GASTEROPODA. 25 



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 compared 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 be- 

 comes double by being folded in order to enter. 



Most of the aquatic Gasteropoda, with a spiral shell, have an oper- 

 culum, 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 withdrawn into it and folded up.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 others of the Gasteropoda the sexes are separate ; others which 

 are hermaphrodite, and some of which possess the faculty of self- 

 impregnation, Avhile others require a reciprocal coitus. 



Their organs of digestion vary as much as those of respiration. 



This class is so numerous that we have been compelled to divide it 

 into a certain number of orders, which Ave have founded upon the 

 position and form of the branchiae. The 



PULMONEA 



Respire the natural air in a cavity, the narrow orifice of which they 

 open and shut at pleasure. Some of them have no shell, others have 

 one which is even very often completely turbinated, but the oper- 

 culum is always Avanted. The 



NuDIBRANCHIATA 



Have no shell, and are furnished with naked branchiae, of various 

 forms, on some part of their back. The 



Inferobranchiata, 



Similar in other respects to the Nudibranchiata, have their branchiae 

 in the margin of their mantle. The 



Tectibranchiata 



Have branchiae on the back and side, covered by the lamina of tlie 

 mantle, which almost always contains a shell more or less developed, or 

 sometimes only enveloped in a recurved margin of the foot. 



These four orders are hermaphrodites, requiring a reciprocal 

 coitus. The 



