The Reactions of Planar/ ans to Light 



77 



is greatest when the worms are pointed toward the Hght, and least 

 when they are pointed in the opposite direction, while an inter- 

 mediate rate occurs when they are started at right angles to the 



This result is at variance with the findings of Parker and Bur- 

 nett ('oo, p. 381), who incidentally reported that Planaria gono- 

 cephala when started away from the light traveled faster than 

 when started toward the light. 



TABLE XIV 



Average rate of locomotion, expressed in mm. per sec, of various species of planarians when started 

 forward, away from, and at right angles to the source of directive light of 2^ c.m. intensity. 



Species 



Direction in which the worm was 

 pointed with reference to the light 



At right angles 



Toward 



Away from 



Dendrocoelum 

 lacteum 



Planaria 

 maculata 



Phagocata I Planaria : Total 

 gracilis gonocephala average 



0.855 

 0.910 

 0-795 



1-475 

 1.505 

 1.440 



r-445 

 1-430 

 1. 310 



0.980 

 1 .205 

 1.090 



1. 19 

 1.26 

 1.16 



It was further found that, regardless of the direction in which the 

 worms were started, there was a gradual decrease of the rate dur- 

 ing the four successive trials. The order in which different worms 

 were oriented during the four trials was arranged so as to neutral- 

 ize the possible effect of the sequence in the direction started. In 

 Table XV the data for 200 trials are arranged to express this 

 slowing down of the rate. 



TABLE XV 



Average decrease in rale of locomotion for 50 planarians during four successive trials while subjected to 



directive light of 2'J c.m. 



Various factors influencing the rate of locomotion, such as the 

 intensity of light, the size and species of the worm, the amount 

 of pigment present in the body and the general physiological state 

 of the animal under experimentation, will be more suitably dis- 

 cussed in other connections. 



