THE SKIN AND SKELETON. 



131 



calcified 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 to- 

 gether, like mosaic-work. These form ten zones, like the 

 ribs of a melon, five broad ones alternating with five nar- 



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

 t, s, ball-aud-sockei joint ; p, pedicellarise ; 2, 3, transverse sections of spines of 

 Cidaris and Echinus. 



rower ones. The former (called interambulacra) are cov- 

 ered 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 project fleshy sucker-feet. 



The skin of the Lobster is hardened by calcareous de- 

 posit 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 segments, or 

 rings, is usually twenty five in the head, eight in the 

 thorax^ and seven in the abdomen. 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 



