390 



THE SENSORIAL FUNCTIONS. 



cessive additions of semicircular canals, fenestra 

 ovalis, tympanic cavity, ossicula, ear-drum, meatus 

 auditorius, cochlea, and concha, till we arrive at 

 the combination of all these parts in the higher 

 orders of the Mammalia, The simpler forms are 

 generally met 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 appa- 

 ratus for their concentration. The Lobster, for 

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

 39y), containing a membranous sac, with a striated 

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

 tory 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 there- 

 fore be considered as corresponding to the fenestra 

 ovalis. The outer side of this membrane in the 



399 



401 



Astacus Jiuviatilis, 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 distributed on the sacculus. 



It appears from a variety of observations that 

 Insects, both in their larva and their perfect state, 

 possess the faculty of hearing ; but no certain 

 knowledge has been obtained of the parts which 



* This groove is represented magnified in Fig. 400. 



I 



