ECHINODERMATA 



257 



is down and the oral aboral axis is often nearly equal to a 

 horizontal diameter so that the body is nearly spherical. The 

 spines are often long and are movable and are important in 

 locomotion. The tube feet are much as in the starfish. The 

 true sea urchins are vegetable feeders and the mouth is pro- 

 vided with a set of five jaws which form a complicated ap- 

 paratus known as Aristotle's lantern. The intestine is long 



FIG. 147. The basket fish, Astrophyton. X 1/2. 



and coiled and opens by a vent on the aboral surface. In 

 some groups of Echinoidea a secondary bilateral symmetry 

 appears and in these the vent is at the posterior margin of 

 the oral surface. 



556. Class V. Holothuroidea. The sea cucumbers have 

 the principal axis horizontal so the oral-aboral axis is at right 

 17 



