REACTIONS TO LIGHT IN CILIATES AXD FLAGELLATES. 65 



NATURE OF AGENT CAUSING THE REACTION. 



(i) The primary and essential cause of the reaction is a change of 

 illumination. The change of illumination must take place with some 

 suddenness, but need not be very great in amount. The change in 

 illumination acts as an effective stimulus even though the degree of 

 illumination preceding the change and that following it would, when 

 acting continuously, produce no such result. This is shown by the 

 experiments on Euglena, in which the light coming from one side was 

 decreased a certain amount. The orientation of the organisms and 

 their direction of movement was the same before and after the change, 

 but at the moment the change occurred there was a marked reaction. 

 Other experiments detailed above demonstrate the same thing. Further, 

 the change in illumination acts independently of the direction of the 

 rays of light. This is shown by the experiment just cited, in which 

 the effective direction of the rays of light was the same before and 

 after the reaction ; it is also shown in the reaction caused when the 

 light is decreased from below, in the case of Euglenae swimming 

 toward a window (p. 50), and in the reaction of Stentor on passing from 

 a shadow to a lighted region even when the animal is oriented with 

 anterior end away from the light (p. 39). The change in illumination 

 acts equally whether it affects the entire organism or only the anterior 

 end. The evidence indicates that in all cases it is really the change at 

 the anterior end which induces the reaction. 



(2) The absolute intensity of the light affects the reaction by deter- 

 mining in a given case whether a reaction shall be caused by an 

 increase or a decrease in illumination. Through this action it also 

 determines, in the way to be mentioned in the next paragraph, whether 

 in a continuous light the sensitive anterior end shall be directed toward 

 or away from the source of light ; that is, whether the response shall 

 be "positive" or "negative." 



(3) Indirectly, and through the factor set forth in paragraph (i), the 

 direction from which the light comes is a determining factor in the 

 reactions. Through the spiral course in which the organisms swim such 

 conditions are furnished that in a field continuously lighted from one 

 side the sensitive anterior end of the unoriented organism is subjected 

 to repeated changes in the intensity of illumination. As a result, 

 organisms which respond by the motor reaction to an increase in illu- 

 mination at the anterior end must become oriented with anterior end 

 directed away from the light; organisms which react to a decrease in 

 illumination must become oriented with anterior end directed toward 

 the light. (Details in the account of Euglena, pp. 60, 61, and Figs. 

 23. 24.) 



