210 LIGHT AND THE BEHAVIOR OF ORGANISMS 



of the beam in which they are exposed, so as to shade one 

 side, they turn directly from the shaded side, indicating 

 that the orienting stimulus in positive Planaria, as in Eu- 

 glena, is due to a decrease in effective intensity on some part 

 of the sensitive surface and that orientation is brought 

 about directly by differential response to localized stimuli. 



Summary 



(i) Planaria may collect in regions of optimum light 

 intensity either by wandering into such regions and coming 

 to rest or by orienting and crawling directly toward such 

 reg ons and coming to rest. The latter method is of course 

 more effective than the former. 



(2) In some forms orientation is very indefinite; in others 

 it is fairly accurate. 



(3) In locomotion there are frequent lateral head move- 

 ments. These may be accelerated either by sudden increase 

 or by sudden decrease in light intensity. They appear to 

 be independent of the location of the stimulus. 



(4) Orientation at least in some forms is due largely to 

 differential response to localized stimulation. The lateral 

 head movements no doubt function by increasing localized 

 stimulations, and thus make it possible for the organism to 

 direct its cour-e more efficiently than it otherwise could. 



(5) Light acts on Planaria by virtue of both changes of 

 intensity and constant intensity. Responses to changes 

 of intensity are comparatively rapid ; responses to constant 

 intensity comparatively slow. The effect of constant light 

 is similar to the effect of constant temperature. 



(6) The orienting stimulus appears to be due to changes 

 of effective intensity on some part of the sensitive surface. 

 In positive specimens it is, as in Euglena, probably due to a 

 decrease, in negative specimens to an increase of intensity 

 on one side. 



(7) There is no evidence indicating that light owing to 

 constant intensity is functional in the process of orientation. 



