888 
Birge—The Crustacea of the Plankton. 
tober, 1896, when great numbers of young appeared on several 
occasiocs, and when the old animals were nearly all full grown, 
so that there were very few half developed individuals. This is 
given on p. 398. 
During November and December the population of the lake 
falls off pretty uniformly in all levels, more rapidly in Novem¬ 
ber than later, and at this time the distribution of the animals 
may be more even than at any other period. If Daphnia puli- 
caria is present it rises toward the surface in December and in¬ 
creases the population of the upper strata. This occurred in 
1895. In all years the distribution in November is more uni¬ 
form than that of December, in which month the population of 
the lower levels of the lake seem to decline more rapidly than 
that of the upper stratum. 
Table XXXI .—Average percentile distribution Oct. 1 — Dec. 31. 
Average 
No. 
Per cent, in each 3 m. level. 
0-3 m. 
3-6. 
6-9. 
9-13. 
13-15. 
15-18. 
1894 . 
595,000 
25.8 
18.8 
16.0 
15.7 
14.0 
9.8 
1895 . 
436,000 
29.7 
18.3 
14.3 
14.9 
12.2 
10.6 
1896. 
759,000 
25.9 
21.0 
15.3 
13.9 
12.4 
11.4 
Figures 22 and 23 represent the total population of each of 
the 6 levels into which the lake was divided. The scale is 
100,000 Crustacea to each vertical interval. If the scale be di¬ 
vided by 3 the same diagrams will serve to show the population 
of each level per cubic meter. The relations of the increase 
and decrease of the population in the several levels are shown 
very plainly from these diagrams. For instance in 1895 it will 
be seen that while the two upper levels began to increase dur¬ 
ing the latter part of April, the population of the lower levels 
scarcely changed from the winter condition until about the first 
of May. The population of the three upper levels reached its 
maximum in the latter part of May, while in the lower part of 
t he lake the population went on increasing, or at least remained 
.stationary, until near the middle of June. The 6-9 m. level 
