474 
RADIATA; ECHINODERMATA. 
ECHINID^. 
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. / 
h. b. Avenues of pores complete. 
Conulus. 
Y (Echinoneus) 
a, a. Mouth not central. 
Echinocorys. 
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