236 LEE RAYMOND DICE 



of positive phototaxis, but that in weak light these tendencies 

 are overpowered by the normal positive phototaxis. 



The temperature at which these animals become indifferent 

 to light varies with the light intensity. Below 15° C. they seem 

 to be positive in any intensity. To weak light they seem posi- 

 tive at all temperatures. Above 25° C. in very strong light 

 there may be a slight negative phototaxis, but if the ultra- 

 violet rays are screened off this seems never to be at all strong. 



PHOTOTAXIS IN RELATION TO THE CHEMICAL CONTENT 



OF THE WATER 



Oxygen: No observable change in phototaxis was produced 

 by changing daphnids from tap water containing much oxygen 

 to the same kind of water from which the oxygen had been driven 

 off by boiling for a half hour, or by the. reverse change. 



Carbon dioxide: Loeb (1906) has found that carbon dioxide 

 will make neutral daphnids positive to light. Moore (1912) 

 has further shown that this substance will make negative daph- 

 nids positive. This chemical then has a tendency to render 

 Daphnid pulex positively phototactic. 



Food: No observations are at hand bearing on the effect of 

 food substances on phototaxis. 



Waste products: A very large number of daphnids were 

 placed in a small vessel and exposed to weak light. They 

 remained positive to this light until the excess of organic waste 

 products in the medium grew so great that all died. 



The tendency of carbon dioxide to produce positive phototaxis 

 seems to be the only factor in the relationship between the 

 chemical content of the water and phototaxis that is likely to 

 be of importance in the normal movements of Daphnia. 



PHOTOTAXIS IN RELATION TO MECHANICAL STIMULATION 



Yerkes (1900) states that some individuals of Daphnia pulex 

 may be made temporarily weakly negative to light by mechan- 

 ical stimulation, such as picking them up in a pipette, but with 

 many individuals he failed to obtain this result. I have been 

 unable to produce negative phototaxis by any amount of gentle 

 or rough handling, either by picking the animals up in a pipette 

 or by shaking the dish. 



