76 PULMONOBRANCHIATA. 



Order II. PULMONOBRANCHIATA*. 



BODY spiral : respiratory apparatus principally consisting 

 of an internal cavity or sac, formed by a fold of the mantle, 

 and lined with a network of vessels, which serves the pur- 

 pose of lungs. 



SHELL usually external and spiral, but sometimes internal 

 and rudimentary or wanting. In the two latter cases the 

 mantle is external and forms a shield on the back. 



With respect to the reproductive system of the Mol- 

 lusca comprised in this Order, it may be observed that 

 each individual of those kinds which do not possess an 

 operculum has both sexes united in itself, but requires 

 to be fertilized by another individual, while those which 

 have an operculum are of different sexes. The former 

 are androgynous, answering in some respects to the 

 botanical term " monoecious ; " and the latter are strictly 

 "dioecious." 



The Pulmonobranchs, Pulmobranchs, Pulmonifers, Pul- 

 monates, or Pneumonobranchs, by all of which various 

 names these mollusks have been called, on account of the 

 analogy which their organ of breathing bears to the 

 lungs of vertebrate animals, respire for the most part 

 atmospheric air. The aquatic kinds obtain also some air 

 from the water by means of auxiliary branchial organs. 

 All the land mollusks, or Snails, belong to this Order. 

 The other members of it are aquatic ; but none of them 

 inhabit the open sea, although a few, belonging to the 

 genera Melampus or Conovulus and Otina, are amphi- 

 bious. These last I propose to include in the marine 

 Mollusca, as they live in the sea for many hours out of 

 the twenty-four, and are only met with on dry land 



* Having a lung-like gill. 



