9(3 



STARFISH, SEA URCHIN, BRITTLE STARS 



month. On ixMiiovin"; tlic spines and skin, a beautiful 

 globular shell (Fig. 49) is exposed, within which are the 

 organs of the body. The shell is composed of limestone 

 plates which ht very closely t(jgether and are ornamented on 

 the surface, with rounded protujjerances, for the articula- 

 tion of the si)ines. Tlie plates composing the shell are 

 grouped together in sucli a manner that they form ten dis- 



ElG. -IS. -Sea urcliin, .showing spines on outsii.Ie of slje 



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

 bear none, and eacli zone alternates witli the other. Tliere- 

 foi'c, although the sea urchin does not possess fi\'o distinct 

 arms bearing tube feet, yet the shell ])resents five distinct 

 areas bearing tul:)e feet, which is couiparable with the 

 sti'ucture of the stai'fish. The sea. ui'chiu ])ossesses five 

 long, curved teeth which lie mostly within the body, only 

 the pouits projecting. 



