74 INTRODUCTION. 



The important function of vision is imparted 

 to fishes to a greater extent, and if perhaps the 

 range of seeing be not great, when within its 

 bounds it is apparently acute and distinct ; and as 

 among the higher vertebrata we have some which 

 are nocturnal in their habits, as well as those 

 which seek their prey by day, so we find among 

 fishes a difference of form in the large eyes of 

 many species which constantly remain at a depth 

 of many hundred fathoms below the surface, and 

 where it has been proved that the influence of 

 light could not extend. In some, again, the eyes 

 are remarkable for their minuteness, and to 

 several species the specific name Caca, or blind, 

 has been applied. These, like the mole in her 

 dark galleries, live in the banks of muddy rivers, 

 and are no doubt furnished with some more 

 exquisite sense to supply their wants, and minister 

 to their sustenance. In the Gastrobranchus, a 

 fish remarkable in all its structure, no trace what- 

 ever of eyes has yet been discovered. 



Water, the medium through which fishes hear, 

 has been proved to be a better conductor of 

 sound than air ; and from a variety of experi- 

 ments, sounds produced under water, have a loud 

 and clear impression on the human ear, placed in 

 the same situation. In fishes there is no external 

 ear, except in a few where a very small cavity is 

 discernible. They want the tympanum, the small 

 bones, and the eustachian tubes ; but the semi- 



