184 THE LIFE OF CRUSTACEA 



genus Potamon ; in Tanganyika, in addition to some 

 of these, there are three species of a remarkable 

 genus, Platytelphusa, not known from any other 

 locality. The Copepoda and Ostracoda of Tangan- 

 yika comprise a remarkably large number of species, 

 many of them peculiar to the lake. A most unusual 

 feature is the entire absence of Cladocera. It is not 

 easy to explain the occurrence of this remarkable 

 fauna in Tanganyika, but the evidence from other 

 groups of animals, such as Mollusca and fishes, tends 

 to suggest that the lake must have been, until recently, 

 completely isolated from the other lakes and river- 

 systems of Africa, that it had no outlet, and that the 

 water was consequently more or less brackish. 

 Under these conditions the fauna of the lake, 

 originally similar to that of the other African lakes, 

 has evolved along lines of its own. 



A very interesting division of the fresh-water fauna 

 is constituted by those animals which inhabit under- 

 ground waters. In the South of England there is 

 found not unfrequently in the water of wells a small 

 colourless transparent Amphipod known as the 

 " Well Shrimp " (Niphargus aquilex Fig. 62), distin- 

 guished from the common fresh- water Gammarus by 

 the slenderness of its body, by the elongation of the 

 last pair of tail appendages (uropods), and by the 

 absence of eyes. The proper habitat of Niphargus is 

 not actually in the wells, but in the subterranean 

 reservoirs and streams by which the wells are fed. 



