ON MOLLUSCA. 197 
which is capable of being elongated or shortened, according 
as the longitudinal or transverse fibres act, we find at its base 
some extrinsic muscles which facilitate this action, some in 
drawing it backwards and others in carrying it forward. 
In the species which are provided with this sort of pro- 
boscis there has not been seen any lingual swelling, properly 
so called, and consequently no corneous hooks. But this 
swelling is pretty frequently replaced by a double group of 
hooks placed at the right and left, and which are more or less 
deeply sunk into the proboscis, so that they do not become 
marginal but when it is strongly turned back. ‘This takes 
place in buccinum and neighbouring genera. In the vis 
maculata, which has also a very long proboscis, there is no 
trace of these hooks. 
Other species have been remarked whose palate is armed 
with a plate of corneous teeth, like the tongue; such are 
many monopleurobranches, and among others bulla and um- 
bracula. 
No acephalous molluscum presents any traces of teeth, nor 
any lingual swelling whatever. But the aperture of the 
mouth, of variable form, though ordinarily very large, and 
almost always inferior, is accompanied with two lips, most 
frequently simple, sometimes fringed, which are prolonged at 
their angles into labial or tentacular appendages. ‘These ap- 
pendages, of a triangular form and very variable size, are 
striated, especially at their internal face, so as a little to re- 
semble gills, with which their connexion is always tolerably 
intimate. They are almost always very soft, and directed 
backwards. In the nucula they are, on the contrary, stiff, 
and directed towards the mouth, so as to represent sorts of 
jaws. 
The salivary apparatus, deficient in all the acephala, pre- 
sent in most of the cephalophora, is ordinarily simple, that is 
