346 MOLLUSCA. 



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, hut 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 sinistral^ . 



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 the aperture when the snail has withdrawn within the shell.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 exj)edient 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 will : 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, 

 which almost always contains a shell more or less developed ; or sometimes the branchiae are 

 enveloped in a narrow fold of the foot. 



These four orders are hermaphroditical, with reciprocal copulation. 



The Heteropodes 



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. 



