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 Cidaria cut longitudinally; 

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

 Cidaris and 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 



