ECHINODERMA. CYSTIDEA 



147 



triangular plates. Between the mouth and the anus is 

 the madreporite (e), which is the external opening of the 

 water- vascular system; just below it is the small, circular 

 genital aperture. All the plates of the calyx are pierced 

 by canals running perpendicularly to the surface ; the 

 canals are in pairs, and the external openings of each 

 pair are enclosed in a raised or depressed area of oval shape 

 (fig. 59 B). 



b — 



Fig. 59. A, Glyptosphcera leuchtenbergi, from the Ordovician of Russia. 

 a, mouth covered by oral plates ; b, food-grooves ; c, facet for the 

 brachiole ; d, anus ; e, just above this is the triangular madreporite, 

 just below is the circular genital aperture (after Volborth). B, a few 

 plates of the same enlarged, showing the pairs of pores. C, plates 

 of Echinosphcera, with pore-rhombs, enlarged. 



Some Cystideans are more primitive in character than 

 the form just described. For example, Aristocytis, from 

 the Ordovician of Bohemia, has an ovoid or pear-shaped 

 body formed of numerous plates, but possesses no food- 

 grooves, brachioles, or stem, and the pores traversing the 

 plates are single. 



In another group of the Cystidea the plates of the 

 calyx are traversed by canals which are arranged in groups 



10—2 



