THE SKIN AND SKELETON. 129 
The skeleton of the Star-fish is a leathery skin studded 
with calcareous particles. The Sea-urchin is covered 
with an inflexible 
shell of elaborate 
and beautiful con- 
struction. The 
shell is really a 
calcified skin, be- 
ing a net-work of 
fibrous tissue and 
earthy matter. It 
varies in shape 
from a sphere to 
a disk; and con- 
sists of hundreds 
of angular pieces 
Fie. 96.—Shell of Sea-urchin (Cidaris) without its spines. 
accurately fitted together, like mosaic- 
work. These form ten zones, like the ribs of a melon, 
five broad ones alternating with five narrower ones. The 
former (called interambulacra) are covered with tuber- 
Fig. 97.—Structure of Sea-urchins’ Spines: 1, a, spine of Cidaris cut longitudinally ; 
t, s, ball and socket joint; p, pedicellarie ; 2, 3, transverse sections of spines of 
Cidaris and Echinus. 
cles bearing movable spines. The narrow zones (called 
ambulacra, as they are likened to walks through a forest) 
9 
