326 MOLLUSCA. 



its edge, sometimes because the mantle is entirely detached 

 from the body along this edge, and sometimes because it is 

 perforated there. 



It sometimes happens that the margin of the mantle is pro- 

 longed in a canal, in order to allow the animal to seek the 

 ambient element without protruding its head and foot from its 

 shell. The latter, in this case, has also an emargination or ca- 

 nal in its edge, for the purpose of lodging that of the mantle. 

 The canal, consequently, in ordinary species, is on the left; and 

 in the reversed, on the right. 



The animal, however, being very flexible, frequently 

 changes ,the position of the shell, and most commonly when 

 there is an emargination or canal, it directs the latter forwards, 

 which throws the spine behind, the columella to the left, and 

 the opposite margin to the right. It is the contrary in the 

 reversed, for which reason their shell is said to be contorted 

 to the left. 



The aperture of the shell, and consequently the last whorl, 

 are more or less large, in proportion to the other whorls, as the 

 head or foot of the animal, which is constantly protruding 

 from and retracting within them, is more or less voluminous in 

 proportion to the mass of the viscera which remain fixed in 

 the shell. 



This aperture is wider or narrower in proportion to the 

 greater or less degree of thickness of these same parts. The 

 aperture of some shells is narrow and long this is because the 

 foot is thin, and becomes folded in order to enter. 



Most of the aquatic Gasteropoda, with a spiral shell, have 

 an operculum, a part sometimes horny, sometimes calcareous, 

 attached to the posterior part of the foot, which closes the 

 shell when its occupant is withdrawn into it and folded up. 



In some of the Gasteropoda the sexes are separate ; others 

 are hermaphrodite ; some of the latter possess the faculty of 

 self impregnation, while others require a reciprocal coitus. 



Their organs of digestion vary as much as those of respira- 

 tion. 



This class is so numerous that we have been compelled to 



