342 ECHINODERMATA [CH. 



off that the ganglionic swellings of the nerve-cord are produced. 

 The vertebrae and these muscles nearly completely fill the arm, 

 leaving only a small canal above the vertebrae (11, Fig. 156) : this 

 is an outgrowth of the body-cavity or coelom, but there is no 

 branch of the alimentary canal continued into it, as was the case 

 with the star-fish. 



The digestive sac is here a simple flattened bag lined by cells 

 somewhat like those lining the pyloric sac of the star-fish. There 

 is no anus, and the edges of the stomach cannot be pushed out. 

 How then, it may be asked, does the Brittle-star eat and of what 

 does its food consist ? ;v , 



FIG. 158. Oral view of part of the disc and arms of Ophioglypha bullata 

 x 4^. From Wyville Thompson. 



It must be confessed that, in spite of their quick movements and 

 highly developed nervous system, Brittle-stars belong in general to 

 the great army of mud-eaters and scavengers. Where they live 

 usually at the bottom of sea pools and at such depths of the ocean 

 as to be in still water the mud or sand is impregnated with 

 decaying animal and vegetable matter, and most Brittle-stars shovel 

 this material into their mouths by means of the two pairs of tube- 

 feet of each arm which lie nearest the mouth and are called the 



