244 LEE RAYMOND DICE 



The normal vertical position of daphnids exposed to hori- 

 zontal light or to darkness will be determined, first by the geo- 

 taxis, which will vary with the light intensity and the tem- 

 perature; and second, by the rate of locomotor activity, which 

 will also vary with the light intensity and with the temperature. 

 The relations of these factors is very complex, especially in view 

 of the fact that the previous exposure to light and heat must 

 be taken into account. Experiments 13 and 14 show that the 

 tendency to go down on heating and up on cooling is persistent 

 for a number of hours. 



In general, high temperatures produce in Daphnia pulex a 

 tendency to positive geotaxis, while low temperatures produce 

 a tendency to negative geotaxis. These tendencies persist for 

 at least a considerable length of time. 



GEOTAXIS IN RELATION TO THE CHEMICAL CONTENT 



OF THE WATER 



Oxygen: No definite change in vertical distribution could be 

 observed on changing daphnids from water saturated with 

 oxygen to water from which the oxygen had been driven off by 

 boiling or by the reverse change. 



Carbon dioxide: Carbon dioxide gas was bubbled for half an 

 hour through a jar of tap water and this was then supposed to 

 contain an extra amount of the chemical in solution. How 

 ever, no change in the vertical distribution could be observed 

 on changing daphnids into this water or back into normal tap 

 water. 



Food: No experiments were conducted on the effect of food 

 as a factor affecting geotaxis, but it is probably not of impor- 

 tance in this respect. 



Waste: One experiment was performed on the effect of waste 

 materials on geotaxis. A very large number of daphnids were 

 placed together in a tall sealed glass jar in the darkroom with 

 weak horizontal light. The crowding was so great that all the 

 adults had died before the end of 24 hours. During this time 

 no change in geotaxis could be observed so it seems that organic 

 waste products have little effect on geotaxis. 



In so far as these observations and inferences are conclusive 

 we find that changes in the chemical content of the medium 

 have little effect on the geotaxis of the adults of Daphnia pulex. 



