REACTIONS OF DAPHNIA PULEX TO LIGHT AND HEAT. 367 



The three columns of Table I marked "(Left) Water Screen" contain records 

 of the times, in seconds, for ten trips toward the screened end of the trough when 

 the screen was on the left of the experimenter as he faced the trough. The next 

 three columns, marked " (Right) No Screen, " contain results for the ten return trips 

 of the same series. Likewise " (Left) No Screen" indicates that the screen had been 

 transferred to the right end of the trough. The trips are numbered 1, 2, 3, etc., in 

 the order in which they were made. From this description it appears that each animal 

 used was permitted, under the directive influence of the light from a 16-candle-power 

 lamp, to make 40 trips each 50 centimetres in length, in rapid succession. Half of 

 these 40 trips were made under the influence of light accompanied by heat, the other 

 half under the influence of light alone. 



B. Observations. — Table I is a representative series of results gotten with 

 Daphnia Number 1. 



It is to be noted (1) that the time of the trips rapidly decreases throughout the series; 

 it being for the first two trips 125 and 185 seconds respectively, and for the last two 58 

 and 57 seconds; (2) that the time for the "First Half" is always considerably less than 

 that for the "Second Half"; and (3) that the average time for all trips toward the Screened 

 End (both the right and the left), 74.9 seconds, is practically the same as that for all trips 

 toward the Unscreened End, 73.3. 



Similar series of results were gotten with four other animals, and in Table II the 

 averages for the five are given. Each column in this case contains the averages for 

 ten trips for each of the animals. The general averages at the bottom of the table 

 are therefore for 50 trips under each of the four conditions, i.e., toward the left when 

 the screen was on the left, toward the right when the screen was on the left, toward 

 the left when the screen was on the right, and toward the right when the screen was on 

 the right. In all 200 trips were made. Of these 100 were toward the water screen, 

 and 100 away from it. At the bottom of Table II the average for each of these 100 

 trips is given. 



In answer to the first question proposed at the beginning of this section, Does 

 the heat accompanying light influence the movements of Daphnia? the experiments 

 do not furnish a definite reply. From the general averages it appears that the time 

 for the "Whole Trip" toward the Unscreened End (73.8 seconds) was 3.8 seconds 

 less than for that toward the Screened End (77.6 seconds). One might conclude 

 from this that the heat increased the rate of movement. But examination of the 

 individual averages shows that in two cases out of five the average time of movement 

 toward the Unscreened End was longer than that toward the Screened. For this reason, 

 and also because of the slight difference of the general averages, we are forced to 



