360 ANIMALS IN INLAND WATERS 



the Ganges-Brahmaputra. There are between 150 and 200 species of 

 exclusively fresh-water fishes in the island of Java, and 404 species 

 in Borneo, in contrast to a total of only 126 species in all Europe. 

 The fresh-water fishes of temperate North America, however, number 

 more than 600 species, and the fauna of eastern Asia is also rich in 

 species. 3 



There are a large number of recent immigrants from the ocean in 

 tropical fresh waters, where the transition from the ocean seems to be 

 facilitated. Just why this should be so warrants further careful study. 

 It is doubtful if this can be ascribed to temperature relations. The 

 fact that heavy tropical rains at certain seasons of the year reduce 

 the salt content of the surface layer of the ocean in certain regions 

 for considerable lengths of time, affording opportunity to many marine 

 animals for adaptation to less salty water, may be of some importance. 

 The fresh-water snails and mussels prevalent in the Malay Archi- 

 pelago, and characteristic of it, are more closely related to those in 

 the ocean than the characteristic fresh-water mollusks of colder re- 

 gions. 4 Nowhere is so great a number of marine mollusks to be found 

 in transition into the fresh-water habitat as in Burma and the neigh- 

 boring Indian and Siamese regions. 5 Fresh-water amphipods, isopods, 

 and crabs of the warmer zones are also often closely related to those 

 of the ocean. The same is true to a great extent for fishes, as, e.g., in 

 east Australia, Java, the Barbary coast, and tropical South America. 6 

 Sharks and rays inhabit fresh water only in tropical and subtropical 

 countries. The similarity of the total fresh-water fauna to the ocean 

 fauna increases from the poles to the equator. 7 



As far as the fauna of tropical lakes is concerned, few studies are 

 available. A number of the large lakes of middle Africa have been 

 described as being poor in animal life and especially poor in plank- 

 ton; Lake Kivu has an almost pure copepod plankton, limited, how- 

 ever, to 2 species; cladocerans are missing, and no information is 

 available for rotifers; Cladocera are also missing and rotifers are 

 poorly represented in Lake Tanganyika, 8 while in Lake Albert and 

 Lake Edward the copepods are the prevailing forms in the plankton. 9 

 It is strange that only 1 planarian and 3 annelids (leeches excepted) 

 are known 10 from so large a lake as Tanganyika. This lake, unlike 

 its neighbors, contains 6 known species of sponges. No isopods or 

 amphipods are known from this group of African lakes. 8 



In the waters of the East Indian islands, Entomostraca appear in 

 much smaller numbers than in our regions. 11 In Java the number of 

 phyllopods, ostracods (6 species), and copepods is small, and in the 

 lakes of Sumatra, though the number of individuals is great, the num- 



