FAUNA OF THE AFRICAN LAKES. 



561 



All exaiiiiiiatioii of this lon^- li.>t of forms reveals in the first 

 place the large proportion of the recorded species which has been 

 observed in Tanganyika. The latter lake contains 31 out of a 

 total of 54 different forms, Nyasa contains 23, Victoria Nyanza 

 13, and the remaining lakes still smaller numbers. The contrast 

 between the lakes is greater than is disclosed by these figures, for 

 out of 31 species known from Tanganyika, 22 are peculiar to that 

 lake, and in comparison only 4 out of 23 for Nyasa and 3 out 

 of 13, for Lake Victoria. In the three smaller lakes there is only 

 a single recorded species which is endemic, and that occurs in 

 Albert Nyanza. Expressed in percentage form, th's means that 

 nearly 71 per cent, of the Copepods of Tanganyika are found in 

 that lake alone, while of the species found in Nyasa only some 

 17 per cent, are peculiar to the lake, in Victoria 23 per cent., and 

 in Albert 14 per cent. Excluding Tanganyika, it will be observed 

 that only 8 of the forms recorded are endemic, so that with 22 

 endemic species, Tanganyika has nearly thi-ee times as many 

 endemic Eucopepoda as the other lakes can muster between them. 



Turning now to the distribution of families and genera in the 

 lakes, the genus Dicqytomus, sole representative of the Centro- 

 pagidte, calls for little comment. A single species of this genus 

 occurs ill Tanganyika,, to which it is peculiar : it appeal's to be 

 a most characteristic plankton form in that lake. Lakes Victoria 

 and Nyasa each contain endemic species as well as types of wider 

 distribution, so that the three big lakes may each be said to have 

 their own particular forms. None of the species of Diaptomus 

 in this list have been ol)served outside the continent of Africa. 



The representatives of the TIarpacticidse are of far greater 

 interest. An endemic species of Canthocamptas has been re- 

 corded from Victoiia Nyanza,, and v/idely distributed species of 

 Attheyella and Dactylopus from Nyasa. It is on the geneia 

 Schizopera Ilyoph 'dus, at one time regarded by Sars as strictl}" 

 marine, that interest really centres. The genus Schizopera is 

 represented in the collections made by the writer by no less than 

 8 species, of ^vdnch 6 are peculiar to Tanganyika and one to 

 Nyasa, while one occurs in both Lakes Tanganyika and Victoria. 

 The genus was established by Sars for the reception of a species 

 found in a brackish- water lasfoon on the Chatham Islands, while 

 subsequently species have been obtained from the Caspian and 

 from Egypt. It is obvious that such a genus can hardly be 

 regarded as strictly marine. Three questions suggest themselves 

 in this connection. How does the genus come to be represented 

 in the heart of Africa ; how is it that brackish -water types are 

 living there in perfectly fresh- water ; and how is it that so many 

 different species are recognisable in Tanganyika ? Sars points out 

 (151, p, 69) that the most obvious explanation would be to regard 

 these as genuine "relict" forms, in whicli case the Tanganyikan 

 species Avoiild be important members of Moore's so-called " halo- 

 limnic" series. He rejects on general grounds the view advanced 

 by Moore, and indicates transport by the aid of migratory 

 aquatic birds as sufficient explanation for this remarkable 



