260 



SUBKIKGDOM ECHIHODEBMATA. 



. IAS. 



CLASS II. ECHINOIDEA. 



General Characteristics. The Echinoids (hedgehog- 

 like) are covered with spines, moved by the general envelop- 

 ing membrane. 

 Their shell is 

 composed of reg- 

 ularly-shaped 

 plates, arranged 

 in radiating 

 zones, every al- 

 ternate plate per- 

 forated for the 

 passage of suck- 

 ers (ambulacra), 

 which can be ex- 

 tended beyond 

 the spines,* and 



Echinus spfmra, Sea Urchin. Spines removed from left half. "J Adhesion en- 

 able the animal 

 Mingled with the spines are stems 



to move over the rocks. 

 (pedicillarice), each 

 ending in forceps, 

 apparently for pre- 

 hension and for rid- 

 ding the animal of 

 parasites. The 

 mouth is armed 

 with five sharp 

 teeth, which con- 

 tinually wear upon 

 one another, but, 



* A full-grown Urchin 

 may have five hundred 

 plates, four thousand 

 spines, and perhaps two 

 thousand suckers. 



Fig. UUQ. 



Meltita qmnqueftira, K^y-hole Urchin. 



