FAUNA OF THE AFRICAN LAKES. 



581 



Victoria ISTyanza 21, and Tanganyika only 8. Clearly there is 

 little disproportion between Lakes Victoria and ISTyasa, but Tan- 

 ganyika shows a marked reduction iti number. It is probable 

 that the low figure for Tanganyika proper has some relation to 

 the nature of the lake water, which appears to be somewhat un- 

 suited to these organisms, liousselet points out in his report on 

 my collections (150, p. 794) that there is a striking difference 

 between the scanty Rotiferan fauna of the lake and the far 

 richer fauna which he observed in a small quantity of material 

 from the Lofu River. Tanganyika water contains an unusual 

 amount of magnesium salts, and though very little is really 

 known concerning the influence of such salts on fresh-water 

 organisms, this seems likely to be the cause of the dissimilarity 

 between lake and river in this respect. The matter has already 

 been discussed in some detail in connection with the complete 

 absence of Cladocera from the lake (c/. p. 569). 



On examining the list of Rotifera more closely, it will be 

 observed that with three exceptions, the forms are all known 

 from other parts of the world, many of them, in fact, being 

 cosmopolitan in their distribution. A single endemic species is 

 enumeratetl from each of the three bigger lakes, but apart from 

 Lecane lofitana described from the Lofu River, these are of only 

 doubtful value. Tiie facilities for dispersal wliich the Rotifers 

 possess are well known, and quite account for this wide distri- 

 bution. In the case of these organisms, therefore, no deductions 

 of any value can be made from the presence or absence of a 

 species in a particular lake, and the interest attaching to a com- 

 parative table of distribution is, in consequence, small. 



Nor does a study of the actual genera and species occurring 

 disclose features of much significance, although one or two com- 

 ments may be offered. In all, 42 genera are enumerated, of 

 which again the largest number is found in Nyasa. Those best 

 represented are Brachionus with 12 sjDecies, or well-marked 

 varieties, TricJiocerca with 9 and Lecane with 6, while nearly all 

 the more important genera are known from at least two of the 

 lakes. Certain cosmopolitan species have been identified from 

 all the four lakes under consideration. There is little to note in 

 the way of unexpected forms or of types unaccountably absent. 

 A subspecies of Brachionus — B. capsicUflorus bidentatus, — which 

 was isolated from the Albert Nyanza, material, is apparently very 

 rare, having been observed only in Calcutta and more recently in 

 Bulgaria. Furthermore, Rousselet has pointed out that the 

 genus Asplanchna is recorded from the lakes, but has not been 

 obtained in South Africa. — In conclusion, it will be observed that 

 as in the case of the Cladocera — which are also of extended distri- 

 bution, — there is no indication of an exceptional fauna peculiar 

 to Tanganyika, or indeed to any of the lakes. 



Gastrotricha. 



Representatives of this group may eventually prove to be 

 widely distributed in the African lakes, but at present there is 



