Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/658

63(5 ECHINODERMATA tuberculatus. Goldfuss.) (After the pentacrinoid is at first, while yet included within the pseud- erabyro, and during its earliest fixed stage, surrounded and inclosed by plates of the perisomatic system alone." The predominance of the perisomatic system in the calyx of the older Crinoids and forms allied to them is hence a fact of considerable interest to the embryo- legist. The stem is made up of numerous ossicles articulated and interpenetrated by elastic fibres and soft connective tissue. It is attached at its distal end by a root-like expansion, or by- numerous, filamentous, branched cirri, having joints similar to those of the stem. Other and uubranched cirri are attached in whorls to many of the ossicles of the stem. Through the centre of the stem runs a canal contain ing a soft solid substance. In the adult Antedon, as has been pointed out by Dr Carpenter (Proc. Roy. Soc., 1876), the medullary portion of the Crinoidal axis passes up through a FIG. 22. Cyathocrinus pentangular five-chambered dilatation of its cortical portion within the centro- dorsal plate, and, reaching the cavity of the calyx, forms the pedicle by which chiefly the basal or dorsal surface of the visceral mass is attached to the calyx. The pedicle enters into the axial canal and passes through out its entire length, more or less imbedded in its walls, to the commencement of the subtentacular canals, where it apparently becomes continuous with the generative plexus of the disk. The stem varies in length, being short in Apiocrinus, long in Peutacrinus ; it is round and sometimes moniliform in most Palaeozoic Crinoids, but in Platycrinus compressed as it nears the calyx ; in Woodocrinus it tapers from the calyx downwards. In the Palaeozoic Crinoids the articulations of the ossicles radiate from the central canal, which is larger than in the more modern forms. In the Mesozoic genera the articular facets are commonly united by crescentic or stellate ridges. In the Silurian genus Periechocrinus the ossicles of the stem are alternately thicker and thinner. The calyx, which may be regarded as formed of the upper most ossicles of the stem, is composed of several series of plates. The lowest of these is commonly formed by 2-6 pieces, the basalia, which in Rhizocrinus appear to be re presented by a single central plate. The basalia in the Tesselata are succeeded by the parabasalia or sub-radialia ; after the basalia or these come one or more rows of plates (in Rhizocrinus three), the radial ia (see fig. 23). In Pentacrinus the radialia seem to form the com mencement of the calyx. Supported by the radialia are series of arm plates, or brachialia, from the upper most of which, as in Pentacrinus, may bifurcate the palmaria. The ossicles of tlie arms are sometimes single, sometimes united by syzijgies, or immovable sutures. In the Silu rian genera Anthocrlnus and Crota- locrinus the subdivisions of the arms are very numerous, and by their lateral articulation form web-like expansions. The arms of Poteriocrinus plicatus Fio. 23. Dissection of bifurcate 4 times, giving 80 rays ; the cal y x of , Lecanocrinus . , , , .!_. macropetalus (after total number or plates in that species Hall) has been estimated at 1300 (see J. c , robrad iaiia mcceedtag the G. Greilfell, Rep. Brit. ASSOC., 1875, central basalia; d, d , d", (!~ -r .1 * j.1. /n radialia ; /, /, intcrradiala. p. 60). lu the calyx of the Tesse lata there are plates, interradialia, present between the radialia. In Antedon the central portion of the viscera is contained in a basin formed by the 1st, 2d, and 3d radialia, and by the 1st and 2d pairs of brachialia, and the basal segments of the pinnules borne by the second; and the calycine cavity is completed by the perisome uniting the basal segments of the arms. Where, as in the Articulata, represented by the modern Crinoids, the disk is more or less membranous, four or five deep furrows radiate upon its sur face from the mouth, which pass on to the oral surface of the arms and extend to the extremity of their pinnules. They carry the hollow ambulacral tentacles, which pass out through pores in the perisome. In 1865 Mr J. Rofe demonstrated (Geol. Jfuy., ii. 245) in the case of seve ral genera .of Tesselate Crinoids from the Mountain Lime stone (Actinocrinus, Ampkoracrinus, Cyathocrinus, and Platycrinus) that the groove on the upper surface of the arms divides at their base into two channels (1) a superior channel passing up beneath (in some cases partly within) the plates of the dome or di.sk to its apex, and there uniting with an internal circular aperture, probably the mouth ; and (2) an inferior channel which goes direct into the visceral cavity. These channels, since their discovery by Mr Piofe, have been shown to be generally present in the Tesselate Crinoids. The superior channels, on the supposi tion that the central opening is a mouth, doubtless served for the supply of food and of water for respiration ; whilst the inferior channels probably gave passage to the motor muscles of the arms, and placed the visceral cavity in con nection with the ovaries, if the latter, as in modern Crinoids, were situated in the arms. In the Paleozoic genus Rhodocrinus the arm is cylindrical, and without a groove on the upper surface, but immediately below its base is situated the orifice of a passage which turns upwards under the dome. What in the Palaeozoic Crinoids is commonly regarded as the anal opening, is situated at the extremity of a proboscidiform tube (fig. 24) inter- radially placed, and often of great length as much as 4-J inches in Poteriocrinus plicatus. In existing Crinoids there are two apertures in the disk the mouth, usually central, as in Rhizo crinus, and the interraclially situated anus. The mouth is closed by lobes of the perisome, the oral valves, which may contain calcareous plates. Between these run the oral or ambulacral grooves from the mouth to the arms. In Antedon (Comatula) the alimentary canal passes obliquely downwards from the mouth, then horizontally, and after more than a complete turn bends up wards again, and ends in a rectal cham ber terminating in a spout-like promi nence. Between the exterior of the mucous wall of the alimentary canal and its peritoneal covering is the in tramural space. The double wall of the canal is strengthened by calcareous disks; and it is by the folding of the inner side of the wall, and the resultant piling together of layers of these plates that Fia. 24. the vertical columella is produced. The body cavity is lined by a smooth peritoneal membrane. The ambulacral furrows are bordered by plates, the ambulacral or marginal lamella , as in Rhizocrinus and Pentacrinus, or, as in Antedon,^ by elevated ridges of the perisome, produced at the edge into a series of small lobes or valvules, and having grouped on their inner side the pedicels. The epithelial fioor of the grooves, there is good reason to believe, is lined with cilia, which, like those of the gullet, serve to create currents in the water and thus to bring into the mouth Diatomacese, spores of Algoe, minute Entomostraca, and other nutritive material. In Antedon, as has been shown by Dr Carpenter Dendrocri-

A, calyx; B, proi