362 REACTIONS OF DAPHNIA PULEX TO LIGHT AND HEAT. 



reactions are in all probabilty frequent because of difference of sensitiveness to light 

 in different regions of the body.* 



II. STATEMENT OF PROBLEMS. 



Davenport and Cannon ('97), Yerkes (:00), and Towel (:00) have shown that 

 Daphnia is positively phototactic. 



The experiments reported in this paper bear upon the following problems: 



1. Does Daphnia react photopathically as well as phototactically? Is there any 

 evidence of an "optimal intensity"? 



2. Does the radiant heat accompanying light influence the movements of the 

 animals? 



3. Does heat, in the absence of light, have a directive influence upon the move- 

 ments of Daphnia? 



4. How does light in its effects upon Daphnia differ from heat? Are both 

 effective only as they change the temperature of the organism? 



III. EXPERIMENTS. 



i. Tests for Photopathic Reactions A. METHOD. As a means for discovering 

 whether there is an "optimal intensity" of light for Daphnia an apparatus was 

 devised which enabled the experimenter to observe, in a dark chamber, the movements 

 of the animals in a glass dish the middle region of which was illuminated by a band 

 of light which varied in intensity from zero at one end to as much as 20 candle- 

 power at the other. 



Figure A is a vertical section of the apparatus used for this work. It consisted 

 of a cylindrical lens, L, 25 centimetres long, 10 centimetres wide, with a radius of 

 curvature of 25 centimetres. One surface of the lens (Fig. B) was flat. This lens 

 was fastened, for convenience of focussing, in a movable holder, H, and so placed 

 that it focussed certain of the parallel rays proceeding from two 16-candle-power lamps 

 (in some tests a 100-candle-power lamp was used), C. B., in a line upon the bottom 

 of a glass box, T, in which the Daphnia were placed. Just above the lens and in 

 contact with its plane surface was a screen of black cardboard containing a triangular 

 opening through which the rays reached the lens. This triangular opening permitted 

 many rays to fall upon the lens at one end, and at the other none, as a result of which 



* Parker and Arkin (: 01, p. 156) call attention to the possibility of this kind of reaction in case of the earthworm. 



