426 Birge—The Crustacea of the Plankton. 
organs, in the diatoms, which are more uniformly distributed 
in the water. The young, therefore, are freed in part during 
the daytime, by the action of light, from the competition of 
most of the older forms of the same species for the food which 
is especially adapted to the young. 
On August 26th, 1895, there was an alternation of cloud and 
sun, which made the day especially favorable for the study of 
the relation of light and the vertical distribution of Daphnia . 
It was found by numerous observations that the adult and half- 
grown Daphnias were approximately one meter below the sur¬ 
face during the sunny periods, but rose to about one-half meter 
during the cloudy intervals. The rise immediately followed the 
obscuring of the sun and the return was as prompt when the 
sun again shone. It was as though the Daphnias were depressed 
by a force against which they were contending, and they rose 
when the sun disappeared with the promptness of a compressed 
spring when relieved of weight. 
In laboratory experiments Diaptomus and young Daphnias 
move quite to the light end of the box in which they are placed. 
If sunlight is reflected by a mirror, they still move toward it 
and find no light too strong which can thus be sent to them. 
It would seem, however, that the direct sunlight of the open 
lake is too strong for them, or they would be present in larger 
numbers in the upper centimeters of the lake. If the warmth 
of the water repelled them we should expect this stratum to be 
tenanted as the lake cools in the fall, and should also expect 
that the young Crustacea would gradually withdraw during the 
day as the surface warms. Neither in autumn nor in early morn¬ 
ing, however, do we find the Crustacea close to the surface. The 
withdrawal from the upper quarter meter or so continues at 
least until the first of November, and the Crustacea descend 
from the surface very promptly after sunrise. As already 
stated, the old nauplii are the only Crustacea which I have 
found in large numbers immediately at the surface on calm, 
bright days. A high temperature, however, increases the neg¬ 
ative action of light and a low temperature lessens or reverses it. 
In early winter when the ice is transparent, D. pulicaria and 
j D. hyalina may often be seen in large numbers immediately 
