324 



ZOOLOGY 



The Cetacea 1 have taken to an aquatic life, for it is 

 certain that their ancestors were land animals. The sea- 

 cows (manatees 2 ), found in rivers in various parts of the 

 world, seem 8 to show a transition to the marine forms, 



FIG. 299. Choloepus, the unau or two-toed sloth. After Vogt and Specht. 



such as the dolphins, the toothed whales (Fig. 300), 

 and the toothless or whalebone whales. The largest of 

 these whales - - the Greenland whale - - reaches an ex- 

 treme length of twenty-two to twenty-four metres, and a 



1 cetus, whale. 2 From a native name. 



3 The relation to Cetacea is not close. 



