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- 
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 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 u 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 cejphalo-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 
