The Reactions of Planarians to Light 



57 



that cumulative effects and the influence of previous exposures 

 were largely avoided. 



The results obtained in 259 trials are condensed in Table III. 



TABLE III 

 Rate of locomotion in millimeters per second of Plana-ia gonocephala in various intensities of non-directive 



listht 



The mechanical stimulus resulting from the removal of the 

 worms, by means of a camel-hair brush, from their individual 

 aquaria to the observation aquarium was practically the same in 

 all cases as were all the other external stimuli except light. The 

 difi^erence in the rate of locomotion appearing in these averages 

 is, therefore, clearly due to diff'erences in the light intensity em- 

 ployed. 



It will be seen also that rate does not increase progressively with 

 intensity. The series of rates and intensities under Table III, 

 if plotted in a frequency curve would give two modes, one at 39 

 and the other at 217 candle meters, with a slight depression be- 

 tween the two. Still, as has been already pointed out, any inten- 

 sity of light gives a faster rate than no light at all. 



The slowest average rate was made under the highest intensity 

 of light employed. Certain facts to be brought forward later 

 favor the opinion that this was not an accidental result. 



Under continuous exposure to one intensity of light the rate of 

 locomotion decreases. The worms seem to " run down " gradually, 

 so that at the end often minutes their rate is only about half that 

 during the first minute. Data illustrating this point are given in 

 Table IV. 



The rate of locomotion depends not so much upon the intensity 

 of light as upon other factors which tend to produce individual 

 behavior upon the part of each particular worm. Stated in another 

 way, there is greater variation between different individuals in the 

 average rate of their locomotion under all intensities than there 



