GASTEROPODA. 



205 



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 small, and is concealed within 

 the substance of the short 

 mantle. But in general, as in 

 the snail and the whelk, the 

 shell is capacious, capable of l 

 receiving and concealing the 

 whole body. Its 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, distin- 

 guished by the structure and position of their organs of respira- 

 tion, as in the following tabular arrangement : 



Breathing air Pulmonifera, p. 206. 



FIG. 210. VOLUTE CRAWLIN 





 l< 



ll 



o 



K 



In a dorsal 

 cavity . . 



Shell 



Almost al- *) 



ways turbi- > Pectinibranchiata, p. 208. 

 nate . . . ) 



Tubiform . . Tubulibranchiata, p. 213. 

 Very open, J 



the form of a [ Scutibranchiatia, p. 213. 



shield. . .1 



Beneath a fold of the man- 



Tectibranohiata, P . 



a straight 'edge of the foot 



Beneath (United !" | Cyclobranchiata, p. 215. 

 the edge of 1 

 themantle (Without a j Infero branohiata, p. 215. 



w 



V Upon the back .... Nudibranchiata, p. 216. 

 vertically compressed, only fit} Heteropoda> p . 2 



