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 

 D. hyalina may often be seen in large numbers immediately 



