378 Birge — The Crustacea of the Plankton. 



thought wise to attempt to distribute the Crustacea among the 

 three meters of each level on any other assumption than that of 

 uniform distribution. 



THE GENERAL VERTICAL DISTRIBUTION OF THE CRUSTACEA. 



Figs. 22-28, Tables B and C, Appendix. 



Winter — January, February, March. 



The months during which the lake is covered with ice show a 

 great equality of distribution on the part of the Crustacea. This 

 is due to several facts. First, the lake is thoroughly homo- 

 thermous, at least in a biological sense. Differences exceeding 

 a degree between the temperature of the water at one meter from 

 the surface and at the bottom of the lake are only found in late 

 winter. Second, the food has no such concentration toward the 

 surface as is found in the summer, though the algae are more 

 abundant in the upper strata. Third, the action of the wind 

 is removed, and the influence of the sun is greatly reduced, 

 both by the snow and ice and by the low temperature of the 

 water. Fourth, there is no reproduction of most species of 

 Crustacea and consequently no difference in age to influence dis- 

 tribution. 



A few forces act in the other way: First, the food is more 

 plentiful near the surface, as the algae reproduce more abund- 

 antly there. Second, when Daphnia pulicaria is present it is 

 far more abundant in the upper strata of the water than below. 

 Third, Cyclops often appears in swarms near the bottom of 

 the lake. Fourth, If Cyclops reproduces during the winter 

 the young are more numerous toward the surface. 



Tables B and C of the appendix show that during January, Feb- 

 ruary, and the early part of March, 1895, there was very little dif- 

 ference in the population of the four upper levels. In January of 

 that year the lower strata were decidedly poorer in number than 

 those above; while in the latter part of the winter they were 

 the most populous, owing to the accumulation of Cyclops in 

 those levels. In the winter of 1896, the 0-3 m. level was at 

 least twice as populous as any below, owing to the large num- 



