrophy or fusion with the proximale, but the aboral prolongations of the chambered organ are always radial; new columnals may or may not be introduced at the proximal end of the stem.