This 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-