Juday—Diurnal Movement of Plankton Crustacea. 563 
It is evident that the effect of light is modified more or less 
by other factors. It has been noted above that temperature 
affected the depth migration of Daphnia pulicaria very mate¬ 
rially, changing it from; twelve meters in October to one meter 
in June. In three lakes, temperature apparently affected the 
extent of the movement of D . hyalina as it moved down deeper 
in the daytime as the water grew warmer so that the difference 
in temperature at the lower limit of the deserted stratum did 
not exceed 2° C., while the change in temperature at the surface 
was very much greater. In Rainbow lake, for example, the 
depth of the migration of this form changed from two meters 
in July to six meters in September. This increase was not due 
to the fact that the water was more transparent because the 
transparency was practically the same' on both occasions;. These 
conditions agree with Birge’s statement (2b, p. 426) that a 
high temperature increases the* negative action of light and a 
low temperature lessens or reverses it. In three other lakes, 
however, temperature did not affect the extent of the migration 
of D. hyalina. In Okauchee lake, for example, it was found at 
a depth of one meter in the day catches of August, October, and 
June and the temperature at this depth was 27°, 15°, and 21° 
respectively. 
The quantity of phytoplankton in a lake affects diurnal move¬ 
ment. Fuhrman (7) says that, where phytoplankton is abun¬ 
dant at the surface, it forms a, veil or covering which absorbs 
very much light. Under cover of this, the organisms may come 
close to the surface without being exposed to direct sunlight. 
With two exceptions, my observations show that the Crustacea, re¬ 
mained closer to the surface in the daytime in those lakes which 
were rich in phytoplankton than in those which were poor in 
phytoplankton. As a result, the movement was not so great in 
the former class of lakes as in the latter. It seems probable 
that the various substances dissolved in the water exert an in¬ 
fluence on diurnal movement. These substances affect the 
amount of light absorbed by the water and also 1 modify, in a 
slight degree at least, the character of the light transmitted. 
If diurnal movement were purely a light phenomenon, there 
ought to be a direct relation between the transparency of the 
water and the depth to which the Crustacea, descend in the day¬ 
time. Burckhardt (4) found that this was true for Daphnia 
hyalina in Vierwaldstattersee. My results show no such rela- 
