346 MOLLUSCA. 



a canal, through which they can reach and receive the circumflaent medium without 

 extruding their head or foot iroin tlie tlielL 'ihe shell has then, also, in its margin, 

 near the end of the columella, 0|")poscd to that towards wdiicli the sjjire tends, an emar- 

 gination, or furrow, wherein to lodge the canal of the cloak. Consetiuently, the canal 

 is to the left in common, hut to the right in the reversed species. 



Further, the animal being very Hexile, can vary the direction of the shell, and oftenest 

 when there is an emargination or furrow, it directs the canal forwards, wdience it 

 happens that the spire points to behind, the columella to the left, and the opj/osite 

 margin to the right. The contrary of this occurs in the reversed sorts ; and this is the 

 reason that we sav 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 fither 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 tixtd within the 

 sh.ell ; and the mouth is wider or narrower just as the same parts are more or less 

 broad. There are shells wdiose 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 the aperture when the snail has withdrawn within the shell.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, wdiile, 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 numenws that we have deemed it expedient to divide it into a certain 

 number of orders, the characters of which we have drawn from the p(jsition and the 

 form of the branchia?. 



The Pulmoxe.\ 

 Breathe the atmosphere, receiving the air within a cavity whose narrow oi-ifice they can open 

 anil close at will : they are hermapbroditical, with reciprocal copulation ; some have no shell, 

 others carry one, which is often truly turhin.ite, but never furnished uith an opercuhiui. 



The Nudibraxchiata 

 Have no shell, and cany their variously-figured branchiiB naked upon some part of the back. 



The Inferobranchiata 

 Are similar, iu some respects, to the preceding, but then- branchiie are situated under the 

 margins of the cloak. 



The Tectibranchiata 

 Have tlieir hraucliiic upon the liack. or up<Jii the side, covered liy a lamina, or fold of the cloak, 

 wliicli ahiiDSt always contains a shell more or less developed; or sometimes the braucbuc are 

 enveloped in a narrow fold of the foot. 



These four orders are hermaphroditic. d, with reciprocal co]udation. 



The IIeterocodes 

 Carry their hranchis 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. AVhat 

 best distinguishes them is the foot compressed into a thin vertical tin, ou tlie margin nf which a 

 httle sucker often appears, — the only trace left of the horizontal foot of the other orders of 

 the class. 



