THE CR1NOIDEA 



plates, deltoidea (A), here regarded as synonymous with oralia (0), 

 alternating with the RR (Fig. Y. 1). But there are nearly always 

 also present " ambulacralia " (Amb or c.p), covering the grooves 

 that lead between or over the apposed edges of the A to the 



P 



Three stages in the evolution of the Teginen. 1, orals only; 2, orals and ambulucruls ; 

 3, orals, ambulacrals, and enlarged peristoniial ambulacrals. 



brachia (Fig, Y. 2). The mouth is either beneath the A or in a 

 space between them ; in the latter case it is covered by ambulacrals, 

 often 5 in number and iuterradial in position, and taken to be 

 orals by some writers (P in Fig. Y. 3). The posterior A in many 

 primitive forms seems to have been pierced by a 

 hydropore, the walls of which may be folded so as 

 to form a madreporite (cf. Stelleroidea and Echi- 

 noidea). The anus (As) lies between post. A and 

 the adjacent RR, and is closed by a valvular 

 pyramid, often surrounded by, or raised on, small 

 plates. 



The Brachia in their simplest form consist of a 

 series of ossicles called brachialui (Br), which con- 

 tinue straight up from the radials (Fig. VI.). The 

 surface of the radial to which the proximal brachial 

 is attached is called the "radial- or arm -facet." 

 Each brachial is rounded on the outer or dorsal sur- 

 face, and grooved on the inner or ventral surface. 

 The ventral or brachial groove contains the follow- 

 A simple, un- j n or so ft parts, taken in order from ventral to 



branched arm, that i -m -TT-T^ r\ i 



of Hybocrinus. R, dorsal (rig. VII.). On the surface, the food-groove 

 chiai ; ' p.p, r 'cover- (f-ff), lined with ciliated epithelium (et), which directs 

 bifiacrais 68 r *'" a stream of water towards the mouth ; an epi- 

 thelial nervous band (nl) stretching from the 

 superficial or oral nerve -system ; a blood-vessel (b), " radial 

 pseudhaemal canal"; a water - vessel (w), which gives off tubes 

 (p) to a series of podia, or " tentacles " (t) that fringe the 

 food-groove and subserve sensation and respiration ; two " sub- 

 tentacular canals " (s.t.c), extensions from the body-cavity ; a canal 



FIG. VI. 



