154 



ZOOLOGY 



times of the year. The average for all localities and periods 

 is about 40,000 individuals per cubic metre of water in the 

 lakes. In the late spring the population of the lakes may rise 

 to 250,000 per cubic metre near the surface; near the bottom 



Fig. 153. — A typical lake, expo.sied to the sunlight and swarming with algae 

 and Entomostraca. 



it maj' fall to 500 or even less. In a lake of 80 square Idlometres 

 there may be 160 iDillion Entomostraca weighing all together 

 about 20 tons, and this mass of organisms is renewed several 

 times each surnmer. 



The abundance of Entomostraca in a lake varies with the 

 time of year. The numbers are relatively small during the 

 winter, but with the melting of the ice at the end of March 



