414 
Birge—The Crustacea of the Plankton. 
Table XXXVIII—Continued. 
-ii 
A 
0, 
<D 
A 
Temperature. 
Diaptomus. 
Cyclops. 
Adult D. hya- 
lina. 
Young D. hya- ' 
lina. 
D. Retrocurva. 
Diaphanosoma. 
Ergsailus. 
Total Crustacea. 
Nauplii. J 
0.1 
10.5 
22.0 
3.5 
4.5 
2.5 
12.5 
35.5 
0.5 
7.5 
13.5 
5.5 
12.5 
10.0 
11.5 
60.5 
J. 
Sept. 13. 9 p. m. - 
1.0 
6.0 
20.5 
7.0 
3.5 
1.0 
14.5 
53.5 
. 
2.0 
12.5 
11.5 
6.0 
3.5 
5.0 
13.5 
52.0 
3.0 
. 
7.5 
11.0 
6.0 
4.5 
8.0 
32.0 
67.0 
. 
These observations (and I could adduce many more) show that 
there is a clearly marked diurnal movement of the Crustacea 
in lake Mendota but that it is confined within the narrow limits 
of the upper meter, or meter and a half. The day population 
of the upper centimeters, especially in bright, calm weather, is 
very small, but the number at one-half meter, even under such 
conditions, is nearly or quite as large as that at any greater 
depth, and may be the maximum number. The day population 
of the upper meter consists chiefly of young and immature Crus¬ 
tacea; most of the older individuals of all species being found 
at greater depths. This relation of age to distribution is most 
marked in the Daphnias and Diaptomus and least marked in Cy¬ 
clops. At night the population of the upper meter agrees in 
general character with that of the water below, the older indi¬ 
viduals ascending, and the younger descending. I have found 
no evidence of an aggregation of adult Crustacea close to the 
surface at night, but my observations have been confined to the 
hours before midnight. 
In general, these conclusions regarding the diurnal movement 
of the Crustacea agree with those of France, (’94, p. 35), with 
the important difference that while the movements described by 
him are measured by meters, those which I have observed take 
place within the narrow limits of the upper meter, or even 
within a smaller distance. There are, however, some note¬ 
worthy exceptions to the agreement. I do not find that the 
