ON MOLLUSCA. 179 
turbines, monodontes, and nerites, which are on each side of 
the pedicle of the foot. Frequently, when these appendages 
are broader, they answer the purposes of swimming. 
This sense becomes less delicate in the species whose ex- 
ternal envelope, being always uncovered, is more or less tuber- 
culous, and appears to be almost lost in those whose envelope 
is more or less solidified. 
The organ of taste, when it exists, is doubtless, as in supe- 
rior animals, situated at the lower part of the buccal cavity, 
where may be frequently remarked a lingual swelling; but 
it must be owned that the skin which invests this part does 
not appear to differ much from that which is at the orifice of 
the mouth itself, and in many other portions of the body. We 
shall presently see, however, that this skin is furnished with 
little corneous hooks, arranged symmetrically, which have 
some analogy with those which are observed at the superficies 
of the tongue of certain mammifera, and that it receives a 
great number of nerves. 
The acephalous mollusca have no trace of this swelling. 
The seat of the sense of smelling, which appears to exist 
only in the cephalophorous mollusca, is not, perhaps, as yet 
sufficiently determined ; and in fact the nature of the skin of 
the mollusca, resembling in general, in its structure, the olfac- 
tory membrane of vertebrated animals, many persons have 
thought that the mollusca might have the power of smelling in 
all parts of their skin.. Others having assumed, as a princi- 
ple, that an odorant molecule must be suspended in a gaseous 
vehicle, have supposed that it was only the species which 
breathe the atmospheric air that could smell, and that conse- 
quently the seat of the function must be the edge of the respi- 
ratory orifice? But this is to deny without any reason the 
existence of this sense in the aquatic species, who questionless 
must smell as well as the others. Another opinion has ob- 
tained with some naturalists, and one which is sanctioned 
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