194 SENSORIAL FUNCTIONS. 



the experiments of others, its transmission through water 

 is more than four times more rapid than it is through 

 the air. These facts assist us in understanding why it is, 

 that no part is required in aquatics, like the tympanum 

 and little bones, to increase the sonorous undulations, and 

 also why these organs in other respects, may be reduced 

 to their utmost simplicity, since the water in which they 

 live transmits sound with so little diminution of its inten- 

 sity. 



Hearing in the Lobster. The simple auditory appa- 

 ratus, as it is found in the Lobster, is represented by 

 Fig. 117. It consists of a vestibular cavity at r, con- 

 Fig. 117. 



taining a membranous sac^, which is furnished with the 

 filaments of the auditory nerve. This vestibule is pro- 

 tected on all sides by solid matter, (as the same is by 

 bone in the human ear,) except at one part, e, where it 

 is closed by a membrane, like the fenestra ovalis, to 

 which part therefore it corresponds. The water com- 

 ing in contact with this membrane, the sound is trans- 

 mitted through it, to the nerves of the vestibule and so to 

 the brain. 



The Mollusca appear to be entirely destitute of the 

 sense of hearing, except perhaps in the highly organized 

 Cephalopoda or Cuttle Fish tribe. In these, there exists 

 a tubercle containing two membranous sacs, which cor- 

 respond to the vestibules of other animals. 



Hearing in the Frog. In the Frog, the ear is entire- 

 ly closed on the outside by a membrane, situated over a 

 little cavity on each side of the head, but on a level 



Why is it supposed unnecessary that the vibratory apparatus should 

 exist in aquatics ? 



