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328 THE TANGANYIKA PROBLEM. 
invertebrates belonging to which have this peculiarity, they 
are rigidly restricted to the confines of Lake Tanganyika, 
or to the waters which are, or have been, in direct connec- 
tion with it. 
In attempting now to form some definite conception of 
what this halolimnic group really is, its geographical 
isolation and restriction to one special African depres- 
sion is among the most remarkable features that the 
group presents, and it is all the more remarkable 
because, as we have seen, the halolimnic group does 
not hold possession of Tanganyika to the exclusion ol 
the general African fresh-water fauna. On the contrary, 
the normal African fresh-water fauna is found to co-exist 
along with it in full force, the members of this normal 
fresh-water fauna of Tanganyika presenting no greater 
specific differences from the animals constituting the fresh- 
water fauna of Nyassa than we find also to subsist 
between the fauna of Nyassa and that of the Victoria 
Nyanza, or between the Victoria Nyanza and Lake Rudolf. 
Further, the geographical isolation which characterises the 
more conspicuous members of the halolimnic group such as 
the Medusa and the gastropods, characterises also, as we 
saw in Chapter VII., a number of animals which might 
under certain conditions be considered as belonging to the 
secondary fresh-water series of a continent. To this 
section of the halolimnic fauna of Tanganyika belong the 
sponges, the protozoa, the prawns and the crabs, and the 
majority of the fishes characteristic of the lake. In the 
case of the invertebrate section of this class, they have, 
however, none of them been found elsewhere, and as 
they can be viewed as marine organisms, since they have 
closely allied forms which habitually live in the sea at the 
present time, it was pointed out that they are to be 
