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 and Echinus. 
ones. The former (called interambulacra ) are covered 
with tubercles bearing movable spines. The narrow 
zones (called ambulacra , as they are likened to w r alks 
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 cembalo-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 
