7 
ON MOLLUSCA. 185 
last sort of muscle the shell does not present any submarginal 
impression, and the mantle is considerably retractile; but in 
the contrary case an impression is very perceptible, in the 
form of a strip, which follows more or less regularly the edge 
of the shell, descending from the anterior muscle, and which 
often forms behind a large flexuosity drawn inwards, and 
‘which indicates tolerably well the size of the posterior and 
tubular. elongations of the mantle. In this last disposition 
the mantle has no part contractile but that which is between 
its edge and this line of insertion. 
We often find, besides, that the middle of the abdomen is 
occupied by a muscular mass more or less thick, polymor- 
phous, and which, besides its intrinsic contractile fibres, has 
also its extrinsic muscles. This mass has received the name 
of foot, as that which occupies the lower part of the gaster- 
opods, of a form and size extremely variable, to such a degree 
that there sometimes exists no trace of it, as in the oysters, it 
is attached more or less in front, which depends upon the 
habitual position of the animal; but, besides, it may be carried 
in different directions by true muscles, which divided into one 
or a greater number of bundles, direct themselves to different 
points of the shell, and especially in front, behind, and in the 
intermediate space, as is observed in the mussels, anodon, &c. 
This extensible foot sometimes resembles a sort of cupping- 
glass, as in the nucule, sometimes a sort of tongue, as in the 
mussels where it is channelled behind ; sometimes it is like a 
hatchet, as in Venus, sometimes like a sort of human hand, 
as in camus, and finally, like a sort of whip, as in the lori- 
pedes, &c. 
Besides these muscles of the foot, which have evidently 
some analogy with those which we have seen serve for its 
retraction in certain gasteropod mollusca, as the patella, 
there are some others directed transversely, that is, from 
one side to the other of the animal, and each extremity of 
