MOVEMENTS OF DAPHNIA 263 



7° C, while it is negative at from 15° to 20° C. These organisms 

 are found on the surface in summer, but in winter they gather 

 at the bottom of the body of water in which they occur. The 

 relation of temperature to geotaxis in this case is the reverse 

 of the relation found to occur in the other cases mentioned 

 above. 



These observations on plankton organisms of several distinct 

 orders and classes go to show that reversal of geotaxis through 

 the agency of changes in light and temperature is a rather wide- 

 spread phenomenon. The importance of such a reversal of 

 geotaxis in determining the vertical movements of plankton 

 animals must be very great. 



SUMMARY 



In the experimental part of this paper it has been shown 

 that the individuals of Daphnia pulex are normally positive to 

 weak light at a temperature of 20° C. At that temperature 

 they are normally indifferent to light intensities stronger than 

 weak diffuse daylight. 



Increase of temperature has a tendency to decrease the posi- 

 tive phototaxis, while decrease of temperature tends to make 

 the animals more strongly positive. At temperatures below 

 12° C. they are positive to all light intensities. 



Increase of light intensity causes a tendency to positive geo- 

 taxis, while decrease of intensity causes a tendency to negative 

 geotaxis. These tendencies seem to persist for at least a con- 

 siderable period of time. 



High temperatures produce a tendency to positive geotaxis, 

 while low temperatures cause a tendency to negative geotaxis. 

 These tendencies seem to be very persistent. 



The diurnal movements of Daphnia pulex are caused chiefly 

 by variations in geotaxis induced by changes in light intensity. 

 The limits of the diurnal movement are determined by the 

 factors effective in producing the seasonal movements. 



Change in geotaxis caused by changes in the temperature of 

 the lake at different seasons of the year is the principal factor 

 determining the seasonal movements of this species. The effects 

 of the changes in temperature on phototaxis, and of the seasonal 

 changes in the midday light intensities on geotaxis and on pho- 

 totaxis are minor factors in this movement. 



