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 oi smell, which seems to be very perfect in the 

 decapod Crustacea, appears also to be sufficiently delicate in 

 many isopods. Its seat i.s 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 antennae. 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- 

 tennae, 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 wdiich 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, 



