LIFE IN PONDS AND STREAMS 



mature and reproduce in one summer. A single female may 

 lay a thousand or more eggs, and as Needham has pointed 

 out, by the fourth generation the possible offspring may num- 

 ber one hundred and twenty-five billions. Even this record 

 hardly equals that of the army of minute crustaceans. These 

 are eaten by many carnivores, yet they have overtopping 

 advantages. Their minute size and their transparency are 

 protections, and like mayflies they feed on plants and have 

 a great reproductive capacity. It has been calculated that 

 in sixty days there might be thirteen billion descendants 

 from one female Daphnia pulex (Fig. 124) if no accident 

 befell any of them. 



Life in lakes. — Lakes are but larger editions of ponds. In 

 the protected shallows of both, the communities are almost 

 alike. But on the open lake surface where the wind blows 

 unhindered, it sweeps water against the shores. There, 

 clinging forms are found: snails, net-building caddis worms, 

 and at a depth of three feet or more the large burrowing may- 

 fly nymphs. Ephemera and Hexagenia (PI. XIII), whose 

 cast skins are often washed up into windrows upon the beaches. 



In lakes there are great areas of deep, open water where 

 there are no hiding places for small animals like those pro- 

 vided by the trash and vegetation in shallow waters. But 

 the lake surfa.ce harbors a great floating or plankton popu- 

 lation, sometimes miles of it, billions of microscopic animals 

 and plants which are not only constantly increasing but also 

 constantly being eaten, or dying and dropping to the bottom. 

 Fishes also live in the open usually a few feet below the sur- 

 face. They are not minute or transparent like the plankton 

 but the}'- are swifter and stronger than any other water 

 animals. They stay near the top where the foraging is 

 good, though in large lakes this means ten or fifteen feet 

 beneath the surface. Below this depth there is a lonesome 

 reach of water, generally barren of plant and animal life. 

 And finally beneath this is the deep soft ooze of the barely 



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