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 peculiar brine-shrimps (Artemia) not found 
in the sea. A still further complication 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- 
