REACTIONS OF DAPHNIA PULEX TO LIGHT AND HEAT. 371 



zontally upon a table in a dark room. At 5 centimetres from one end of it was an 

 electric heater, consisting of a metal frame which held coils of high-resistance wire. 

 The heater presented to the end of the trough a radiating surface 10 centimetres 

 square, and the heat from this surface could be felt by the hand at a distance of 

 40 centimetres. 



The trough was filled with water to a depth of .5 centimetre, and the partitions 

 lowered; then two Daphnia were placed in each of the spaces except those at the 

 ends. In the accompanying Tables III, IV, and V the spaces are numbered 1 to 6, 

 and the number of animals in each is indicated. When all was in readiness for an 

 experiment the partitions were raised and the animals permitted to swim freely, 

 or as the stimulus to be tested might direct them, for a period of three minutes. The 

 partitions were then lowered, and an electric light turned on so that the positions 

 of the animals could be recorded. This record having been made, the animals were 

 again distributed as at the beginning of the experiment, i.e., two in each of spaces 

 2 to 5.* 



It was necessary, in order that no undetected factors enter as directive influences, 

 that check experiments be made just before each of the series of experiments to test 

 the influence of heat. In these preliminary observations the conditions were, so far 

 as determinable precisely the same as in case of the immediately following series, 

 except for the absence of heat, the influence of which it was the purpose of the 

 experiments to determine.! 



B. Observations. — Table III is a record of the check series of ten observations 

 preliminary to the experiments of Table IV. The vertical columns in the table give 

 the number of animals in the various spaces at the end of each experiment. The 

 result of the experiments is expressed by comparing the mean position of the animals 

 at the beginning with the mean position at the end of the experiment; the differ- 

 ence of these means expresses the amount of movement toward either end (+ or — ) 

 in terms of spaces. At the beginning of an experiment, since the spaces 2, 3, 4, and 

 5 each contained two animals, the mean position as determined by the formula 



Product of space X Nu mber of animals 

 Total number of animals 



* All of the observations were made at night in a room from which light was excluded. 



t As it seemed possible that heat might cause currents in the water of the trough which would change the posi- 

 tions of the animals, check experiments were made by placing a number of Daphnia in a glass tube, filled with water, 

 one end of which was placed in snow, and the other over the electric heater. Under these conditions the animals 

 collected near the snow end, although no movement of light particles of material suspended in the water could be 

 detected. 



