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 
peculiar 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 fertilization—enables the animals to 
reproduce themselves very rapidly when the conditions 
