The Reactions of Plaiiarians to Light 



51 



Turning. That planarians do more turning in the dark than 

 they do in various intensities of non-directive Hght is apparent from 

 the following table of percentages. 



TABLE I 

 Percent of turnings of Planaria gonocephala in the dark and in various intensities of light 



Light in candle meters. 



431 lAv. of all 

 intensities 



Per cent of turning 



Per cent of straight paths. 

 Number of observations. . 



72 

 28 



Furthermore, out of a total of 46 cases of turnings made by dif- 

 ferent individuals of Planaria gonocephala in the dark 23 were 

 clockwise and 23 contra-clockwise. This perfect balance in be- 

 havior did not recur when the same worms performed turning 

 evolutions in the light. 



Change of Course. As to what constitutes "definite" and what 

 "indefinite" changesof path, an S-shaped course is to be regarded 

 as an indefinite aimless wandering, whereas angles in a straight 

 path or tangents in a curving path are classed as definite responses 

 because they are what would normally occur if some directive stim- 

 ulus were interposed. It was found that P. gonocephala made 

 indefinite changes in its course more frequently in the dark than 

 in any series of light intensities to which it was subjected for an 

 equal length of time. On the other hand definite changes occurred 

 oftener in the light, although the factor of directive light had been 

 excluded. 



Table II summarizes 350 records on 10 different worms with the 

 results reduced to percentages. 



It will be seen that the percent of S-shaped ("indefinite") paths 

 in the dark decidedly eclipses that which was made in any inten- 

 sity of light, while the per cent of angular and tangential paths 

 ("definite") laid in the dark is exceeded in every instance by that 

 made in any intensity of light with one exception, viz: 11 cm., 

 which, however, is not sufficient to change the average result. 



Summary. Planarians move about in the dark but at a slower 

 rate than in non-directive light whatever the intensity. They 



