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 3 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. 



