COMMUNITIES IN STANDING WATERS 353 



nourishment, and delivers the first brood two days later, with addi- 

 tional broods every three days ; the number of young is increased from 

 12-16 at first up to 60 or more at each delivery. 82 The rate of repro- 

 duction is increased in many cases through parthenogenetic develop- 

 ment; then all individuals are female and each one produces young, 

 e.g., many Phyllopoda and rotifers at least in temperate regions. The 

 descendants of any one Daphnia magna during one month have been 

 estimated to be 30,000,000 individuals. 82 



Drought resistance. — Finally, a number of inhabitants of small 

 pools are so organized that they may dry up into a small particle of 

 dust without actually dying; when the water supply is renewed they 

 swell up again and live on. This capability of anabiosis is found in 

 many rotifers {Callidina, Philodina) , bear animalcules, nematodes, 

 and a few copepods (Harpacticidae). Many of these are limited to 

 temporary pools, like Philodina roseola. Rotifers, on the other hand, 

 which are transported into such waters in the resting-egg stage, are 

 usually accidental, not regular, inhabitants. 83 



Another way of escaping the handicaps of drying up of temporary 

 water basins is by burrowing or burying in the mud. This occurs 

 regularly even among many of the smallest animals in pools, e.g., 

 copepods and ostracods. This power explains the fact that an abun- 

 dance of life can be awakened in specimens from bottoms of temporary 

 water basins by adding water. Dried mud from the meadows of the 

 flood area of the lower Danube produces a rather large number of 

 animal species of the groups mentioned above (Apus, Cladocera, Os- 

 tracoda, bear animalcules, a number of flatworms, a nematode, as well 

 as several annelids). 84 Many species of these groups of animals (from 

 central Africa and Australia) have so far been studied only from such 

 cultures. 85 These facts help to explain the extensive distribution of 

 such forms, since they are certain to be carried on bits of mud on the 

 feet of birds. 



Some larger animals are also capable of living through temporary 

 droughts, e.g., some insect larvae, and especially mollusks and fishes. 

 The hardened mud which surrounds them forms an impenetrable case 

 which is often strengthened by a secretion of slime which prevents 

 desiccation. Such protection is especially prevalent in the tropics. 

 Spatha, a genus of mussel thriving in the temporary basins and 

 streams of African steppes, lies buried in the mud during the period 

 of drought. The long interruption of growth makes the annual lines 

 of growth stand out prominently on the shell. The Pectinibranchiata 

 are especially characterized by this habit among snails, for they have 

 in their operculum an exceptionally efficient protection against dry- 



