MOLLUSCA. 
346 
a canal, through which they can reach and receive the circumfluent medium without 
extruding their head or foot from the shell. The shell has then, also, in its margin, 
near the end of the columella, opposed to that towards which the spire tends, an emar- 
gination, or furrow, wherein to lodge the canal of the cloak. Consequently, the canal 
is to the left in common, but to the right in the reversed species. 
Further, the animal being very flexile, can vary the direction of the shell, and oftenest 
when there is an emargination or furrow, it directs the canal forwards, whence it 
happens that the spire points to behind, the columella to the left, and the opposite 
margin to the right. The contrary of this occurs in the reversed sorts : and this is the 
reason that we say that their shell turns to the left, [or is sinistraT ] . 
The mouth of the shell, and consequently also the last whorl, is greater or less, in 
relation to the other whorls, according as the head or the foot of the animal is more 
or less voluminous in relation to the mass of viscera which remains fixed within the 
shell ; and the mouth is wider or narrower just as the same parts are more or less 
broad. There are shells whose mouth is narrow and long ; and there it is that the foot 
is thin, and doubles on itself before it can be retracted. 
The greater number of the aquatic Gasteropods with a spiral shell, have an operculum, 
or a corneous or calcareous plate, affixed upon the posterior part of the foot, to close 
the aperture when the snail has withdrawn within the shell. 
There are Gasteropods with separate sexes, and others which are hermaphrodites ; 
and of these some are capable of self-impregnation, while, in others, the copulation of 
two individuals is required. 
Their organs of digestion do not vary less than those of respiration. 
The class is so numerous that we have deemed it expedient to divide it into a certain 
number of orders, the characters of which we have drawn from the position and the 
form of the branchiae. 
The Pulmonea 
Breathe the atmosphere, receiving the air within a cavity whose narrow orifice they can open 
and close at wall : they are hermaphroditical, with reciprocal copulation : some have no shell, 
others carry one, which is often truly turbinate, but never furnished with an operculum. 
The Nudibranchiata 
Have no shell, and carry their variously-figured branchiae naked upon some part of the back. 
The Inferobranchiata ; 
Are similar, in some respects, to the preceding, but their branchiae are situated under the 
margins of the cloak. 
The Tectibranchiata 
Have their branchiae upon the back, or upon the side, covered by a lamina, or fold of the cloak, v 
which almost always contains a shell more or less developed ; or sometimes the branchiae are ■ 
enveloped in a narrow fold of the foot. ,, |j 
These four orders are hermaphroditical, with reciprocal copulation. '■ 
The Heteropodes II 
Carry their branchiae upon the back, where they form a transverse row of little tufts, and are, 
in some instances, protected, as well as a portion of the viscera, by a symmetrical shell. What 
best distinguishes them is the foot compressed into a thin vertical fin, on the margin of which a 
little sucker often appears, — the only trace left of the horizontal foot of the other orders of 
the class. 
