HEARING. 



435 



with in aquatic animals, probably because the 

 sonorous undulations of water are communicated 

 more readily, and with greater force, than those 

 of air, and require no accessory apparatus for 

 their concentration. The lobster, for instance, 

 has a vestibular cavity (seen at v, in Fig. 399), 

 containing a membranous sac, with a striated 

 groove (g)*, and receiving the filaments of the 

 auditory nerve. This vestibule is protected by 

 the shell on all sides, except at one part, where 

 it is closed only by a membrane (e), which may 

 therefore be considered as corresponding to the 

 fenestra ovalis. The outer-side of this mem- 



F F 



401 



brane in the Astacus fluviatilis, or cray-fish, is 

 seen at f in Fig. 401 ; while Fig. 402, shows 

 an interior view of the same membrane (f), with 

 the vestibide (v) laid open, and the auditory 

 nerve (n) passing through the shell to be dis- 

 tributed on the sacculus. 



It appears from a variety of observations that 

 Insects, both in their larva and their perfect 



This groove is represented magnified in Fig, 400. 



