ARCHICCELOMA TA. 



Fig- 99. — A Common Sea-Urchin (Echinus 



Microstoma). 



Natural size (After Wyvili.e Thomson). 



The animal is seen from the aboral surface, from the left 

 half of which the spines have been removed. The plates can be 

 identitied from the next figure. 



Fig. 100. — Diagram of Dorsal View of Echinus 



SHOWING THE PlATES. 



Genital Plate. - 

 Madreporite. — 



Antambulacral Plates. 



Anus. 

 _ Ocular Plate. 



Ambulacral Plates. 



In the sand-dwelling types, or Heart-urchins, such as Spatangus, 

 there are no lantern nor teeth, and the body becomes piano- 

 symmetric. 



