324 MOLLUSCA. 



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 abdomen, which sometimes however assumes the shape of 

 a sulcus, 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, situated above the mouth and do not surround it, 

 varying in number from two to six ; sometimes they are want- 

 ing ; their function is that of touch, or at most that of smell. The 

 eyes are very small, here adhering to the head, and there to 

 the base, side, or point of the tentaculum; sometimes they are 

 wanting. The position, structure, and nature of their respi- 

 ratory 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, however, 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 receive 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 



