ECHINODERMATA. 261 



inverted, stemless Crinoids. The digestive sac is confined 

 to the disk, and the raadreporic tubercle is concealed. 



Fig. 213. - Ophiocoma 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, 



