372 



ANIMALS IN INLAND WATERS 



species. 41 Rotifers are also quite numerous. Of crustaceans, Artemia 

 is a salt-water inhabitant of world-wide distribution, which usually 

 occurs in great numbers. Resting stages, in all these forms, can survive 

 periods of increased concentration; thus the Protozoa have cysts and 

 the rotifers and Artemia have hard-shelled winter eggs. This is an 

 important point in their adaptation to their environment. The winter 

 eggs of Artemia, like those of other Euphyllopoda, are very resistant; 

 they may develop even after a period of 8 years of drought. 42 



Artemia is very variable in its appearance; every habitat has its 

 own form which can be distinguished by color, size, location and num- 



a 



Fia. 108. — Caudal fork of Artemia salina from waters of varying salt content, 

 near Odessa: a, from water of 8°; b, 14°; c, and d, 18°; e, 23.5°; and /, 25° 

 Baume. After Schmankewitsch. 



ber of setae, form of the posterior end, etc. A large number of these 

 changes are due to the influence of the salt content; 43 since this is 

 constantly changing in nature, reliable information concerning these 

 phenomena can be obtained only by breeding experiments in water of 

 constant concentration. With an increase of salt content the animals 

 become smaller, the length of the narrow post-abdomen in comparison 

 with the abdomen increases, the furca (tail fork) becomes shorter and 

 is set with fewer hairs (Fig. 108). 



The appearance of males also varies with the habitat; in many 

 places there are only females, in others both sexes. One copepod, 

 Diaptomus salinus, is also rather resistant against fluctuations of salt 

 concentration; this is rare in fresh water, but may be abundant in salt 

 lakes such as Lake Aral. 



Insects and insect larvae are the most numerous among Metazoa. 

 A number of genera and species of hydrophilid water beetles and their 



