260 LEE RAYMOND DICE 



or from a source of stimulation. Whether this is due to a tropism 

 caused by unequal stimulation on the two sides of a bilaterally 

 symmetrical organism, or whether the orientation to the stim- 

 ulus results from the action of trial and error need not concern 

 us here. 



It seems that most of the behavior of Daphnia is determined 

 by orientation of the body to different sources of stimulation. 

 However, the random movements which occur on exposure to 

 strong light, great cold, or great heat give a means of escape 

 from unfavorable conditions, but secure this result without a 

 definite orientation of the body. 



It may be observed that while many of the previously given 

 experiments show tendencies to upward movement it is very 

 seldom that all the animals go to the upper part of the dish. 

 In the experiments showing a tendency to go toward the bot- 

 tom this tendency is likely to be very strongly marked. It 

 might be argued, then, that the upward movement is not due 

 to a negative geotaxis, but is merely an irregular wandering 

 which naturally would carry many of the animals toward the 

 surface when the factors holding them down were no longer 

 operative. However, it may be observed in many cases, par- 

 ticularly on shading a tall jar of daphnids exposed in sunlight, 

 that there is an immediate and vigorous upward movement. 

 The failure of some to move to the top when the tendency is 

 in that direction must be due in part to the weight of the body, 

 which has a constant tendency to drag the animal down. It is 

 probably also due in part to the unnatural conditions under 

 which the animals were kept. It was observed that daphnids 

 freshly collected from natural ponds showed a greater tendency 

 to upward movement than those kept for a long time in the 

 laboratory. There is the further fact that the jars used for 

 experimentation have a very slight depth in comparison with 

 the average depth of a lake so that the available range of move- 

 ment was very small. In nature the daphnids do not go to the 

 very surface of the water or they would be caught on the sur- 

 face film. 



The reactions of the animals in moving upward or downward 

 in response to changes in light intensity or temperature might 

 be considered to be due to a change in the reaction of the organ- 

 ism to pressure rather than to gravity. It can be observed that 



