FAUiSA OF TUE AFRICAK LAKES. 569 



investigations on the Copepoila and Ostracorla of the expedition, 

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

 Cl.idocera from Victoria Nyanza and N^'nsii., and these forms are 

 included in my list of species without furthei' comment, altliough 

 they constitute in some cases new records of distribution. 



The reason for the absence of Cladocera 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 Cladocera 

 in the river Lofu and also b}- the marked difference in nature of 

 the Rotatorian 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 diiierent 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 salinitjr 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 difierence 

 between lake and river in this i-espect. Further, it may be 

 pointed out that a, somewhat similar state of afiairs has been 

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

 i-eport on the fresh-water Algpe 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 iniluenced by the nature of the 

 water. 



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

 known of the 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 Thames water, 

 but is not excessive. The a,mount of calcium salts is, however, 

 low, while salts of magnesium are present in much larger 

 quantities than 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 sufiicient to afl:ect these 

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



It becomes necessary in this connection to refer to the natui'e 

 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 hy Hundeshagen (103) it appears that while the water 

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

 amount of salts of sodium and magnesium — particularly salts of 

 magnesium, 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 



