HEARING 177 



The ears of the reptiles, the amphibians and the fishes are 

 simpler in structure than those of the mammals and birds. 

 In the first place not only do none of them possess an ear- 

 pinna, but none of them possesses an external ear ; the ear- 

 drum, in those that have one, is on the surface and practically 

 continuous with the general surface of the skin — at most it 

 lies in a shallow depression, except in the crocodiles where it 

 is protected by a movable flap of skin. The inner ear, too, 

 is less complicated, for there is no cochlea, and the sound- 

 receiving part is the lagena alone. It is thus probable that 

 the hearing in all these animals is much less efficient in 

 discriminating between the quality of different sounds 

 than in the mammals and birds, and that it is less sensitive 

 to the higher notes. For all that, the hearing of some of these 

 creatures must be very much more than rudimentary — most 

 frogs are noisy creatures, at least during the breeding season, 

 and all species have their own characteristic songs, some of 

 them with sweet bell-like notes. The hearing of lizards 

 appears to be acute ; though they are not vociferous animals 

 some of them have voices. Although snakes have no ear- 

 drum they are probably not, as popularly supposed, deaf, 

 but then fishes have no ear-drum and they are certainly not 

 deaf. 



The fishes have neither external nor middle ear, no ear- 

 drum nor special bone to conduct sound to the inner ear. 

 The inner ear is primitive compared with that in the other 

 vertebrates, for only a small part of it is concerned with 

 hearing. This part is the lagena, a patch of hair-cells covered 

 with jelly containing limy particles called the otoliths or ear- 

 stones. In the sharks and rays the stones are minute and 

 numerous, but in the ordinary scaly fish they are large and 

 few, though only one, and that a small one, lies in the lagena 

 — the rest may serve other functions as we shall see later. 

 Water is a better conductor of sound waves than air because 

 it is incompressible. The body of a fish consists of over ninety 

 per cent water and consequently sound waves are conducted 

 directly into the inside of the fish without the necessity for 

 any amplifying mechanism to convey them to the ear. Vibra- 

 tions carried directly to the inner ear in this way are sufficient 

 to agitate the otoliths and stimulate the sensory cells into 



