94 THE WORLD OF ANIMAL LIFE 



are closed. So it is witii the ears of the seal, which close as 

 soon as the animal dives, and prevent the admission of even a 

 single drop of water. 



If we were to dissect a seal, we should iind that the outward 

 ear is situated at some little distance from the internal organs 

 of hearing, with which it communicates by means of a narrow 

 tubular passage. The reason for this peculiar structure is evident. 



If a human swimmer dives under water, he at once becomes 

 perfectly deaf to sounds in the air above. He cannot hear the 

 loudest shouting, or even the explosion caused by the discharge 

 of a gun. But he can hear sounds in the water very distinctly. 

 He can distinguish, for instance, the beating of the oars in a 

 boat at a considerable distance away. And if a heavy blow 

 were to be struck upon the surface of the water exactly above 

 him, he would be almost stunned by the shock. 



Of this fact the North American Indians take advantage when 

 they are engaged in hunting the beaver. As they walk over the 

 glassy, frozen surface of a river, they catch sight of one of these 

 animals swimming below. One of the hunters then raises his 

 club, and strikes a heavy blow upon the ice. The beaver is 

 stunned by the shock, floats motionless to the surface, and is 

 extracted through a hastily-cut hole before it can recover its 

 senses. 



The nostrils of the seal are formed upon somewhat the same 

 principles as the ears, for they, too, close tightly when the animal 

 dives, and effectually prevent the admission of water. 



Even yet we have not exhausted the interesting features of 

 a seal's structure. How, for instance, does it contrive to remain 

 so long under water? Between the moment at which it disappears 

 beneath the surface and that at which it rises again to breathe, 

 twenty minutes or half an hour are said to elapse. How is this? 

 The seal is a warm-blooded animal, and therefore its blood must 

 be incessantly purified. And we know that a human swimmer 

 cannot remain submerged for more than two minutes at a 

 time. 



The seal, however, is remarkable for a singular development 

 of the veins near the liver, which are dilated in such a manner 



