230 LEE RAYMOND DICE 



INTRODUCTION 



Vertical movements of the plankton Crustacea have been 

 noted by many observers. The published results show that 

 there is great variation in the movements made by different 

 species, and that the same species in different lakes shows great 

 differences in its vertical migrations. These facts indicate that 

 there is a considerable number of factors which operate together 

 to determine the movements. This paper is presented with the 

 hope that it will help to make clear the relative importance of 

 the factors determining the movements of a common fresh water 

 entomostracean . 



This study has been carried on under the direction of Dr. 

 S. J. Holmes to whom the author is indebted for constant advice 

 and criticism. To Professor E. P. Lewis of the Department of 

 Physics thanks are due for information concerning the absorption 

 of ultra-violet rays in passing through the atmosphere and 

 through water. Dr. A. C. Chandler assisted in taking observa- 

 tions in some of the longer experiments. 



In the following experiments only one species of crustacean 

 was used, Daphnia pulex De Geer, which was found abundantly 

 at times in the lakes and ponds about Berkeley. The specimens 

 used were obtained from several artificial ponds on the Uni- 

 versity Campus. Many were kept for some time in glass aquaria 

 in the laboratory before being used for experimentation in order 

 to accustom them to laboratory conditions; others were used 

 immediately after being collected from the pond in order to 

 ascertain how far the behavior had been modified by the arti- 

 ficial conditions. It has been observed that daphnids in different 

 stages of their life history show differences in their vertical 

 movements. To avoid this complication only adult individuals 

 were used. 



In all these experiments the animals were kept in water taken 

 from the same aquarium as the animals unless a statement is 

 made to the contrary. In the experiments on geotaxis the jars 

 were filled completely and the top sealed by a ground glass 

 stopper to exclude the possibility of a difference of oxygen content 

 between the top and the bottom. To avoid the possibility of 

 chance in the animals remaining constantly at one end or the 

 other of the dish, the ends were occasionally reversed or the 

 contents of the dish stirred up. 



