226 ANIMAL BIOLOGY 



buried deep within the flesh. In the whalebone whales the 

 teeth have entirely disappeared in the adult, but they ap- 

 pear during the early development of the embryo, thus 

 indicating the descent of these whales from toothed ances- 

 tors. What is commonly called whalebone is not really 

 bone, but a horny substance that occurs in the form of a 

 fringe of plates attached to the upper jaws. This fringe 

 serves as a strainer to hold in the creatures the whale 

 catches in its capacious mouth while allowing the water to 

 pass through. Whales feed upon fish, squid and various 

 small animals that swim in the open ocean. Hairs in the 

 whales are almost entirely absent. Heat is retained in the 

 body by means of the very thick layer of fat, or blubber, 

 beneath the skin. It is this fat that yields whale oil, the 

 pursuit of which led to an extensive whaling industry which 

 was carried on until the supply of whales became greatly 

 reduced. Spermaceti is a product of an oil which comes 

 from a large cavity in the head of the sperm whale. The 

 latter differs from the whalebone whales in having numer- 

 ous conical teeth instead of plates of whalebone. The 

 nostrils of whales are united to form a single aperture on 

 the upper surface of the head. As air is blown out of this 

 opening, from the lungs, a column of spray, the con- 

 densed moisture of the breath, appears which has given 

 rise to the erroneous notion that the whale spouts out the 

 water taken in through the mouth. Whales may remain 

 under water for a long time, but like all animals that de- 

 pend upon their lungs for their supply of air they are com- 

 pelled to come to the surface to breathe. 



The sea cows constitute a small order of aquatic mam- 

 mals, called the Sirenia. These animals live in rivers or 

 near the shore of the ocean where they feed upon aquatic 

 vegetation. One of the largest species, Steller's sea cow, 

 was an animal of twenty to thirty feet in length, inhabiting 



