204 THE ANIMALS OF LAKES AND RIVERS 



characters. Later, the ordinary fresh-water fauna of 

 Africa gradually reached the lake, giving the present- 

 day minghng of special types with marine affinities 

 (halohmnic forms) and ordinary fresh-water types. This 

 hypothesis has not been universally accepted, and the 

 evidence upon which it is based depends upon detailed 

 anatomical points which cannot be considered here. 



As the figures already given suggest, Tanganyika is 

 an enormous lake when compared with the lochs of 

 Scotland, and it is of course very inaccessible. We 

 cannot, therefore, hope to have the detailed information 

 in regard to its fauna which is available for the Scottish 

 lochs or for Lake Balaton. It has never been systematic- 

 ally sounded, so that it is not possible to distinguish 

 between the faunas of the different depths in detail. 

 The great point of interest is the occurrence of inverte- 

 brate animals of definitely ' marine ' type, notably of 

 a fresh-water jellyfish. Invertebrates of marine type 

 are of course not confined to Tanganyika. We have 

 already mentioned their occurrence in other lakes, e.g. 

 in Lake Baikal, but they seem to be especially numerous 

 and pecuKar in Tanganyika, and, as already stated, 

 they coexist there with ordinary fresh- water forms. 



No less than fifty species of molluscs have been 

 described from Lake Tanganyika. Of these a con- 

 siderable number belong to the genera commonly 

 represented in masses of fresh- water elsewhere. Thus 

 we have species of Limnaea, Planorbis, Bithynia, Unio, 

 &c., the first two genera being cosmopolitan and very 

 old. But in addition there are a number of gastropods 

 of pecuhar and primitive characters, with strong marine 

 affinities, constituting part of Moore's ' halolimnic ' 

 fauna. Some of these live at depths of 600 feet and 

 upwards, which is in itself an exceptional feature, for, 



