396 THE OCEAN WORLD. 



CHAPTER XIV. 



PULMONARY GASTEROPODS. 



THE Pulmonary Gasteropods comprehend those molluscs which, 

 as we have said, live in the air and breathe the natural atmosphere. 

 The respiratory organ is a cavity in the walls of which the blood- 

 vessels form a complicated network. The air enters this cavity 

 through an orifice, which the animal opens and shuts at will a species 

 of lung, in short, which is placed upon the back of the animal. They 

 are both terrestrial and aquatic animals. In the latter case, they must 

 come to the surface of the water in order to breathe, like the phocas 

 and cetacea among the Mammifera. 



The Pulmonifera, the second order of Gasteropods, comprehends 

 those animals which live in and breathe the air. 



It is divided into four sections ; the Operculata, or animals whose 

 shells are closed by an operculum, and the In- Operculata, or animals 

 without operculum. 



Operculata is divided into two families ; first, Aciculidsc, types, Geo- 

 melania and Acicula ; and second, Cyclostomidae, types, Pupina and 

 Cyclostoma. Cyclostoma is perhaps the best known ; the mouth is 

 circular, the name being derived from cyclos, circle, and stoma, mouth. 



The second section, In-Opercula, contains five families. 



First, Auriculidse, types, Conovulus and Auricula. 



Second, Limnseidse, types, Planorbis, Physa, and Limnaea. 



Third, Oncidiadse, types, Vaginulus and Oncidium. 



Fourth, Limacidse, types, Testacella and Limax. 



Fifth, Helicidae, types, Clausilia, Pupa, Achatina, Bulimus, 

 Succinea, Yitrina, and Helix. 



