CRINOIDEA 



579 



radiate across the tegmen, each running to one of the pairs of arms, 

 to supply which it bifurcates. The groove on each arm gives a branch 

 to each pinnule. Along their whole course the grooves bear a row of 

 finger-shaped podia on each side, and they and the podia are ciliated. 

 Down this system a current set up by the cilia conveys particles 

 gathered from the water to the mouth for food. The podia can only 



trb. ^'^•' ^i^"*' 



cir. 



cir.g. 



I^ig' 434- A transverse section through the disc and base of an arm of 

 Antedon rosacea. After Ludwig. «/. various sections of alimentary canal; ax.coe. 

 axial part of perivisceral coelom, free from trabeculae; Br^, Br^, first and 

 second brachial plates; CD. centrodorsal piece; ch.on. "chambered organ" 

 in centre of aboral nervous system; cir. cirri; cir.g. branches of chambered 

 organ and genital stolon in cirri; coe. coelomic canals of arm; d.ra.n. radial 

 nerve of aboral nervous system; epm. epithelium of ambulacral groove; 

 g.sto. genital stolon; M. mouth; rn. muscles connecting radial and brachial 

 plates; n. nervous layer of ambulacral groove; or.wv. circumoral water 

 vascular ring, giving off, on the left in the figure, a canal to one of the tube 

 feet around the mouth; por. pore canals; R^, R^, R^, first, second and third 

 radial plates; ra.wv. radial water canal; stc. stone canals; trb. trabeculae tra- 

 versing the coelom. 



serve for respiration and to increase the ciliated surface : they are not 

 prehensile, and it is said that only those around the mouth are sensory. 

 Everywhere except in the grooves, the ectoderm is vestigial and 

 cuticulate like that of the Ophiuroidea. The dermis, which on the 

 oral side is merely leathery, contains on the aboral side a skeleton of 

 large ossicles. This consists of: (i) the centrodorsal; (2) the small 

 rosette (formed by the fusion of five larval pieces known as basals), 

 which is internal and roofs a cavity, presently to be described, in the 



37-2 



