CLASSIFICATION OF THE MOLLUSCA. 



393 



In the highest mollusks, the sexes are distinct ; but 

 united in Land-Snails, and in some kinds of bivalves, etc. 



According to the most generally received classification, 

 the Mollusca include six Classes : 



I. CEPHALOPODA or cephalopoda ; with distinctly 



marked head, which is furnished with fleshy 

 appendages or arms ; two prominent eyes ; two 

 stout horny jaws ; and the body naked, or 

 protected by a shell. 



II. GASTEROPODA or Gasteropods ; with a more 



or less well defined head ; and with the lower 

 portion of their body formed for a locomotive 

 organ ; and in most cases with a univalve 

 shell. 



III. ACEPHALA or Conchifera or Bivalves ; with 



apparently no head, and with a shell com- 

 posed of two pieces or valves. 



IV. TUNICATA or Ascidians ; with a soft elastic 1 



covering instead of a shell. 



V. BRACHIOPODA or Arm-footed animals ; with 



two ciliated arms ; a shell of two valves, a 

 dorsal and a ventral, and the two valves of 

 unequal size, but equal sided.* 



VI. POLYZOA or Bryozoa or moss-like Mollusks, 



growing in clusters. 



MOLLUS- 

 ^CAYERA. 



MOLLUS- 

 C01DEA. 



* See foot, note in regard to Bracliiopods, on p. 370. 



17* 



