THE SKIN AND SKELETON. 131 



skin, being a net- work of fibrous tissue and earthy matter. 

 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 



FIG. 97. Structure of Sea-urchins' Spines: 1, a, spine of Cidaris cut longitudinally; 

 t, s, ball-and-socket joint ; p, pedicellariae ; 2, 3, transverse sections of spines of 

 Cidaris aud Echinus. 



ones. The former (called interambulacra) 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 the animal sends out fleshy sucker-feet. 



The skin of the Crab and Lobster is hardened by cal- 

 careous deposit into a "crust," or shell; 73 but, instead of 

 forming one piece, it is divided into a series of segments, 

 which move on each other. The number of these seg- 

 ments, or rings, is usually twenty-one to the head, tho- 

 rax, and abdomen, seven each. In the adult, however, 

 the rings of the head and thorax are often soldered to- 

 gether into one shield, called cephalo-thorax ; and in the 

 Horseshoe Crab the abdominal rings are also united. The 

 shell of Crustaceans is periodically cast off, for the ani- 

 mals continue to grow even after they have reached their 



