CLASS MAMMALIA 



443 



called blowing. They have to come frequently to the surface for the 

 purpose of obtaining air, and each time they come they "blow." Among 

 the whales are the largest animals that have ever lived, reaching a 

 maximum length of 100 feet. The teeth of the whalebone whales are 

 rudimentary and functionless and are replaced by whalebone, or haleen. 

 Whalebone is a horny material developed from the epidermal lining 



Fig. 342. — Ground squirrels, showing hibernating position in nest in the ground. In 

 this state the functional processes are greatly reduced, the body is cold to the touch, and 

 the animal is helpless. Upper, gray ground squirrel, Citellus franklinii (Sabine). Lower, 

 Golden-manteled ground squirrel, C. lateralis (Say). {From Wade, Journal of Mammalogy.) 



of the mouth, which is arranged as a series of curtain-like plates to form 

 a sieve. These whales feed on relatively small animals which occur in 

 large numbers and which they strain from the water with this sieve. 



448. Hibernation. — Hibernation has previously been noted as an 

 adaptation to terrestrial life and was briefly discussed in connection 

 with Amphibia (Sec. 405). Among the reptiles, turtles bury themselves 

 during winter in the mud of the shores and bottoms of the bodies of water 



