200 THE ANIMALS OF LAKES AND RIVERS 
and the other the sterlet (Acipenser ruthenus). The 
cat-fish constitute a large family of fresh-water fish 
especially characteristic of the equatorial region. The 
wels is found only in Eastern Europe, and is the only 
member of its family in European waters. The sterlet 
is a near ally of the sturgeon, and is a member of 
a small group of old-fashioned fishes which used to 
be called ganoids. The name has been abandoned in 
more recent classifications of fish, for the ganoids are 
not a homogeneous group, but their interest for us is 
that they are fish of a primitive type, which once lived 
in the sea, but have been driven into fresh water 
(though the sturgeon also occurs in the sea) by the 
competition of the more highly organized bony fish. 
Of the invertebrates it is sufficient to say that 
with considerable general resemblance to those of the 
Scottish lochs, they display certain minor differences. 
Thus, the Arctic types of crustacea are absent, and 
such forms as the fresh-water crayfish (Astacus), found 
in England, but not in Scotland, are present here. 
Similarly there is a greater wealth of molluscs, a con- 
siderable number of genera of more or less southern 
facies being here represented, though they are absent 
from the lochs of Scotland. On the other hand, the 
rotifers are far less abundant than in the Scottish lochs, 
but comparisons of this sort do not profit very much, 
as it is difficult to be sure that the investigations have 
been conducted along exactly similar lines in the two 
cases. 
As a third example of a lake fauna we may take that 
of Lake Tanganyika in tropical Africa, which has been 
the object of careful study by Mr. J. E. 8. Moore 
and others. 
Tanganyika has an area of 12,700 square miles. It 
