ON’ MOLLUSCA. 181 
tentacula, for they may be anterior to them, posterior, exte- 
rior, or interior, all which positions furnish good characters in 
zoology. 
The organ of hearing presents much fewer differences in the 
mollusca, and is found no farther than in the brachiocephala 
and a few other genera, where it is reduced to a small sac, 
hollowed at the lower lateral part of the cephalic cartilage, 
and which has not even any immediate communication with 
the exterior. 
We shall now advert to the apparatus of locomotion. 
We have observed, in treating of the structure of the skin in 
the mollusca, that the contractile fibre is not often distinct from 
the dermis itself, properly so called ; hence it happens that all 
the points of this skin are susceptible of contraction in all 
directions. Itis, in fact, a thing perfectly certain, that all the 
external parts of a molluscum, and even the gills, can execute a 
number of vibratory movements; but that would produce lit- 
tle more than a sort of partial locomotion. ‘The general loco- 
motion is determined by a true distinct muscular fibre, visible 
at the internal face of the skin, and disposed in bundles, hav- 
ing a determinate form and direction. It sometimes has its 
point of rest on the solidified part of the skin, but nevertheless, 
it is very seldom that this part can, in reality, serve for the 
purposes of locomotion, except in the articulated mollusca. 
The disposition, the number, and even the form of the mus- 
cles, in this type of animals, are necessarily in relation with 
their general form. ‘Thus, wherever there is a well-marked 
separation between the head and trunk, there are superior 
lateral and inferior muscles, as is observed in the brachioce- 
phalous mollusca; but in all the rest of the trunk this distinc- 
tion does not take place. In the oscabriones it is different, 
each articulation of the back having its particular muscles ; 
but they can scarcely be considered as genuine mollusca. 
The mantle which envelopes the body of the mollusca, 
