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- 



401 



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

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

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

 the vestibule (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. 



