142 THE TANGANYIKA PROBLEM. 



are nevertheless highly remarkable from the fact that they 

 are only found in Lake Tanganyika among the fresh 

 waters of the African interior. To this category of 

 animals belong the prawns, some crabs, the sponges, 

 some protozoa, and a host of Cichlid fishes. Fishes of 

 course are highly migratory forms, and, once established 

 in fresh water, are sure to spread to a large extent 

 through the rivers and lakes of the continental land-mass 

 in which they happen to be ; still there is this peculiarity 

 about the fishes which are at the present time en- 

 countered in Tanganyika ; some of the cichlids, as Mr. 

 Boulenger has shown, are primitive representatives of 

 the group ; while, in Tanganyika, but not in Nyassa and 

 the lakes to the south, there exists the ancient ganoid 

 Polypterus, and several characinid fishes which although 

 not restricted to Tanganyika, do nevertheless share the 

 somewhat primitive characters which the other halolimnic 

 animals undoubtedly possess. On account of this, I am 

 inclined for the present to regard the African Polypteroids 

 and at any rate the genera of Cichliclse peculiar to 

 Tanganyika, as a piscine portion of the halolimnic group. 

 The reasons for the incorporation of these fishes which 

 are not wholly restricted to Tanganyika is again discussed 

 further on, and, as thus defined, the halolimnic fauna 

 of Tanganyika contains the following forms : — among the 

 Teleostei fourteen genera of Cichlida^, Polypterus among 

 the ganoids, seventeen genera of gastropods, possibly two 

 Lamellibranchs, two crabs, two prawns, one gymnokematus 

 polyzoan, one medusa and some endemic protozoa. All 

 these forms, with the exception of Polypterus, which is ob- 

 viously an animal capable of migration, are the peculiar and 

 characteristic feature of Lake Tanganyika, and it will be 

 admitted that if the invertebrates of this halolimnic fauna 



