GASTEROPODA. 327 



divide it into a certain number of orders, which we have found- 

 ed upon the position and form of the branchiae. Tlie 



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 which is even frequently turbinated, 

 but the operculum is always wanting. The 



NUDIBRANCHIATA 



Have no shell, and are furnished with naked branchiae, of va- 

 rious forms, on some part of their back. The 



Inferobranchiata, 



Similar in other respects to the Nudibranchiata^ have their 

 branchiae on the margin of their mantle. The 



Tectibranchiata 



Have branchiae on the back and side, covered by a lamina of 

 the mantle, which generally contains a shell more or less de- 

 veloped, or sometimes only involved in a recurved margin of 

 the foot. 



Thesie four orders are hermaphrodites, requiring a recipro- 

 cal coitus. The 



Heteropoda 



Have their branchiae on the back, where they form a trans- 

 verse range of small panaches, protected, as well as part of 

 the viscera, in some species, by a symmetrical shell. They 

 are particularly distinguished however by the foot, which is 

 compressed into a thin, vertical fin, on whose margin is fre- 

 quently observed a small cup, the only vestige of the horizon- 

 tal foot of the rest of the class. In the 



Pectinibranchiata 



The sexes are separated ; the respiratory organs almost always 

 consist of branchiae, composed of lamella?, united in the form 



