40 MOLLUSCA. 



The third class, which includes a great number of naked 

 and testaceous mollusca, and to which Cuvier gives the name 

 GASTEROPODA, from the circumstance of the belly being 

 formed for crawling, has been subdivided into eight orders, 

 from circumstances connected with the organs of respira- 

 tion. 



In the first order, termed Pulmones^ which breathe air, 

 he has constituted two divisions, the terrestrial and the aqua- 

 tic. The animals of the former live on land, and were in- 

 cluded by Linnaeus in his genera Umax and turbo. They 

 are the land shells of most authors. Those of the division, 

 termed aquatic, live in the water, but require at intervals to 

 come to the surface to obtain fresh air. They constitute, 

 with a few exceptions, the fresh water shells of naturalists. 

 In the second and third orders, or Nudibrmiclies and Infero- 

 branches, the species consist almost entirely of genera formed 

 from the animals which Linnaeus and many others included 

 in the genus Doris. They are naked mollusca, and are like- 

 wise destitute of any internal testaceous plate. The fourth 

 order, termed Tectibranches, contains animals whose branch- 

 iae, like small leaves more or less divided, are situate on the 

 right side, or upon the back. The animals of this division 

 possess a shell, but it is in general placed beneath the com- 

 mon integuments, such as the genus Aplysia and several 

 species of the genus Bulla. .The fifth order, termed Heter- 

 opodes, have the gills plumore and dorsal, with the foot 

 compressed and vertical like a fin, with a small portion of 

 it only formed to act as an organ of adhesion, as in the other 

 gasteropoda. The Pectinibranches form the sixth order, 

 and are distinguished by the branchiae, which are like leaves 

 or threads placed parallel in one, two, or three lines, on the 

 surface of the pulmonary cavity, and by having the sexes 



