222 TEXTBOOK OF ZOOLOGY 



circumoral canal. Each foot is connected with a bulblike ampulla. 

 These tube feet along- with some mobile spmes constitute the loco- 

 motor system. Five interradial pouches or branchiae or "Polian 

 vesicles" are in communication with the circumoral canal. The 

 esophagus leads from the aboral part of Aristotle's lantern into the 

 flat, dilated stomach. The stomach extends almost around the inter- 

 nal wall of the body. From it the intestine leads in an opposite 

 direction and in sea urchins finally opens externally by the median 

 aboral anus. In the sand dollar the intestine passes along the poste- 

 rior ambulacral plate to the anal opening which is near the margin 

 of the disc-shaped body. A branch from the esophagus runs par- 

 allel to the stomach and finally joins it. This tube is heavily 





Fig. 119. — Thyone, tlie common sea cucumber. 



ciliated internally and is known as the siphon. Its function is con- 

 jectured to be either respiratory or a means of washing refuse from 

 the intestine. The principal organs of respiration are the inter- 

 radial pouches and the tube feet. The nervous system is composed 

 of a circumoral ring with radial cords extending into the ambula- 

 cral areas. Strongylocentrotus, Arbacia, Tripneustes, Clypeaster, 

 and Echinarachnius (sand dollar) are representative genera of the 

 group. 



Class Holothurioidea. — The eehinoderms of this class have only 

 an incomplete skeleton, the body is elongated, the mouth surrounded 

 by tentacles is located at one end and the anus is at the other. 

 These animals are called sea cucumbers because of their shape and 



