500 THE ANATOMY OF INVERTEBRATED ANIMALS. 



this follow five pieces (first radialia) closely united together 

 and with a central piece, which probably represents the basa- 

 lia of other Crinoids. The first radial corresponds in direc- 

 tion with the origin of one of the arms, and is followed by a 

 second and third radial. With the third radial is articulated 

 the first of the brachial ossicles, which constitute the skele- 

 tal support of the unbranched brachia. The pinnules are also 

 supported by a series of elongated calcified joints, the basal 

 joint being articulated with a brachial ossicle and the distal 

 joint pointed. 



The axial canal dilates in the enlarged pyriform ossicle 

 above mentioned ; and, from the dilatation, branches, which 

 traverse the radial and the pinnular ossicles, are given off. 

 There is a calcareous plate in the substance of each oral valve, 

 and minute reticulated calcifications are scattered through the 

 perisoma of the oral face of the disk. 



The sides of the radial grooves are provided throughout 

 with a double series of oval calcareous plates — the marginal 

 lamellce — which are disposed transversely to the groov T e, those 

 of opposite sides alternating with one another. They can be 

 erected or depressed ; and, in the latter case, overlap one an- 

 other like tiles. 



In Pentacrinus, the long stem is fixed by its distal end, 

 and the pentagonal artieidi of its skeleton give off, at inter- 

 vals, whorls of unbranched cirri. No distinct basal piece is 

 known, but the calyx appears to begin with the five first radi- 

 alia. At the third radiale, the series bifurcates into two 

 series of brachialia, and these again bifurcate to give rise to 

 the palmaria, which support the free arms. There are mar- 

 ginal lamellae along the sides of the tentacular grooves, and a 

 longitudinal series of calcareous ossicles occupies the floor of 

 each groove. The anus is situated upon an elevated inter- 

 radial cone. 



The body of an adult Comatula (Antedon) answers to the 

 calyx, with its brachia, in other Crinoids. 



The centre of the skeleton is constituted by a large centro- 

 dorsal ossicle, articulated with the aboral face of which are 

 the numerous cirri, by which the Antedon ordinarily grasps 

 the bodies to which it adheres, though it is able, on occasion, 

 to swim freely about. This centro-dorsal ossicle appears to 

 be the homologue of the uppermost part of the stem in the 

 Pentacrinus. There are five divergent series of radialia, 

 each containing three ossicles. The first radials, or those 

 nearest the centro-dorsal plate, are close'ly adherent to one 



