ON CRUSTACEA. 
249 
and filled with an aqueous fluid, into which a nerve penetrates. 
Its external orifice is applied against a round, thick, white 
membrane, which closes an aperture of the same form, pierced 
at the posterior part by a tubercle of the crustaceous envelope, 
and which is a sort of tympanum. 
In the crabs, and other brachyurous Crustacea, we find at 
the base of the external antennae, the same cavity of the testa; 
but its external projection is much less if it exist. When this 
projection is found, it is altogether strong, and has no posterior 
aperture provided with a membrane analogous to the tym- 
panum. 
The sense of smell, which seems to be very perfect in the 
decapod Crustacea, appears also to be sufficiently delicate in 
many isopods. Its seat is not better known in these animals 
than in many insects ; and from the same reasons, which 
we have already noticed in treating of the last mentioned 
class, it has been supposed to reside in the antennse. It 
has been remarked, that the first pair of nerves proceed into 
these appendages in the same manner as the first pair of 
nerves in vertebrated animals is carried into those organs 
which are so indubitably known to be olfactory ; the analogy of 
function has therefore been inferred from analogy of position. 
This question, nevertheless, remains totally unresolved, for 
if the antennae be the organs of smell in the insects and Crus- 
tacea, where are those of the arachnida, which have no an- 
tennas, and which, nevertheless, exhibit an equal perception 
of odorant emanations ? 
M. Dumeril, in adopting the conjecture of Baster, has en- 
deavoured to demonstrate that the seat of smell in insects 
should be found in the points through which the air necessary 
for respiration was introduced into the body, that is to say, 
towards the entrance of the stigmata. But where should we 
place this seat in the Crustacea which respire by gills ? 
M. Cuvier, who, in his Lessons on Comparative Anatomy, 
