364 



REACTIONS OF DAPHNIA PULEX TO LIGHT AND HEAT. 



a line of light perfectly graded in intensity was obtained. Figure B, an oblique side 

 view of the lens, will make clear the form of the lens and its relation to the screen. 

 On its way to the lens the light passed through a solution of alum, 10 centimetres 

 deep, in a glass dish, A (Fig. A). By this solution the heat-rays were cut off. From 

 the box the rays, in order that the results might not be complicated by reflection, 

 were allowed to pass through a slit, K, in the table, F, to a mirror, M, by which they 

 were reflected to the side of the room. Further, all end and side light was shut off 

 from the experiment-box by black cardboard screens, E, and heavy cloth curtains, D. 

 In the experiments the plate-glass experiment-box, 25 centimetres long, 5 centi- 

 metres broad, and 4 centimetres deep, was filled with water to a depth of .5 centi- 

 metre, and so placed that the band of light from the lens, which was 16 centimetres 



long and about 1 centimetre wide, illuminated the middle 

 region of it, whereas the remainder of the box was dark. 

 The Daphnia were, therefore, free to move either in the 

 relatively dark regions or in the band of light. The 

 smallest angle made by any ray. with the bottom of the 

 box, T, was approximately 81°, hence the animals moved 

 almost at right angles to the rays, so long as their move- 

 ments were parallel to the bottom of the box. 



By varying the size of the triangular opening through 

 which light was admitted to the lens, and by using lamps 

 of different powers, it was possible to increase or diminish 

 the intensity of the band of fight. 



B. Observations. — Daphnias when placed in the 



experiment-box wander into the band of light. Having 



entered the band they move from side to side in it, but 



, , , seldom go outside; as soon as they reach the edge of the 



of light at the + end of the band ' _ J 



of light; o indicates the course of band and the anterior portion of the body comes into the 

 the rays at the - end. fa^ reg i on f ^he box, while at the same time the 



posterior portion is in the band of light, they turn back into the illuminated area. 

 This reaction occurs repeatedly during the progress of an animal from one end of the 

 box to the other. It is strikingly similar to the chemotactic reactions of Paramecium 

 as described by Jennings ('97, p. 269). 



Furthermore, the animals when once they have entered the band of light move 

 very quickly to the region of greatest intensity of illumination, and remain there most 

 of the time. It is to be remarked that the animals in moving from the lower to the 

 higher intensity of light endeavor to orient themselves so that the long axis of the 



Figure B. — Cylindrical Lens and 

 Screen with Triangular Open- 

 ing. 



X, horizontally placed screen of 

 black cardboard resting upon the 

 plane surface of the lens, with a 

 triangular opening, V, beneath 

 which is seen, at t, the curved 

 outline of one end of the lens; 

 n, n indicate the course of rays 



