'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- 
