PRIMARY DIVISIONS AND CLASSES. 223 



cephalopoda. The oyster may represent the type of the first, 

 that is, of the acephala ; the snail that of the gasteropoda ; 

 the hyalsea (Fig. 184) that of the pteropoda ; and the sepia 

 (Fig. 185) the group of the cephalopoda. 



382. Finally, the fourth and last primary division of 

 the animal kingdom, the zoophytes, comprises also very 

 varied animals, and is divided into several classes. In one of 

 these groups, called the class echinodermata, the body is 

 formed to creep on the sand or rocks at the bottom of the 

 sea, and for this purpose the surface is provided with a 

 number of small prehensile appendages; the integuments 

 also are of considerable consistence, and even sometimes of a 

 stony hardness. 



The sea-stars, the holothurw, sea-cucumber (Fig. 186), and 

 the sea-urchins, are types of this class. 



fet 



Fig. 185. The Common Sepia, or Cuttlefish. 



In the second group, formed by the acalepha, the body is, 

 on the contrary, entirely gelatinous, and formed only for 

 swimming. The medusae (Fig. 188), which float in the sea 

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

 are examples of this class of zoophytes. 



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



