ON MOLLUSCA. 209 
nature so absorbent; but as this organ contains a much greater 
number of vessels than any other part, the aerial absorption 
should be much stronger there. 
We also know that the species which are provided with a 
pulmonary cavity die in a short time after they have been 
retained a certain depth under the water, without allowing 
them to re-ascend to its surface; and that, on the contrary, the 
species with gills cannot live for any time in the atmospheric 
air, especially when those gills are uncovered. When they 
are internal, the animal can survive for a long time, in conse- 
quence of the water by which they are moistened, and which 
does not quickly evaporate. 
The mechanism by which the ambient fluid is brought in 
contact with the elaborating fluid, or the blood, is in general 
rather simple. 
In the species whose gills are exterior, as tritonia, scyllea, 
phyllidia, &c., it is sufficient for the animal to swim for the 
purpose of respiring. 
Those, on the contrary, which have the respiratory organs 
formed by the parietes of a cavity, as the pulmobranchia, or 
contained in the cavity, as almost all the other cephalous mol- 
lusca, the ambient fluid (air or water) is introduced or expelled 
by the dilatation or contraction of the cavity, and of its sim- 
ple or tubular orifice ; and these two effects are facilitated in 
all the species, and especially in such as are provided with a 
shell, by the extension or contraction given to the anterior 
part of the body where the apparatus exists, and by its ad- 
vancement into the broadest part of the shell. But in no case 
is there any regularity in the inspiration and expiration: they 
do not even exist (as independent motions) in the brachio- 
cephala, where the water introduced into the cavity of the 
mantle occupied by the gills serves at the same time for loco- 
motion. 
‘he acephalous malacozoaria, which are all aquatic, present 
VOL. XL, P 
