HEARING. 391 



exercise this sense. The prevailing opinion among 

 entomologists is that it resides in some part of tlie 

 antennae ; organs, which are supposed to have a 

 peculiar sensibility to aerial undulations. This 

 hypothesis is founded principally on the analogy of 

 the Crustacea, whose antennae contain the vestibular 

 cavity already described ; but on the other hand it 

 is opposed by the fact that Spiders, which hear 

 very acutely, have no antennae ; and it is also 

 reported that insects, when deprived of their an- 

 tennae, still retain the power of hearing.* 



None of the Mollusca appear to possess, even in 

 the smallest degree, the sense of hearing, with the 

 exception only of the highly organized Cephalo- 

 poda ; and in them we find, at the lower part of the 

 cartilaginous ring, (which has been supposed to 

 exhibit the first rudiment of a cranium,) a tubercle, 

 containing in its interior two membranous vesicles, 

 contiguous to each other, and surrounded by a 

 fluid. They evidently correspond to the vestibular 

 sacs, and contain each a small calcareous body, 

 suspended from the vesicles by slender nervous 

 filaments, like the clapper of a bell, and probably 

 performing an office analogous to that instrument ; 

 for, being thrown into a tremulous motion by every 

 undulation of the surrounding fluid, they must 

 strike against the membrane, and communicate 

 similar and still stronger impulses to the nerves by 

 which they are suspended, thus increasing the 



* Comparetti has described structures in a great number of insects, 

 which he imagined were organs of hearing ; but his observations 

 have not been confirmed by subsequent inquirers, and their accu- 

 racy is therefore doubtful. See De Blainville " De I'Organisation 

 des Animaux," i, 565. 



