94 LIGHT AND THE BEHAVIOR OP ORGANISMS 



this way their reactions during the process of reorientation 

 could be studied in detail. The following description of 

 this process refers to E. x in the crawling state. 



If the light in which positive organisms are oriented is 

 decreased in intensity without a change in the direction of 

 the rays, e.g., by pushing back the Nernst glower on the 

 track, they respond in a characteristically definite way. 

 If the decrease is relatively slight the anterior end is merely 

 turned toward the ventral surface, the whole body becomes 

 more curved and the spiral course of the anterior end 

 becomes wider. If however the decrease is considerable, 

 they frequently stop in their forward motion and turn the 

 anterior end toward the ventral surface to such an extent 

 that the two halves of the organism form a right angle. 

 In this condition they continue to rotate, turning over and 

 over in the same spot, and appear to be squirming and 

 twisting about aimlessly. They soon however straighten 

 again and continue on their way toward the source of light, 

 having apparently become acclimatized to the change in 

 light intensity. If the intensity is increased there is no 

 response in positive Euglenae. Negative individuals, on 

 the contrary, respond precisely as described above if the 

 light intensity is increased, but not at all if it is decreased. 

 If the specimens however are only slightly positive or nega- 

 tive they may be caused to respond with this twisting reac- 

 tion either by increasing the intensity or by decreasing it. 

 In order to induce this reaction it is necessary to change 

 the intensity at a certain rate. If the glower is moved back 

 very slowly and steadily, no reaction whatever is seen. 

 A sudden decrease of intensity then without any change in 

 the direction of the rays produces a definite reaction in 

 positive individuals, and a sudden increase of intensity 

 produces the same reaction in negative individuals. These 

 reactions are in accord with the shock effects of Engelmann 

 and Pfeffer and Unlerschiedsempfindlichkeit of Loeb. They 

 are not due to an absolute change of intensity but to the 

 time rate of change of intensity. The amount of change 



