312 Aqtiatic Societies 



it has at least one great carnivore, that, hke the tiger, 

 ranges the fields, selecting only the larger beasts for 

 slaughter. This is Leptodora (fig. i86). It is of phan- 

 tom-like transparency, and though large enough to be 

 conspicuous, only the pigment in its eye and the color 

 of the food it has devoiured are readily seen. It ranges 

 the water with slow fiappings of its great, wing-like 

 antenna. It can overtake and overpower such forms 

 as Cyclops and Daphne and it eats them by squeezing 

 out and sucking out the soft parts of the body, rejecting 

 the hard shell. Leptodora, in a small way, functions 

 in this society as do the fishes of the necton. 



The total population of plancton in any lake is very 

 considerable. Kofoid ('03) reported the maximum 

 plancton production found by himself in Flag Lake near 

 Havana, 111., as 667 cubic centimeters per square meter 

 of surface: found by Ward ('95) in Lake Michigan, 

 176, do.; found by Juday ('97) in the shoal water of 

 Turkey Lake in Indiana, 1439 do. Kofoid estimated 

 the total run-off of plancton from the Illinois River as 

 above 67,000 cubic meters per year — this the produc- 

 tion of the river, over and above what is consumed 

 by the organisms dwelling in it. 



If we imagine the organisms of a lake to be pro- 

 jected downward in a layer on the bottom, this thick 

 layer would probably represent a quantity of life equal 

 to that produced by an average equal area of dry land. 



