GASTEROPODA. 



well-defined group. Their principal distinction is, 

 that they crawl with an uniform gliding motion 

 upon a fleshy disk, situated beneath the body. The 

 back is furnished with a thick fleshy mantle, gene- 

 rally projecting, very diverse in shape, but in most 

 cases producing a shell ; the shell, however, is not 

 always visible. A distinct head is placed in the 

 front of the body, sometimes partly concealed by 

 the mantle, furnished with fleshy tentacles, above 

 the mouth but not surrounding it. In some of the 

 most familiar species, the eyes are placed on two 

 of these projections, but in others they are wanting; 

 but all the senses seem very obtuse except that of 

 touch, which is extremely delicate. When a shell 

 exists, it is formed in the thickness of the mantle, 

 enveloped in its substance, or on its surface exposed 

 to the air ; its use is to protect the animal from 

 external injury, being in general large enough to 

 receive and conceal the whole body ; and in those 

 cases where it is internal and very small, it is still 

 so placed as to protect the most important organs, 

 the heart and lungs. It is more or less hard and 

 brittle in its texture, acquiring in some of the great 

 marine shells, the Conchs (Strombus) for example, a 

 stony strength and hardness. In a few species it is 

 regular and symmetrical in shape, but in very many 

 it is twisted in a spiral direction around a centre, 

 which is sometimes hollow. The aperture formed 

 by the edges of the shell can, in most cases, be 

 accurately closed when the animal has drawn itself 

 within, by means of a shelly or horny plate (oper- 



