CHAPTER IX 



THE ANIMALS OF LAKES AND RIVERS 



The water of rivers and of very many lakes differs 

 from that of the sea in the absence of any considerable 

 quantity of common salt, making it ' fresh ' instead 

 of salt. Such bodies of water contain a special fauna, 

 which, as contrasted with the fauna of the ocean, may 

 be described as impoverished ; but in addition to this 

 negative character, this fauna possesses also some 

 positive ones, which we shall consider later. Certain 

 other lakes, either because they had a former connexion 

 with the sea, or because they have no outlet, contain 

 saline water. Such masses of water may contain some 

 animals with distinctly marine affinities, e. g. in the 

 Caspian are found a seal and a kind of herring. But 

 the Caspian is far from being as salt as the sea, and in 

 addition to animals of marine type it contains some 

 typically fresh-water fishes. On the other hand, masses 

 of very salt water, such as the Great Salt Lake of Utah, 

 contain pecuKar brine-shrimps (Artemia) not found 

 in the sea. A stiU further comphcation is introduced 

 by the fact that certain lakes, though their waters are 

 perfectly fresh, contain animals of distinctly ' marine ' 

 facies. Thus Lake Baikal, though its waters are not 

 salt, lodges a seal allied to the Caspian seal, and a 

 marine worm ; Lake Tanganyika, in Central Africa, 

 contains several animals of marine type, and so on. 

 In consequence we cannot sharply separate fresh- 

 water animals from marine ones, in the sense of imply- 



