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, Aciculidæ, types, Geomelania and Acicula; and second, Cyclostomidæ, 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, Auriculidæ, types, Conovulus and Auricula.
Second, Limnæidæ, types, Planorbis, Physa, and Limnæa.
Third, Oncidiadæ, types, Vaginulus and Oncidium.