THE ANIMALS OF LAKES AND RIVERS 205 



as we have already explained, there appears to be a 

 deficiency of oxygen in the depths of equatorial lakes, 

 and in Lake Nyasa no life has been found at depths 

 greater than 100 to 150 feet. The occurrence of deep- 

 water forms in Lake Tanganyika is perhaps associated 

 with the antiquity of the lake, which has given time 

 for the evolution of a special deep-water fauna. 



The Crustacea include the usual small copepods of 

 fresh water, and also two crabs and two prawns, all 

 pecuUar to the lake. One of these crabs extends down- 

 wards from 500 to 600 feet, and is thus another example 

 of the deep-water fauna. Among the other interesting 

 forms are a peculiar polyzoon, three fresh- water sponges, 

 and the jellyfish (Limnocnida) already mentioned. A 

 closely related fresh- water jellyfish has been described 

 from Rhodesia, and another apparently very similar 

 form has been found in the tributaries of the Krishna 

 in India, but none of these is well known. 



The protozoa of Tanganyika seem to resemble 

 generally those of other African lakes, and indeed those 

 of lakes in general. 



To these accounts of characteristic lake-faunas, a 

 word or two may be added on the fauna of the Great 

 Salt Lake of Utah. Here the conditions are extra- 

 ordinarily unfavourable to animal life. Only one 

 organism can be said to have solved completely the 

 problem of adaptation to life in the brine. This is the 

 Brine-shrimp (Artemia salina), which in summer is 

 excessively abundant (cf . Scottish Geographical Magazine, 

 vol. xvii. (1901) p. 617). It is very tolerant of cold 

 and of variations in the salinity of the water, and the 

 fact that the eggs can develop parthenogenetically — that 

 is, without previous fertihzation — enables the animals to 

 reproduce themselves very rapidly when the conditions 



