366 REACTIONS OF DAPHNIA PULEX TO LIGHT AND HEAT. 



Daphnia; (2) whether the directive influence of light tends to diminish during the 

 period for which an animal is subjected to it; (3) whether Daphnia shows any signs 

 of fatigue in long-continued motor reaction to light and heat; and (4) whether, 

 under conditions different from those of the previously described photopathic experi- 

 ments, Daphnia exhibits any preference for a particular intensity of light. 



A. Method. — The Daphnias to be observed were placed, one at a time, in a tin 

 trough 60 centimetres long, 6 centimetres wide, and 8 centimetres deep, painted dead 

 black inside and out to prevent reflection. The ends of the trough were glass, so 

 that light from electric lamps could enter parallel to the long axis of the trough. A 

 16-candle-power lamp, in the circuit of which a key enabled the experimenter to turn 

 the light on or off quickly, was placed 5 centimetres from each end of the trough. 

 All side and all end rays, except those passing through the glass ends, were cut off 

 by screens. At one end a glass vessel, having parallel vertical sides 15 centimetres 

 square and 5 centimetres apart, filled with distilled water, was so placed as to inter- 

 pose 5 centimetres of water between the lamp and the end of the trough in order that 

 the heat-rays might not enter. At the opposite end of the trough both heat and 

 light were permitted to enter. 



The apparatus was set up in a dark room. The experiments consisted in placing 

 a Daphnia at one end of the trough, turning on the light at the opposite end, and not- 

 ing, with the aid of a stop-watch, the time taken by the animal to move from a fine 5 

 centimetres distant from the end at which it started to the middle point of the trough, 

 and, again, from there to a line 5 centimetres from the end nearer to the light. The 

 first period mentioned may be designated as the time of migration for the "First 

 Half" of the trip, the second as the time for the "Second Half," and the sum of the 

 two as the time for the "Whole Trip." These expressions are used in the tables 

 which follow. As soon as the animal had reached the light end the light was turned 

 off at that end, and that at the other at once turned on. In the same way as for 

 the first trip the time for the return trip was noted. This was repeated ten times 

 in succession, thus giving records for ten trips toward the end of the trough at which 

 the adiathermal screen was placed, and for ten toward the screenless end. In the 

 tables the trips are marked "Water Screen," indicating those toward the screen, 

 and "No Screen," those in the opposite direction. After an animal had made ten 

 trips in each direction, the water screen was shifted to the other end of the trough 

 and another series of ten trips in each direction recorded. This was done in order 

 to eliminate any differences in the time of the trips toward the screen and those away 

 from it which might arise because of slight intensity differences between the two 

 lights or from other differences in the conditions of the halves of the trough. 



