CRUSTACEA OF FRESH WATERS 183 



direct communication with the sea, although it con- 

 tains many animals, such as seals, which are certainly 

 of marine origin. The chief Crustacea of the lake are 

 numerous species of Amphipods belonging to the 

 genus Gammarus, and other genera closely related 

 thereto, and for these, at all events, there is no need 

 to assume a " relict " origin. 



One of the most remarkable lakes in the world 

 from a zoological point of view is Lake Tanganyika 

 in Africa. When it was found that this lake con- 

 tained a fauna very different from that of the other 

 great lakes of Africa, it was rashly assumed that it 

 must be of relict origin, and some remarkable specula- 

 tions were indulged in as to the former connection 

 between the lake and the sea. Further research, 

 while it has greatly emphasized the peculiar nature 

 of the fauna, has entirely disposed of the view that 

 it originated in this way. The Crabs and Prawns, 

 for example, are not nearly related to marine forms, 

 but belong to groups that are characteristic of fresh 

 waters in the tropics. While Nyassa and the 

 Victoria Nyanza have as yet only yielded a single 

 species of Prawn, and that one of enormously wide 

 distribution (from the Nile to Queensland), Tangan- 

 yika contains no fewer than twelve species, all of 

 which are peculiar to the lake, while all except one 

 belong to genera unrepresented elsewhere. Similarly, 

 the Crabs found in the other great lakes of Africa 

 belong to commonplace types of River Crabs of the 



