COMMUNITIES IN STANDING WATERS 351 



in the unique sink-hole lakes which periodically lose their waters 

 entirely, when they are drained through solution channels into tem- 

 porarily low subterranean tributaries. Temporary water basins are 

 especially numerous in hot regions, where there is a sharp contrast 

 between rainy and dry seasons; but they occur also in the temperate 

 and frigid zones. Running waters may show a similar periodicity, 

 e.g., the wadis of Algeria, the southern tributaries of the Red Sea, 

 or many rivers of Australia. In these rivers the water runs off quickly 

 and the pools and puddles which remain are similar to standing water 

 basins. 



Animals which live in such water basins must be able to do with- 

 out water for long periods of time without perishing. A rigid selection 

 results which accounts for the characteristics of the fauna. The pov- 

 erty in species of such an animal community is shown by a compari- 

 son 77 between temporary and permanent rain-water pools; in the 

 temporary pools 19 species of animals (2 rhizopods, 2 heliozoans, 11 

 ciliates) were found, in permanent water holes (over rocky sub- 

 stratum) more than 88 species (15 rhizopods, 3 heliozoans, more than 

 12 ciliates, 22 rotifers, 13 cladocerans, 3 copepods, 20 insect larvae). 

 The majority of them are small animals, which usually live less than 

 a year, with many generations during the course of that time. All 

 these animals, at some stage in their life history, possess the ability 

 to do entirely without water, either in the egg or in the larva or in 

 the adult animal. Besides these there is another group of inhabitants 

 of temporary water basins, of considerable size and duration of life, 

 which are capable of burrowing in the mud and aestivating during the 

 period of drought. 



Of the short-lived inhabitants of temporary water basins, the 

 Crustacea produce by far the most numerous forms. The survivors of 

 the most primitive group of Crustacea, the Euphyllopoda,* are, in 

 fact, almost entirely restricted to such temporary water basins. They 

 have become adapted to these environmental conditions to such a 

 degree that, so far as they are not cold-water forms, the eggs can 

 develop only after having lain in a dry place for some time; the eggs 

 of the cold-water Euphyllopoda must freeze before they will hatch. 

 This group of Crustacea is best represented in those regions where 

 steppe conditions are accompanied by frequent occurrence of such 

 disappearing water basins, as in Africa and Australia. Seven different 

 species of Euphyllopoda were taken from the mud of a pool in the 

 Sudan. The number of species increases in Europe from west to east. 78 



* Branchipus, Apus, Estheria, Limnadia, and others. 



