Vertical Distribution of the Crustacea — Spring. 881 
production of the species, while the presence of a larger number 
in the bottom water of the lake than in the surface water indi¬ 
cates that the species is past its maximum and is already be¬ 
ginning to decline in numbers. 
In both years the numbers of Crustacea in the upper water 
show an increase during April, due to the multiplication of 
Cyclops. This increase went on, as was shown in the early part 
of this paper, much more rapidly in 1896 than in 1895. As a 
result, the population both of the surface water and of the 
lower levels increased much more rapidly in 1896, and the latter 
part of April, 1896, represents about the same condition of the 
development of the Crustacea, as does the first half of May in 
1895. In each case more than 40 per cent, of the Crustacea 
were present in the upper stratum, while the 15-18 m. level 
had not increased greatly in numbers above its condition in 
winter. In the latter part of April, 1896, the 15-18 m. level 
contained less than 3 per cent, of the whole number of Crustacea 
present; and in the first part of May, 1895, it contained less than 
7 per cent. As the number of Cyclops and Daphniapulicaria be¬ 
came greater, they moved downward into the deeper water, so that 
it became relatively more populous. In the latter part of May, 
1895, the 15-18 m. level contained 10 per cent, of the Crustacea, 
while in 1896 it contained over 40 per cent. This increase in 
the population of the lower strata goes on after a considerable 
decline has come in that of the upper strata. The lower 
water lags behind the upper both in the increase and decrease 
of its population, and the maximum population of the lower 
strata comes from two to three weeks after the maximum popu¬ 
lation of the lake has passed. 
These relations become more obvious if we divide the lake some¬ 
what arbitrarily into three levels, 0-3 m., 3-9 m., 9-18 m. 
The distribution of the Crustacea among these three regions is 
shown in Figs. 24 and 25. By reference to these it will be 
seen that in 1895 the two upper levels increased much more 
rapidly than did the lower half of the lake from the latter part 
of April to the middle of May. In the latter part of May the 
reverse is true; and in early June the population of the lower 
water was stationary, while that of the upper half of the lake 
