62 Herbert Eugene Walter 



where they finally stopped, usually in the angle formed by the 

 paraffine wall and the bottom. 



Loeb's conclusion ('93b, p. 101) that planarians subjected to 

 directive light come to rest in regions of least intensity, seems 

 therefore to be equally true of planarians in non-directive light. 



Summary. In non-directive light Planaria gonocephala moves 

 faster, turns less and makes more " definite " but fewer " indefinite" 

 changes than in the dark. Rate of locomotion; amount of turning; 

 changes in the character of the course, as well as the amount of 

 wandering, do not appear to be correlated with varying light inten- 

 sities, unless in the following instance. Under the highest inten- 

 sity employed, namely, 431 cm., occurred the slowest rate; the 

 least turning; the greatest number of "definite" and the fewest 

 "indefinite" responses, together with the straightest paths. The 

 excess of clockwise over contra-clockwise turnings throughout the 

 series of intensities is probably not attributable to light. 



Continuous exposure to light results in a decreasing rate of loco- 

 motion, although in the second minute of movement as compared 

 with the first an increase in the rate of locomotion takes place, 

 while fewer turnings occur. 



Rate of locomotion is less influenced by differences in light inten- 

 sity than by certain internal factors which go to make up what 

 may be termed the individuality of different worms. Individ- 

 ual worms may sometimes fail to respond for a considerable inter- 

 val of time to light stimuli that ordinarily produce immediate 

 effects. 



Finally, planarians subjected to non-directive light come to 

 rest in regions of lessened light intensity the same as they do in 

 directive light. 



C Abrupt Changes in Intensity 



Abrupt changes in intensity may be of two kinds : either with 

 reference principally to time or to space. First, those changes are 

 abrupt in time in which light or dark is suddenly thrown upon the 

 worm, and secondly, those changes are abrupt in space in which 

 a moving worm passes immediately from an area of one intensity 

 into a sharply defined area of a different intensity. This topic 



