THE SKIN AND SKELETON 



339 



FIG. 293. Shell of Sea Urchin {Cidaris') without its spines. 



earthy plates. It varies in shape from a sphere to 



a disk, and consists of hundreds of angular pieces 



accurately fitted 



together, like 



mosaic work. 



These form ten 



zones, like the 



ribs of a melon, 



five broad ones 



alternating with 



five narrower 



ones. The 



former (called 



inter- ambulacra) 



i . , 



are covered with 

 tubercles bearing movable spines. The narrow zones 

 (called ambulacra, as they are likened to walks through 



a forest) are 

 pierced with 

 small holes, 

 through 

 which pro- 

 ject fleshy 

 sucker feet. 



The skin 

 of the lobster 

 is hardened 

 by calcareous 

 deposit into 



FIG. 294. Structure of Sea Urchins' Spines (magnified) : i, a, a " CRISt," Or 

 spine of Cidaris cut longitudinally; /, s, ball-and-socket joint; , .. *~. , 

 /, pedicellariae ; 2, 3, transverse sections of spines of Cidaris Snell J ' DUt, 



instead of 



forming one piece, it is divided into a series of seg- 

 ments, which move on each other. The number of these 

 segments, or rings, is usually twenty five in the head, 



