474 RADIATA; ECHINODERMATA, 



ECHINIDJi. 



The covering consists of numerous angular plates, adhering by 

 simple or serrated sutures, and pierced by numerous pores. 

 These pores give passage to the canal of the tentacular pro- 

 cesses whieh are connected with the investing integument. 

 The plates are also covered with tubercular processes, sup- 

 porting moveable spines. The body, when at rest, is sup- 

 ported on the oral disc. 



I. Anocysti. — Vent in the dorsal surface. 



a. Vent central, in the axis of the body. 

 Cidaris. 

 Echinus. 



Clypeus. 



a. a. Vent lateral above the margin. 



(Cassidula. \ 

 Nucleolitis. / 



II. Pleurocysti. — Vent marginal. 



Echinarachnius. 

 Spatangus. 



III. Catocysti. — Vent in the under surface. 

 a. Mouth central. 



b. Avenues of pores limited. 

 Echinocyamus. 



(Echinanthus. \ 

 Echinodiscus./ 



b. b. Avenues of pores complete. 

 Conulus. 

 (Echinoneus) 



a. a. Mouth not central. 

 Echinocorys. 



