2o8 LIGHT AND THE BEHAVIOR OF ORGANISMS 



is regulated by the relative intensity of light on opposite 

 sides. 



It may now be asked: What is the cause of the orienting 

 stimulus ? Is it a change of intensity or constant intensity ? 

 There is direct evidence showing that the organism responds 

 both to changes of Intensity and to constant intensity. If 

 a planarian passes suddenly from a region of a given inten- 

 sity into a region of a higher or lower intensity, it responds 

 by suddenly turning the head from side to side; and when 

 subjected to constant light of different intensities it may 

 become more or less active. In working with fresh-water 

 planarlans I frequently observed that if they were exposed 

 to constant illumination after they had been at rest for 

 some time, they did not respond for several minutes; when 

 they did respond they first moved very slowly and very 

 gradually became more active. Walter (1907, p. 63) records 

 similar observations. It may of course be argued that even 

 in this case it is the change of intensity due to turning on 

 the light that arouses the animals. It Is however hardly 

 probable that It Is, since the response Is frequently not 

 apparent until after the animals have been exposed for 

 several minutes. It appears that a difference In constant 

 light Intensity not only causes the planaria to become active 

 or to come to rest, but that it also affects the rate of move- 

 ment after they are active. 



In experiments on Planaria gonocephala in constant illu- 

 mination of different intensities from directly above, Walter 

 (1907, p. 57) found an average rateof locomotion of 0.57 mm. 

 per second in darkness, 0.63 mm. per second in 431 ca. m. 

 The highest rate, 0.75 mm. per second, was in 39 ca. m. 

 It v/ill thus be seen that the greatest increase due to a 

 difference In constant Intensity Is 0.18 mm. per second. 



It is therefore clear that a change of Intensity causes a 

 comparatively rapid response, and difference in absolute or 

 constant Intensity a comparatively slowresponse. This leads 

 to a conclusion arrived at previously several times in these 

 chapters, that while an organism may be stimulated both 



