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 of 

 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 



