MOVEMENTS OF DAPHNIA 253 



in the water at each season, this being near the surface in winter 

 and deeper in the lake during the summer. Diurnal movement 

 may occur at all seasons, but is confined within the limits of 

 the range for the given season, so that at some seasons the 

 diurnal movement does not reach the surface at any time during 

 the day. 



There is a marked difference between the movements of the 

 adults and the young daphnids. The young animals are nearly 

 always found nearer the surface than the adults, and move 

 upward more promptly than the latter. 



Storms cause the plankton Crustacea to be found at lower 

 depths according to some observers, but this does not seem to 

 be always the case. 



Cloudy weather causes the daphnids to be found nearer the 

 surface than would be the case in clear weather. 



Moonlight has been stated to result in driving the plankton 

 Crustacea to lower depths than the normal for a dark night. 



These, then, are the movements of Daphnia whose causes are 

 the object of study in this paper. While it will be impossible 

 to discuss the factors governing the movements in each lake, 

 because the physical factors have not been studied in sufficient 

 detail, still, on the basis of the experiments described in the 

 first part of this paper, it will be possible to suggest in many 

 cases the probable relationship between the physical factors and 

 the observed movements. 



VERTICAL MOVEMENTS CAUSED BY CHANGES OF LIGHT 



INTENSITY 



As sunlight passes down through the water of a lake it is 

 gradually absorbed until at great depths there is absolute dark- 

 ness. This result gives, in deep lakes, a range of light intensi- 

 ties from full sunlight to darkness. We have seen that Daphnia 

 pulex is positively phototactic at intensities below the intensity 

 of diffuse daylight. To intensities above this, at a temperature 

 of 20° C, the animals are normally neutral. Only in high 

 temperatures and strong light do they show a negative photo- 

 taxis. The positive phototaxis of the daphnids will prevent 

 them from going much below the lower limit of neutral intensity, 

 but, as far as the phototaxis is concerned, they might occur at 

 any level in the neutral intensity. The range of light intensity 



