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. S. Moore 

 and others. 



Tanganyika has an area of 12,700 square miles. It 



