VERMES, FLY LARVAE, AND ECHINODERMS 209 



by a sudden change of light Intensity and by constant inten- 

 sity, the processes involved are different. A sudden change 

 of intensity acts much like mechanical contact or a change 

 in an electric current. Constant intensity on the other 

 hand acts like constant temperature. 



Our original questions still remain: Is orientation due to 

 light acting through change of intensity? Or is it due to 

 constant intensity, both sides of the organism being stimu- 

 lated constantly, but unequally when one side is more 

 highly illuminated than the other, thus causing difference 

 in rate of movement of the two sides ? The maximum dif- 

 ference in rate due to absolute difference of intensity, as we 

 have seen, is only 0.18 mm. per second. It is evident then 

 that the greatest difference in rate of locomotion on the 

 two sides due to absolute difference of intensity could not 

 be more than 0.18 mm. per second; and if orientation is 

 due to this it is clear that the orienting process would be 

 exceedingly slow, very much more so than it actually is. 



Our evidence then indicates that orientation in these 

 forms is due not to light acting constantly as a directive 

 stimulus similar to the action of a constant electric current 

 in accord with Loeb's theory, but to reactions caused by 

 intermittent action of light through changes of intensity 

 on some part of the sensitive surface. These changes may 

 of course be due to the movements of the organism or to 

 changes in the direction of illumination. 



It is evident that if the anterior end turns from side to 

 side, or if the ray direction is changed, the intensity on one 

 side becomes higher while that on the other becomes lower. 

 Is the orientation due to the former or to the latter? In 

 Euglena it was demonstrated that if the specimens are posi- 

 tive the orienting stimulus is due to a decrease of effective 

 intensity; if negative, to an increase of effective intensity. 

 In Planaria there appear to have been no observations bear- 

 ing directly on this point. I frequently observed, however, 

 that when positive polyclads crawling toward a source of 

 light come into contact with a sharp shadow at either edge 



