PRIMARY DIVISIONS AND CLASSES. 



213 



on the contrary, entirely gelatinous, and formed only for 

 swimming. The medusae (Fig. 165), which float in the sea 

 and are frequently stranded on the sandy shores of the coast, 

 are examples of this class of zoophytes. 



Fig. 163. Holothuria. 



In a third class, that of corallines or polyps, properly so 

 called (Fig. 164), there exists no longer any organ of locomo- 



Fig. 165. Rhizostoma. 



Fig. 164. The Coral Polyp. 



tion : the destiny of the animal is to live fixed to the soil, and 

 its mouth is surrounded with moveable tubercles (Fig. 166), 

 by means of which it gathers in the surrounding waters the 

 corpuscles necessary to its nutrition ; in general, a portion of 



