GASTEROPODA. 273 



CHAPTER XVIII. 



Fifth Class of Mollusca. 



Gasteropoda.* 



The Gasteropod Molliisks are so named on account 

 of the peculiarity of their locomotive apparatus. The 

 inferior surface of the body is spread out into a broad 

 fleshy disk or foot, on which the animal crawls with 

 an uniform gliding motion. The back is covered 

 more or less completely with a mantle, which in the 

 great majority of species secretes a shell. In some, 

 as in several of our native slugs, the shell is very 



Fig. 210.— volute crawling. 



small, and is concealed wdthin the substance of the 

 short mantle. But in general, as in the Snail and 

 the Whelk, the shell is capacious, capable of receiving 

 and concealing the whole body. The form is com- 

 monly that of a long cone, twisted in a spiral manner 

 upon itself. 



The innumerable species belonging to this extensive 

 class are distributed by Cuvier under several orders 

 distinguished by the structure and position of their 

 organs of respiration, as in the following tabular 

 arrangement : — 



■^ yaa-TT^p, gaster, the helly ; ttovs, h o5o's, pous, podos, a foot. 



N 3 



