The dorsal surface of the "calyx" of the Crinoidea is corn posed of a number of calcareous plates, accurately fitted together, and having the following general arrangement (fig. 104). Resting directly upon the summit of the highest joint of the column is a series of plates, generally three or five in number, which from their position are termed the "basals" (fig. 104, b). Succeeding to the basals, and alternating with them, there is commonly found a second cycle of polygonal plates, which are generally termed the "parabasals" (fig. 104, p), and which in many forms are never developed.* Succeeding to the parabasals (or, in the absence of these, to the basals) are two or three cycles of plates, which are directly superimposed upon one another in longitudinal rows, and which form the foundations of the arms. These are known as the "radials" (fig. 104, r), and are termed "primary radials," "secondary radials," and "tertiary radials," according to their distance from the basals. The last radial plates, or those furthest from the column, give origin to the plates of the arms. The radial plates are arranged in a series of vertical columns, which radiate from the summit of the basals to the bases of the arms. Between the different columns of radial plates, however, there may be intercalated certain other smaller plates, which, from their position, are termed "inter-radials" (fig. 104, i); while one of the inter-radial spaces, corresponding with the anus, is usually much wider than the others, and is furnished with an additional series of calcareous pieces, which are termed "anal plates "(fig. 104, a). Of the living stalked Crinoids, the best known is the Penta-crinus caput - Medusa of the Caribbean Sea. Another West Indian form is the curious sessile Holopus. More recently a stalked Crinoid has been discovered in the Atlantic and North Sea, and has been described under the name of Rhizocrinus Lofotensis (fig. 101). The chief interest of this form is the fact that it belongs to a group of the Crinoidea hitherto believed to be exclusively confined to the Mesozoic rocks - viz., the Apiocrinidae. or " Pear-encrinites." In fact, Rhizocrinus is very closely allied to the Cretaceous genus Bourgueticrinus, and it may even be doubted if it is generically separable from it. The late remarkable researches into the life of the deeper parts of the ocean have brought to life several new Crinoids, which will doubtless, when fully investigated, still further fill up the interval between the living and extinct Crinoidea. Amongst these may be mentioned Pentacrinus Wyville-Thomsoni, Bathy-crinus gracilis, and Hyocrinus Bethellianus.
Fig. 102. - Portion of an arm of Platycrinus, showing the lateral pinnulae.
Fig. 103. - Platycrinus tricontadactylus. Carboniferous. The left-hand figure shows the calyx, arms, and upper part of the stem; and the figure next this shows the surface of one of the joints of the column. The right-hand figure shows the proboscis.
* According to the high authority of Mr P. H. Carpenter, when there is only one cycle of plates between the top column-joint and the primary radials, it is the so-called "basals" (or "under-basals") which are wanting, and the cycle that is present consists of plates corresponding with the "parabasals " of such Crinoids as have two inferior cycles of calycine plates.
Fig. 104. - Diagram of the dissected calyx of Rhodocrimis, viewed from below (after Schultze). b Basals; p Parabasals; r First radials; i Inter - radials; a Anal plates.
In the second type of the Crinoidea - represented in our seas by the forms which are commonly known as "Feather-stars," and which are grouped together under the general name of Comatula - the animal is not permanently fixed, but is only attached by a stalk when young. Taking the British Comatula (Antedon) rosacea as the type of this group, the larva, after various preliminary embryonic changes, appears as a small stalked Crinoid (fig. 105, b), in which state it was described as a distinct species under the name of Pentacrinus Europaeus.
Fig. 105. - Crinoidea. Comatula (Antedon) rosacea, the Feather-star. a Free adult; b Fixed young. (After Forbes.)
In its adult condition, however, the Comatula (fig. 105, a) is free, and consists of a pentagonal disc, which gives origin to ten slender arms, which are fringed with many marginal pinnulae. The mouth and anus are on the ventral surface of the disc, which in this case is again the inferior surface, since the animal has the power of creeping about by means of its pinnated arms. Though capable of creeping, the animal more usually has recourse to swimming, the five left arms working as paddles simultaneously, and alternating in their action with the five right arms. The arms of Comatula rosacea exhibit-on their ventral surface a deep "brachial groove," the elevated margins of which are cut out into minute crescentic respiratory leaves, at the base of each of which is a group of three tentacles, connected with a cavity in the interior of the respiratory leaf, and communicating by a common trunk with the radiating ambu-lacral vessel. The floor of the brachial grooves is ciliated, and underneath each runs a radiating ambulacral vessel, together with a blood - vascular trunk, and a peculiar fibrillar "subepithelial band," which is supposed to be of a nervous nature. In some Comatulids (certain Actinometrae) there is the curious feature that some of the arms in some individuals may want the ventral grooves, tentacles, and nerves, while in other individuals all the arms possess these structures. The dorsal surface of the calyx, again, carries a tuft of jointed filaments or cirri, by which the animal is enabled to moor itself temporarily to foreign objects.