GASTEROPODA. 67 



coudition they are either predatory and carnivorous, or prefer 

 a vegetable diet, and inhabit, sometimes the high seas, as 

 the Nucleobranchs ; sometimes the forest trees and surface 

 of the earth, as the Pulmonifera ; sometimes the lakes 

 and rivers, as the Amjmllariidce, and sometimes the shallow 

 creeks and bays, as the zoophagous tribes generally. In 

 the Prosobranchiate division the adult animal is provided 

 with a shelly covering, usually more or less spiral, with 

 the aperture either entire, or notched, or produced into a 

 canal in front ; in the Opisthobranchiate division the body 

 is either naked, or with the gills protected by a shelly 

 valve, which is external, or concealed in the mantle. In 

 the Nucleobranchs or the Heteropoda of Lamarck, the 

 foot is modified for swimming, as the animals are pelagic, 

 and the shell is often very thin, rudimentary, or wanting. 

 The animals of the Pulmoniferous division are usually en- 

 cased in spiral shells, which, in some tribes, however, are 

 rudimentary, and in others altogether wanting. 



In the branchiferous Gasteropods the form and position 

 of the gills vary very considerably and offer excellent 

 characters for defining the orders ; in the Nucleobranchs 

 they form a projecting comb-like tuft ; in the Nudibranchs 

 they are exposed upon the surface of the back ; in the 

 Tectibranchs they constitute a plume between the mantle 

 and the foot ; in the Prosobranchs, which include most of 

 the Pectinibranchs of Ouvier, the gills, usually unsym- 

 metrical, are in the form of two comb-like organs in a 

 cavity over the back protected by the mantle ; while in 

 the Pulmoniferous tribes the place of the gills is occu- 

 pied by an air-sac lined with a vascular net-work, which 

 acts like a lung. The modifications which the foot, the 

 mantle, and the other organs undergo, will be mentioned 

 under the various groups which these peculiarities create. 



