GASTEROPODA. 
25 
The aperture of the sliell, 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 narroAV and long — this is because the foot is thin, and be- 
comes dou.hle by being folded in order to enter. 
Most of the aquatic Gasteropoda, with a spiral shell, have an oper- 
culum, a jjart sometimes horny, sometimes calcareous, attached to the 
posterior part of the foot, which closes the shell when its occupant is 
Avithdrawn into it and folded uj:). 
In others of the Gasteropoda the sexes arc separate ; others Avhich 
are hermaphrodite, and some of which possess the faculty of self- 
impregnation, Avhile others require a reciprocal coitus. 
'I'heir organs of digestion vary as much as those of respiration. 
I’his class is so numerous that we have been comijclled to divide it 
into a certain number of orders, Avhich we 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 Avhich is even very often completely turbinated, but the oper- 
culum is always Avanted. The 
Nudibranchiata 
Have no shell, and are furnished Avith naked branchiae, of various 
forms, on some part of their back. The 
Inferobranchiata, 
Similar in other respects to the Nudibranchiata, have their branchicP 
in the margin of their mantle. The 
Tectibranchiata 
Have branchiae on the back and side, covered by the lamina of the 
mantle, Avhich almost ahvays contains a shell more or less developed. Or 
sometimes only enA^eloped in a recurved margin of the foot. 
These tour orders are hermaphrodites, requiring a reciprocal 
coitus. The 
