FAUNA OF THE AFRICAN LAKES. 



605 



peculiar to the lake. Nyasa contains an endemic genus belong- 

 ing to the Argalidfe, and the Ostracoda are well represented, 

 8 species being endemic out of 17. It is strange that no aquatic 

 snakes and no Polyzoa have yet been recorded ; on the other hand, 

 Nyasa alone of the lakes under review contains aquatic tortoises 

 referred to the Trionychid?e. 



The three smaller lakes contain representatives of fewer animal 

 groups, bat it is possible that this is merely due to less syste- 

 matic investigation. — Albert Nyanza displays only 13 per cent, 

 of endemic forms, which are mostly Mollusca, while it has no 

 endemic fish. The genus Limnocaridella (prawn) is the only 

 genus peculiar to the lake. — Edward Nyanza contains a more 

 interesting sei-ies of fish, with one genns and six species endemic. 

 Only 3 molluscs are peculiar out of a total of 15, and there is 

 little else which calls for comment. — In Kivu, representatives 

 of only 6 groups of animals are at present known to occur. 

 While future exploration may bring other forms to light, the 

 poverty of its fauna is probably connected with the exceptional 

 salinity of the water. Out of a total of 23 species, there are 

 13 fishes, 3 being endemic, while the only other endemic type 

 is an Oligochaete worm. The apparent absence of the hippo- 

 potamus and the crocodile is a point of some interest. Similarly 

 no Lamellibranchs appear to occur, although two forms of 

 Gasteropod are known. 



It is obvious that none of these lakes exhibit such striking- 

 forms as Tanganyika, and that while endemic types are not 

 wanting, these are fewer in number and for the most part only 

 specific in character. Apart from the presence of the medusa in 

 Lake Victoria, there is nothing to suggest a special connection 

 with the sea. In broad terms it may be said that the lakes 

 contain the ordinary fresh-water fauna of Africa modified in 

 varying degree in each case. Where sucli modification is con- 

 siderable, as in Victoria Nyanza and less markedly Nyasa, it may 

 well have been caused by a, period of isolation proportional to the 

 relative peculiarity of the fauna. It is more especially the fisli- 

 fauna of these lakes which is rich in endemic species and shows 

 certain endemic genera, and from this evidence it would seem 

 that Lake Victoria remained isolated for a longer period than 

 Nyasa (c/. p. 536). If isolation be accepted as accounting for the 

 remarkable fauna of Tanganyika, it is clear that a still longer 

 period must have been necessary in that case to produce such 

 notable results. 



There are certain other matters concerning the distribution of 

 animals in the African lakes which are brought out by this com- 

 parative survey. While representatives occur of most of those 

 groups which may be expected in tropical fresh-waters, there are 

 some interesting exceptions. It comes as a. surprise to a Euro- 

 pean naturalist to find no fresh-water Isopods such as Asellus, or 

 Amphipods such as Gammarus. yet these familiar forms appear 

 to be conspicuously absent from the tropical parts of Afiica, 



