I02 LIGHT AND THE BEHAVIOR OF ORGANISMS 



be considered a criterion of psychic activity. For all that 

 Is known to the contrary Euglena may be conscious. It 

 may Indeed have anthropomorphic sensations accompany- 

 ing each reaction. But surely no one would consider the 

 fact that It always turns toward the same side and does 

 not respond In accordance with the theory of localized 

 stimulation as Indicating that it has such sensations. 



i. Orientation of Euglena in the swimming state. — 

 Early in December two species, Euglena deses and a form 

 much like viridls but somewhat larger, were found in the 

 free-swimming state. They were however not very active; 

 the specimens of E. deses studied swam only at an average 

 rate of approximately 0.25 mm. per minute and rotated 

 only about eleven times per minute; the other species moved 

 somewhat faster. The reactions In both could readily be 

 followed under a magnification of 150 diameters. Their 

 orienting reactions were studied In the same manner as 

 were those of Euglena In the crawling state, and they were 

 found to be practically the same. 



A decrease of the light Intensity in the field without a 

 change In the direction of the rays produces definite reac- 

 tions, (i) There is a slight bending of the anterior end 

 toward the dorsal surface. (2) The whole organism turns 

 toward the dorsal surface by the action of the flagellum. 

 (3) Their spiral course becomes wider. If the decrease is 

 considerable they may be thrown out of orientation entirely 

 and turn about several times before they become oriented 

 again. 



If the direction of the rays Is changed without the light 

 intensity's being changed, there Is usually no reaction, 

 just as m crawling specimens, until in the process of rota- 

 tion the surface containing the eye-spot comes to face the 

 light; then there is a sudden turning toward this surface, 

 i.e., toward the source of light. In many instances the 

 turning is so sharp immediately after the dorsal surface 

 becomes Illuminated that it may appropriately be desig- 

 nated as a jerk or a twitch. This reaction is repeated 



