ARCHICELOMATA. 173 
Fig. 99.—A ComMMon SEA-URCHIN ( Echinus 
Microstoma). 
Natural size (After WyviLte 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. - ‘ pete 
Fig. 100.—D1acGRraM OF Dorsat Virw or EcHINus 
SHOWING THE PLATES. 
Genital Plate. 
Madreporite. Anus. 
Ocular Plate. 
Antambulacral Plates. 
Ambulacral Plates. 
In the sand-dwelling types, or Heart-urchins, such as Spatangus, 
there are no Jantern nor teeth, and the body becomes plano- 
symmetric. 
