96 



STARFISH, SEA UKCHIN, BRITTLE STARS 



luoutli. On rc'iuox'inti; the .spines ami .skin, a beautiful 

 globular shell (Fig. 49) is exposeil, within wliich are the 

 organs of the body. The .shell is composed of limestone 

 plates which fit very clo.sely together and are ornamented on 

 the surface, with rounded protuberances, for the articula- 

 tion of the spines. The plates composing the shell are 

 grouped together in such a manner that the}' form ten dis- 



FiG. 4S. — Sea urchin, showing spines on outside of sheli. 



tinct zones. Five of these zones bear tube feet and five 

 bear none, and each zone alternates with the otlicr. There- 

 fore, although the sea urchin does not possess five distinct 

 arms bearing tube feet, yet the shell presents five distinct 

 areas bearing tube feet, which is comparable witli the 

 structure of the starfish. The sea urchin possesses five 

 long, curved teeth which lie mostly ^^dthin the body, only 

 the points projecting. 



