204 THE ANIMALS OF LAKES AND RIVERS 
characters. Later, the ordinary fresh-water fauna of 
Africa gradually reached the lake, giving the present- 
day mingling of special types with marine affinities 
(halolimnic 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 peculiar 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 peculiar 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, 
