100 



THF CR1KOIDEA 



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

 alternating with the RR (Fig. V. 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 



As 



Three stages in the evolution of the Tegmen. 1, orals only: 2. orals anil ambulucruls ; 

 8, orals, ambulacrals, and enlarged peristomial ambulacrals. 



brachia (Fig. V. 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 interradial in position, and taken to be 

 orals by some writers (P in Fig. V. 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 brachialia (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. 

 FIG. vi. The ventral or brachial groove contains the follow- 



branched'arm that * n & s0 ^ P arts > ta ^en in order from ventral to 

 of iiybocrinus. ^A 1 , dorsal (Fig. VII.). On the surface, the food-groove 

 (f.g), lined with ciliated epithelium (et), which directs 

 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.f.c), extensions from the body-cavity ; a canal 



covU- 

 r an '" 



