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 Linnzus in his genera limax 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 Nudibranches and Infero- 
branches, thespecies consist almost entirely of genera formed 
from the animals which Linnzeus 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 Tectibranckes, contains animals whose branch- 
ize, 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 branchiz, 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 
