180 SUPPLEMENT 
by the authority of M. de Blainville, that the extremities of 
the true tentacula form the organ of olfaction. In fact, the 
skin there is softer, more smooth, and more delicate than in 
any other part, and the nerve which repairs thither is more 
considerable. 
That certain of the cephala do smell, may be inferred from 
the fact that slugs and snails seek after particular plants in 
perfect darkness. . 
The organ of vision has not given rise to so many opinions, 
because in its structure the relation of cause and effect is much 
more evident. It is wanting in all the articulated mollusca, 
and in all the acephalous. It is,on the contrary, almost certain 
that it exists in all the cephalophorous races, the hipponices 
perhaps excepted. But it is susceptible of very different de- 
grees of development. 
The eyes of these animals are never more than two in num- 
ber, disposed very symmetrically, one on each side of the 
head, or of the anterior part of the body, in case the head is 
not very distinct. 
Certain fibrous, vascular, and nervous coats may be recog- 
nized in the structure of these eyes; but the cornea belongs 
only to the skin. We also see there some humours and a 
crystalline very distinct. Sometimes there are even small 
muscles which can move them a little in a sort of orbit, or pro- 
tecting cavity, as takes place in sepia and other neighbouring 
genera. But in general, these eyes would be immoveable, if 
they were not pretty frequently more or less pedicled ; that is 
to say, carried at the extremity ofa sort of tentaculum, analogous 
to that of olfaction, as may especially be observed in the family 
of the limacines, and by which their eyes can be directed by 
the animal in a great number of ways; or on the olfactory 
appendage itself, in a point of its extent, more or less elevated, 
as in the buccina, strombus, &c. In the instances where they 
are sessile, their position varies much, in relation to the true 
a 
