FAUNA OF THE AFRICAN LAKES. 



569 



investigations on the Copepoda and Ostracoda of the expedition, 

 G. 0. Sars, however, kindly isolated and identified certain 

 Oladocera from Victoria Nyanza and Nyasa, and these forms are 

 included in my list of species w^ithoiit further comment, although 

 they constitute in some cases new records of distribution. 



The reason for the absence of Oladocera from Tanganyika 

 remains a matter for speculation, but the most likely explanation 

 would seem to be that the water of the lake either contains 

 certain salts which are fatal to such organisms, or is perhaps of 

 too saline a nature to permit them to survive. Colour is lent to 

 this view by the already-mentioned occurrence of Oladocera 

 in the river Lofu and also by the marked difference in nature of 

 the llotatorian fauna of the lake proper and the river mouth. 

 Kousselet (150, p. 794) reports that in a small quantity of 

 material from the river he distinguished 23 difierent species 

 of Eotifera, while in a large series of tow-nettings from difierent 

 parts of Tanganyika he was only able to find 10 species, of which 

 but one was common to lake and river. It thus seems highly 

 probable that though the salinity of the lake water has not made 

 it impossible for certain Rotifers to live in Tanganyika, it is at 

 least the factor which has determined the striking difi'erence 

 between lake and liver in this respect. Further, it may be 

 pointed out tliat a somewhat similar state of affairs has been 

 observed in the case of the phytoplankton. G. S. West in his 

 report on the fresh-water Alga3 of my expedition states (200, 

 p, 96): — "Of the thirty species observed in this river-plankton, 

 only one occurred in the plankton of the lake itself." This 

 evidence is very suggestive, and appeals to confirm the view put 

 forward, as it is well known that the distribution of these delicate 

 plant organisms is profoundly influenced by the nature of the 

 water. 



This is not perhaps the place to discuss at all fully what is 

 known of tlie nature of the lake water, but the principal features 

 may be indicated as follows. The total amount of dissolved 

 saline matter is rather higher than in the case of Tham.es water, 

 but is not excessive. The amount of calcium salts is, however, 

 low, while salts of magnesium are present in much larger 

 quantities tlian is characteristic of fresh waters. It is an obvious 

 suggestion that the presence of this excess of magnesium salts — 

 which are characteristically bitter to the taste — while not ren- 

 dering the water unfit to drink, might be sufficient to afifect these 

 organisms and prevent them from obtaining a footing in the lake. 



It becomes necessary in this connection to refer to the nature 

 of the water of Lake Kivu, which is in direct communication with 

 Tanganyika. Here the quantity of saline matter is excessive, 

 and indeed most remarkable for a relatively large lake. From an 

 analysis by Hundeshagen (103) it appears that wdiile the water 

 is almost entirely free of calcium salts, it contains a very unusual 

 amount of salts of sodium and magnesium — particularly salts of 

 mxgnesium, of which there are more than twice the amount 

 present in Tanganyika. 



The plankton of Kivu, among other lakes, was collected by the 

 Proc. Zool. Soc— 1920, No. XXXVIII. 38 



