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CHAPTER XV. 
PULMONIFERA OR PULMONARY GASTEROPODS. 
Tue Pulmonary Gasteropods comprehend those molluscs which, 
as we have said, live in the air and breathe the natural atmosphere, 
Their 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 side 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 seals and whales among the mammals. 
The Pulmonifera, the third order of Gasteropods, comprehends 
those animals which live in and breathe the air. 
It is divided into two sections; I, the Operculata, or land-snails, 
whose shells are closed by an operculum; and (II.) the In-Operculata, 
or land-snails without operculum. 
The first section, Operculata, is divided into two families ; first, 
Aciculide, of which the typical genera are Geomelania and Acicula ; 
and the second, Cyclostomide, of which the typical genera are 
Pupina, Cyclophorus, and Cyclostoma. Cyclostoma is perhaps the 
best known genus; the mouth is circular, the name being derived 
from cydos, circle, and stoma, mouth. 
The second section, In-Operculata, contains five families :— 
I. Auriculide ; typical genera, Conovulus, Carychium, and 
Auricula. 
Il. Limnedie ; typical genera, Planorbis, Physa, Ancylus, and 
Limnea. 
Ill. Oncidiade ; typical genera, Vaginulus and Oncidium. 
IV. Limacide ; typical genera, Testacella, Ation, and Limax. 
V. Heicide; typical genera, Clausilia, Pupa, Achatina, Bulimus, 
Succinea, Vitrina, and Helix. 
It will be desirable to treat of a few of these families a little in 
detail, and we will commence with that of the Limnzidz, which is 
