ECHINODERMATA. 261 
inverted, stemless Orinoids. The digestive sac is confined 
to the disk, and the madreporic tubercle is concealed. 
Fig. 213. -Ojrtiocoma Russei, an Ophiuran; natural size. West Indies. 
Class III.— Echinoidea. 
The Sea-urchin is encased in a thin, hollow shell cov¬ 
ered with spines, and varying in shape from a sphere to a 
disk. 134 The mouth is underneath, and contains a dental 
apparatus more complicated than that of any other creat¬ 
ure. It leads to a digestive tube, which extends spirally 
to the summit of the body. The spines are for burrow¬ 
ing and locomotion, and are moved by small muscles, each 
being articulated by ball-and-socket joint to a distinct tu¬ 
bercle. When stripped of its spines, the shell (or “test”) 
is seen to be formed of a multitude of pentagonal plates, 
fitted together like a mosaic. 135 Five double rows of plates. 
