24. CLASS GASTEROPODA. 
tion to the mass of viscera that remains fixed in the shell. This 
aperture is so much broader or narrower as those same parts 
are more or less thick. ‘There are some shells whose aperture 
is narrow and long. ‘This is when the foot is thin, and folds 
in two to enter. ; 
Most of the aquatic gasteropoda with spiral shells have an 
operculum or a piece, sometimes horny, sometimes calcareous, 
attached on the hinder part of the foot, which closes the shell 
when the animal has entered and is folded up in it. 
There are gasteropoda with the sexes separated, and others 
hermaphrodite, of which some can generate of themselves, 
while others have need of reciprocal coition. Their organs of 
digestion do not differ less than those of respiration. 
This class is too numerous not to be divided into a certain 
number of orders, which we have derived from the position 
and form of the gills. 
The PULMONARIA 
Respire the atmospheric air in a cavity whose narrow orifice 
they open and close at will. They are hermaphrodites, but 
require reciprocal copulation. Some have no shell, others 
have, and often even completely turbinated ; but they have no 
operculum. 
The NUDIBRANCHIATA 
Have no shell, and have naked gills of divers forms, on some 
portion of the back. 
The INFEROBRANCHIATA 
In other respects similar to the preceding, carry their gills 
under the reflected edges of their mantle. 
The TECTIBRANCHIATA 
Have gills on the back or side, covered by a lamina of the 
