REACTIONS OF DAPHNIA PULEX TO LIGHT AND HEAT. 375 



for different regions of the body determines the direction of movement. I have there- 

 fore used the term thermotactic to refer to a motor reaction which is determined by 

 difference in the degree of stimulation of different parts of the animal. 



D. The Relation to Each Other of Heat and Light as Stimuli. — These 

 experiments have some bearing upon the theory that light stimulates an organism in 

 essentially the same way as heat, that is, by altering the chemical processes of the 

 tissues through changes in the temperature of the regions affected. Such a theory 

 receives some support from the fact that the reactions of certain animals to light can 

 be changed from positive to negative, or the reverse, by changing the temperature. 

 Groom and Loeb ('90, pp. 166, 167) found that the nauplii of Balanus react differently 

 to light at different temperatures; Loeb ('93, p. 91) has proved that Polygordius 

 larvae which are positively phototactic at 24° C. are negative at 29°; Massart ('91, 

 p. 164) states that a flagellate, Cnromulina, is positive at 20° C. and negative at 50°, 

 and Strassburger ('78, p. 605) noticed that certain swarm-spores that were positive 

 at 16° to 18° were negative at 40°. 



Thus far in experiments with Daphnia I have been unable to change the sense, 

 of the reaction to light by changing the temperature. Yet, if light affects the organ- 

 ism as does heat, one would expect that an organism which avoids high temperatures, 

 as these experiments have shown that Daphnia does, would become negatively photo- 

 tactic in the presence of strong light. But even light which is accompanied by suf- 

 ficient heat to kill the animals is sought by Daphnia. For this reason, and also be- 

 cause the reaction to light is exceedingly quick, whereas that to heat is slow, I feel 

 justified in concluding that light acts upon Daphnia otherwise than does heat. It 

 may well be, however, that the difference is one of degree rather than kind. 



IV. SUMMARY. 



1. Daphnia pulex is strongly positively phototactic to all intensities of light 

 from to 100 candle-power. 



2. There is no evidence of preference for a certain intensity, the "optimal." 



3. So far as the experiments described in this paper indicate, the heat accompany- 

 ing the light from a 16-candle-power incandescent lamp does not have any noticeable 

 influence upon the direction or rate of movement of Daphnia. 



4. Subjection of Daphnia to 16-candle-power light for a period of one hour, dur- 

 ing which interval the animal was kept in almost constant motion, showed a gradual 

 increase in the rate of movement, and in the sensitiveness to changes in the intensity 



