80 -PULMONIFERA. 
ORDER PULMONIFERA. 
(Lung-breathing Mollusks.) 
Tus extensive Order contains Mollusca which 
differ from all the rest of the Class, by breathing 
atmospheric air. There is an orifice situated on 
the right side, beneath the margin of the mantle, 
which opens into a chamber lined with a delicate 
net-work of vessels. This lined cavity is analogous 
to the lungs of vertebrate animals, and its muscular 
floor is said to perform alternate motions, answering 
to those of the diaphragm, by which the lungs are 
filled and emptied by turns. 
Many of the species inhabit fresh waters ; but the 
greater number are denizens of the land, requiring, 
however, a damp atmosphere, to preserve them in 
health and vigour. The close, humid forests of 
tropical countries sustain the terrestrial species in 
immense number and variety: the shells of many 
of these—distinguished by their curious forms, by 
their large size, by their rich and brilliant colours, 
often arranged in finely-contrasted bands, and by 
their delicately sculptured surfaces,—are among the 
most precious ornaments of conchological cabinets. 
The aquatic species form, notwithstanding the 
element in which they live, no exception to the 
leading character of the Order; they also breathe 
air, which they obtain by coming periodically to 
the surface. 
In general, the subjects of this Order are protected 
by an ample spiral shell ; but in some this appen- 
