244 SUBKINGDOM MOLLUSCA. 



animal increases in size, it partitions off the part previously 

 occupied and moves forward. There are over two thousand 

 fossil species, but only few living. 



Fig. MB. 



Nautilus pompitius, Pearly Nautilus. |. 

 Section showing Chambers and Siphuncle. 



The Nautilidae (sailors) have about a hundred short 

 tapering tentacles, which are quickly withdrawn into the 

 first chamber on an alarm, so that their habits are little 

 known. 



CLASS II. GASTEROPODA. 



General Characteristics. The Gasteropods (belly- 

 footed) move from place to place by a single fleshy disc of 

 foot placed under the abdomen. Most inhabit univalve 

 shells, though some species are naked. The viscera occupy 

 the upper part of the shell, which is secreted by the mantle 

 in various ways, according to the" habits of the different 

 forms. 



arm and endeavored to reach him with its beak. Its slimy grasp sickened, while its 

 threatening look so alarmed him that he loudly called for assistance. His release 

 was at last effected only by killing the cuttle-fish with the boat-hook, and cutting off 

 its arms bit by bit. 



