212 



ZOOLOGY. 



othrr animals of the same primary division, and which gene- 

 rally have the body supported by a ><>rt of interior shell. 

 Such are the various modes of conformation serving as a 

 basis of tin' division of the mollusca, properly so called, into 

 four closso, railed acephala, gasteropoda, pteropoda, and 

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

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

 the hj'alea (Fig. 161) that of the pteropoda ; and the sepia 

 (Fig. 162) the group of the cephalopoda. 



Fig. 162. The Common Sepia. 



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 holothuria (Fig. 163), and the sea- 

 urchins, are types of this class. 



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



