Vertical Distribution of Individual Species. 397 
Table XXXIII. — Cyclops, 1896. Number per cubic meter stated in thou¬ 
sands. 
Depth, meters. 
0-3. 
3-6. 
6-9. 
9-12. 
13-15. 
15-18. 
April 1-15. 
17.2 
11.7 
18.9 
20.3 
12.8 
15.0 
April 16-30. 
109.4 
84.1 
52.5 
28.8 
18.8 
9.6 
May 1-15 .. 
190.2 
124.9 
117.4 
84.5 
52.9 
42.7 
May 16-31 .. 
37.0 
37.3 
34.3 
35.2 
42.1 
64.8 
June 1-15. 
20.5 
13.7 
7.6 
6.7 
5.7 
14.1 
June 16-30 ... 
59.2 
32.4 
17.9 
13.4 
6.7 
9.5 
Marsh (’97, p. 204) finds that Cyclops ftuviatilis is present in 
great numbers near the surface. Its distribution, therefore, 
agrees more nearly with that of Diaptomus than it does with 
C. brevispinosus. The latter species is present in Green lake 
in very small numbers apparently in and below the thermocline 
in summer. 
Daphnia hyalina. 
Figure 29.— Table F, Appendix. 
There are two facts which give the peculiarities of vertical 
distribution of Daphnia hyalina and the allied species D. retro- 
■curva. These are: First, a decided tendency of the young 
animals to accumulate in the superficial strata of the water, 
frequently in the upper meter. Second, a tendency on the part 
of the older animals to settle toward the bottom. These species, 
therefore, show a very high percentage in the upper levels of 
the lake in periods when they are increasing, and especially at 
those times when the broods of young appear. On the other 
hand, when the species is declining in numbers, and in the in¬ 
tervals between the appearance of broods, the distribution may 
be comparatively equal throughout that part of the lake inhab¬ 
ited by the species. As examples, compare the table on page 
398, and the detailed figures of Table F, Appendix. 
The percentage in the upper level rarely falls below 25, even 
in the winter. In May, when the spring broods appear, the 
average number in the 0-3m. level ranges from 45 to 55 per 
cent., and the same ratio is found during the summer when the 
species is increasing in numbers. On the other hand, when the 
