FISH IN GENERAL. 77 



known; it is conjectured merely to be an erectile texture, and 

 to have some influence in adapting the visual organ to different 

 densities of medium and different distances. 



From the above remarks on the spherical form of the crys- 

 talline, the immobility of the pupil, and the difficulty of 

 changing the length of its axis, it may be assumed that the 

 sight of fishes is very imperfect. The images upon their 

 retina must appear confused, and it is therefore probable that 

 they are not susceptible of receiving very precise impressions 

 of forms of objects. It is nevertheless true that they perceive 

 their prey even at some distance, by the eye alone, since arti- 

 ficial flies can deceive them. 



The ear of fishes consists almost solely of the labyrinth ; 

 and even that is much less complicated than in quadrupeds 

 or birds. There is no external ear, unless the small cavity 

 slightly turned in a spiral direction, which is placed before 

 the meatus externus in rays, and is entirely covered by the 

 skin, deserves that name. Osseous fishes have no such cavity 

 nor meatus ; a few only, lepedoleprus, macrourus, and some 

 mormyri, have openings in the cranium closed by the skin, 

 and capable of communicating mediately with the labyrinth. 

 Others like myripristes have the cranium open below, but 

 covered by a membranous partition which adheres to the 

 swim bladder ; but these communications are very different 

 from those which take place by the tympanum or by the 

 eustachian tubes \ Fish are in fact deprived of both these 

 organs, and of the bonelets appropriated to the tympanum. 

 In most osseous fishes all the membranous labyrinth is sus- 

 pended in a chamber of the cranium, formed by a lateral 

 excavation in the great chamber of the encephalon. Of the 

 bony labyrinth there remain no other vestiges than some 



1 The connection between the auditory apparatus and the swim bladder 

 is very remarkable in several species of fresh water fish of the Ganges, and 

 others from the salt water estuaries of its Delta. 



