OASTEROPODA. gj^ 



CLASS III. 



GASTEROPODA. 



The Gasteropoda constitute a very numerous class of the Mollusca, 

 an idea of which is afforded by the Slug-. 



They generally crawl upon a fleshy disk, situated under the abdo- 

 men, which sometimes however, assumes the shape of a furrow, or 

 that of a vertical lamina. The back is furnished with a mantle which 

 is more or less extended, takes various forms, and in the greater 

 number of genera, produces a shell. Their head placed anteriorly, 

 is more or less visible, as it is the more or less involved under the 

 mantle ; its tentacula are very small, they are situated above the 

 mouth but do not surround it, varying in number from two to six ; 

 sometimes they are wanted ; their function is that of touch, or at 

 most that of smell. The eyes are very small in some species, ad- 

 hering to the head, in others to the base, side, or point of the tenta- 

 culum ; sometimes they are wanted. The position, structure, and 

 nature of their respiratory organs vary, and afford the means of 

 dividing them into several families ; they never, however, have more 

 than a single aortic heart, that is to say, one placed between the pul- 

 monary vein and the aorta. 



The position of the apertures, through which the genital organs, 

 and that of the anus project, varies ; they are almost always, how- 

 ever, on the right side of the body. 



Several are entirely naked; others have merely a concealed shell, 

 but most of them are furnished with one that is large enough to re- 

 ceive and shelter them. 



The shell is formed in the thickness of the mantle. Some of them 

 are symmetrical and consist of a single piece ; others are non-sym- 

 metrical, which, in those species where they are very concave, and 

 where they continue to grow for a long time, become necessarily 

 obliqviely spiral. 



If we figure to ourselves an oblique cone, in which other cones, 

 always wider in one direction than in the others, are successively 

 placed, it Will be easily seen that the convolution of the whole takes 

 place on the side which enlarges the least. 



This part, on which the cone is rolled, is termed the columella ; 

 it is sometimes solid, and sometimes hollow. When hollow, its aper- 

 ture is called the umbilicus. 



