200 COLLEGE SOOLOGY 



The water-vascular system di fers in several respects from 

 that of the starfish. The madreporite is on the oral surface. 

 The tube-feet have lost their locomotor function and serve 

 as tactile organs ; the ampulla have consequently disap- 

 peared. 



Nutrition. — The food of the brittle-stars consists of minute 

 organisms and decaying organic matter lying on the mud of the 

 sea bottom. It is scooped into the mouth by special tube-feet, 



PiG. 138. — Aboral view of Ophioglypha bullata, a brittle-star. (From 

 Shipley and MacBride, after Thompson.) 



two pairs to each arm, called the oral tube-feet. The rows of 

 spines which extend out over the mouth opening serve as strainers 

 (Fig. 140). The stomach is a simple sac without caeca; it can- 

 not be pushed out of the mouth. There is no anus. 



Behavior. — The locomotion of brittle-stars is comparatively 

 rapid. The arms are bent laterally, and enable animals 

 belonging to certain species to " run," or climb, and 

 probably to swim. Apparently they cannot be taught like 

 starfishes. 



