MOVEMENTS OF DAPHNIA 245 



GEOTAXIS IN RELATION TO MECHANICAL STIMULATION 



It could not be determined that mechanical stimulation has 

 any effect on the geotaxis of these daphnids. If such an effect 

 is present it is either very weakly marked or is very temporary 

 and can have little value in determining the vertical movements 

 of the animals. 



GEOTAXIS AT DIFFERENT TIMES OF THE DAY 



A number of experiments were performed to determine if the 

 geotaxis varies at different times during the day. The method 

 used was to place a number of daphnids in a tall jar marked off 

 into five divisions. The jar was completely filled with water 

 and the top sealed by a glass stopper. In the darkroom the 

 jar of animals was exposed to weak horizontal light. A count was 

 made each hour of the number of individuals in each division of the 

 jar, during a period of 30 hours. No periodicity in vertical 

 distribution could be found, either in daphnids taken from 

 daylight at the beginning of the experiment or in daphnids 

 previously exposed to weak light for a day or more. We may 

 then conclude that there is no diurnal rhythm of geotaxis in this 

 species, except that dependent upon rhythmical change in the 

 external physical factors. 



LOCOMOTOR ACTIVITY IN RELATION TO LIGHT INTENSITY 



Daphnids under natural conditions assume a position in the 

 water with the head pointing upward. When no locomotor 

 movements are being made the drag of the antennae on the 

 water, as the weight of the body pulls the animal down, causes 

 the daphnids to assume this posision. When strokes are made 

 with the antennae the tendency then is to raise the animal 

 vertically through the water. 



The rate of sinking of adult daphnids 3 to 4 millimeters long 

 with expanded antennae is stated by Birge (1897) to be at the 

 rate of 20 to 30 centimeters per minute. Young daphnids 

 1 millimeter or less in length sink much more slowly at the rate 

 of 5 to 10 centimeters per minute. This slower rate of sinking 

 makes it much easier for the young daphnids to remain near 

 the surface than it is for the adults. 



It will be seen that it requires a certain rate of activity to 

 maintain an individual at any certain height in the water. If 



