ECHINODERMA. ECHINOIDEA 



117 



periproct is encircled by a ring formed of ten plates, five 

 are called genital (g) and five ocular (o). The genital plates 

 form the inner part of the ring ; they are often more or 

 less hexagonal in outline, and are usually provided with a 

 perforation which serves as the opening for the genital 

 ducts — whence their name ; one is pierced by numerous 

 pores and is the madreporic plate (m). Outside the genital 

 plates and alternating with them are the ocular plates ; 

 these are smaller than the genital and usually triangular 

 or pentagonal, and each has a perforation through which 



■— m 



s.— m 



Fig. 47. Some types of apical disc. A. Peltastes wrighti, Lower 

 Greensand. B. Echinocorys vulgaris, Upper Chalk. C. Collyrites 

 bicordata, Corallian. D. Palceechinus, Carboniferous Limestone. 

 E. Galerites subrotundus, Chalk. In the figures the ocular plates 

 are distinguished by dots, the genital plates by lines, m, madreporic 

 plate; a, anus; b, sur-anal plate. All enlarged. 



the termination of the radial water- vessel projects, bearing 

 at its tip a rudimentary visual organ 1 . 



1 The genital plates are sometimes termed basals and the oculars are 

 also known as radials, since, by some authors, they have been considered 

 to represent the plates which bear those names in other groups of the 

 Echinoderma. It is more probable that, although occupying similar 

 positions, they have originated independently in the different groups. 



