258 LEE RAYMOND DICE 



VERTICAL MOVEMENTS CAUSED BY CHANGES IN THE 

 CHEMICAL CONTENT OF THE MEDIUM 



The chemical content of the water of an ordinary lake will 

 be subject to changes in the proportion of dissolved food sub- 

 stances, in oxygen content, in the amount of dissolved carbon 

 dioxide, and in the proportion of organic waste products. 



Changes in the proportion of carbon dioxide is the only factor 

 likely to be of importance in the vertical movements of Daphnia. 

 This substance has a tendency to produce positive phototaxis. 

 In the stagnant water found at certain seasons below the ther- 

 mocline in some moderately deep lakes, the excess of carbon 

 dioxide will perhaps increase the positive phototaxis of the 

 daphnids enough to cause them to rise to a better ventilated 

 stratum. Otherwise changes of the chemical content of the 

 medium do not seem to be of great importance in determining 

 the vertical movements of Daphnia. 



VERTICAL MOVEMENTS CAUSED BY AGING OF INDIVIDUALS 



Although no careful study has been made of the reactions of 

 any age of the daphnids except the adults, it can easily be 

 observed that the young daphnids show marked differences in 

 behavior from the behavior of the adults. In general the young 

 are more strongly positively phototactic and more strongly 

 negatively geotactic than the adults. They sink less rapidly 

 through the water than the adults. All these features tend to 

 cause the young daphnids to be found on the average nearer 

 the surface than the adults. In many cases the adults reach 

 the surface at some time in the day and in winter they may 

 remain near the surface all day, but in general the adults are 

 found at deeper levels than the young. In the life of the indi- 

 vidual there must then be a movement of the average daily 

 position downward through the water until death occurs and 

 the dead body sinks to the bottom. In lakes where the lower 

 levels are stagnant the daphnids will likely be killed soon after 

 they drop below the thermocline. 



Reproduction seems to occur in periods. Birge (1897), in 

 Lake Mendota, found one period in the fall and several in the 

 spring. During a period of reproduction young daphnids appear 

 in great numbers and crowd toward the surface. Being in much 

 greater numbers than the adults the percentages of distribution 



