372 



REACTIONS OF DAPHNIA PULEX TO LIGHT AND HEAT. 



was 3.5. After experiment 1 of Table III the mean position was 4, which indicates 

 that there had been an average movement of half a space toward the end of the trough 



TABLE III. 



CHECK EXPERIMENTS IN DABKNESS. NO HEAT. 



Period 3 min. Dec. 12, 1901, 7 P.M. 



Movement for series +.3125 (i.e., .3125 of a space toward the + end). 



marked space 6. Since in the experiments heat was applied at the end next space 1, 

 it will be convenient to designate that as the + end, and the opposite as the end, 

 in order that without confusion movement of the animals toward the heat may be 

 designated as +, and away from it as (see Table III). For this reason the move- 

 ment in experiment 1, Table III, is expressed as 0.50 (i.e., the average movement 

 of the eight Daphnia was a half -space toward the end of the trough). 



The check series of ten experiments in Table III resulted in six + movements, 

 with an average for the series of +0.3125. There is therefore indication of a slight 

 tendency toward the + end. This may have been due to undetected rays of light 

 or to slight jarring of the trough. The movement is so slight, however, that it is 

 not of great importance ; a large number of observations would in all probability have 

 resulted in an average of 0. 



As soon as this check series was ended the heat was turned on and a similar series 

 of experiments made to test the influence of heat upon the movements of the animals. 

 At the beginning of the heat series the temperature of the water throughout the trough 

 was about 21 C. At the end of the series it was 28 at the + end of the trough, and 

 25 at the - - end. 



Table IV gives the result of this test of the effect of radiant heat. The first ex- 

 periment resulted in a marked movement toward the heated ( + ) end, but thereafter 

 every experiment resulted in a movement in the opposite ( ) direction. In experi- 

 ment 1 it is almost certain that the heat had not yet had time to affect the water to 



