192 THE ANIMALS OF LAKES AND RIVERS 



not very profound, not sufficient to cause the animals 

 to diverge very markedly from their terrestrial allies. 

 This suggests that in most cases the modification is 

 relatively recent. Among the more frequent modifica- 

 tions are webbed feet, used in swimming, close dense 

 fiir which prevents the water reaching the skin and so 

 chiUing it, and a broad flattened tail used as a rudder. 

 Of mammals with more profound adaptations to 

 aquatic life, few occur in inland waters. We have 

 already spoken of the Lake Baikal seal {Phoca sibirica) 

 and the Caspian seal {Phoca caspica), both closely 

 related to the Ringed seal of northern waters. The 

 Baikal seal also occurs in Lake Oron, on the course of 

 the river Vitim, and in addition to its seal Lake Baikal 

 shows certain ' marine ' features in its moUuscan fauna. 

 The explanation is perhaps that the lake had once 

 a connexion, direct or indirect, with the great inland 

 sea of Tertiary times. Among the Sirenia the manatee 

 is largely fresh-water, ascending such rivers as the 

 Amazon almost to their sources. But these animals 

 thrive equally well in the ocean, and their presence in 

 large rivers requires no special explanation. 



The large Cetaceans are exclusively marine, but 

 certain dolphins occur in fresh water. The one which 

 is most definitely adapted for this habitat is the blind 

 dolphin of the Ganges, Brahmaputra, and Indus 

 {Platanista gangetica). This animal occurs from tidal 

 waters to the upper reaches of the rivers, as high up 

 as the depth of the water permits, but it has never 

 been seen in the sea. These dolphins feed upon fresh- 

 water fish and river prawns, and are apparently primi- 

 tive forms which, like Ganoid fish, have found a refuge 

 in fresh water. Other fresh-water dolphins occur in the 

 large rivers of South America, especially the Amazon. 



