87 



Class VI. GASTROPODA. 



(Snails and Slugs.) 



The under side of tlie body in tliis extensive 

 Class forms a fiat, usually broad, fleshy disk, which 

 is the organ of locomotion. Every one is familiar 

 with the mode in which a Snail or a Periwinkle 

 glides along on this broad "belly-foot;" and, by 

 watching the motion on the opposite side of a 

 plate of glass, we readily discern the minute 

 wrinkles produced by the contraction of numerous 

 muscular fibres in succession^ which proceed from 

 the hind parts forward in waves_, by which means 

 the foot slides uniformly along over solid bodies. 



The upper parts, in the majority of species, are 

 covered with a fleshy cloak or mantle, the edges 

 of which are free, in a gi-eater or less degree, and in 

 some cases are expanded into great lateral wings. 

 In the Sea-hare {A2)lysia), these expansions serve, 

 by their waving motion, as swimming-fins. The 

 proper function of the mantle-edge is, however, 

 the formation and increase of the shell ; and hence 

 it is always found, in shell-covered species, on that 

 \ art of the body which is near the orifice. In a few 

 species, the shell is included within the substance 

 of the mantle. 



A distinct head is always present, more or less 

 conspicuous according to the degree in which it 

 projects from beneath the front of the mantle. It 

 is furnished with various organs of sense. 



