34 Descriptive Zoology. 



joint, and is capable of a limited motion in any direction 

 by means of muscles attached around the base. Between 

 the spines are the pincherlike pedicellariae, as in the star- 

 fish, only here they have three blades instead of two. 



The Water Tube System. The sea urchin has a water 

 tube or water vascular system essentially like that of the 

 starfish. There is a water ring around the gullet, with 

 radiating tubes between the rows of feet along the ambu- 

 lacral rows. Inside the shell are water bulbs, or ampullae, 

 as in the starfish, and the mechanism for the operation of 

 these parts is as described for the starfish. 



How the Sea Urchin Locomotes. The injected feet are 

 extended and attached by means of the suckerlike action 

 at the end of each foot. Then the muscular shortening of 

 the feet pulls the sea urchin along. This means of pro- 

 gression is also aided considerably by the movements of the 

 spines. The sea urchin can climb perpendicular surfaces. 

 When placed on the aboral surface, it can turn over, though 

 it is a very slow process. Sea urchins are sometimes found 

 in deep holes in rocks, and it is believed that they have 

 gradually made the holes. 



Digestive System of the Sea Urchin. Projecting from 

 the mouth are usually to be seen five hard, white teeth. 

 These teeth are movable, not being set in sockets, but held 

 in a very complicated apparatus, somewhat like a five-angled 

 top, known as Aristotle's lantern. The whole apparatus is 

 under muscular control. Through the center of the whole 

 runs the gullet. Above the tooth apparatus the intestine 

 runs around the body wall, then reverses, making in all 

 about two and a half turns around the body wall. It the 

 extends up to the apex, where it ends in the anus, guard( 

 usually by four calcareous plates. It is held in place 

 a thin membrane, the mesentery. 



