The Reactions of Planarians to Light 8 1 



a point directly opposite the light was reckoned as o per cent of 

 error, whereas emergence at a point directly toward the light was 

 reckoned as ioo per cent, or a maximum of error in orientation. 

 The orientation value at these two extremes having been estab- 

 lished, the percentage of error which occurs when the worm 

 emerges at any intermediate position on the circumference of the 

 circle may be easily determined. 



TABLE XVI 



Average degree of error in orientation made by various species of planarians during 360 trials in directive 





light 



of different intensities 













Percentage of error in orientation 





When started to- 

 ward the light 



When started 



away from the 



light 



Average 



3 . 3 candle meters 



34 

 3 2 

 3 1 



11. 5 



12 

 10 



22.7 

 22.0 





20.5 







3 2 



11 









From this table it appears that there is three times as much 

 wandering, or error of orientation, by worms headed toward the 

 light, as by those headed away from it. This doubtless indicates 

 that orientation is a more complicated process in the former case 

 than in the latter. 



Duration of Activity. Superficial observation is sufficient to 

 establish the fact that different species of planarians when set into 

 activity in directive light show decided differences with regard to 

 the length of time they normally continue in motion before com- 

 ing to rest. Among the forms experimented upon, Bdelloura 

 came to a stand-still in light soonest and Phagocata latest.'' Fatigue 

 in itself is by no means the inevitable result of continued activity 

 on the part of an organism. For instance, Hodge and Aikens ('95) 

 observed a Vorticella continuously for 36 hours, during which 

 time its regular ciliary and contractile movements continued unin- 

 terruptedly, while Radl ('01) found that the eye of Daphnia when 



