42 THE BEHAVIOR OF LOWER ORGANISMS. 



most of the individuals have gathered at the side next to A^ as in 

 Fig. 15. Then B is extinguished, while at the same time A is illumi- 

 nated. The Stentors then turn and move toward B. They may be 

 stopped at any point in their course and the direction of swimming 

 reversed by simultaneously turning off one light and turning on the 

 other. With a sensitive culture the phenomena take place with con- 

 siderable precision, about four-fifths of the individuals responding 

 quickly to every reversal of the direction from which the light comes. 



Under these circumstances it is easy to observe the individuals at the 

 moment of the reversal of the course. The observation already made 

 is confirmed ; the animals always turn at the moment of reversal toward 

 the right aboral side. The reaction is thus of the same sort that occurs 

 when there is a sudden increase in illumination. After the first reaction 

 the anterior end is pointed in a new direction. If this new direction 

 is away from the source of light the animal swims forward in the 

 course so laid out. If, as is usually the case, the first reaction does 

 not result in directing the anterior end away from the source of light, 

 the reaction is repeated, and this may occur several times. Thus the 

 anterior end becomes directed successively toward every quarter ; as 

 soon as it lies toward the side opposite the light the reaction ceases. 

 The animal now swims straight ahead (that is, in a spiral with a 

 straight axis) away from the source of light. 



Thus while it is clear that light falling from one side produces a 

 well-defined orientation, this orientation does not take place in such a 

 way as to be in accordance with the tropism theory as set forth, for 

 example, by Holt & Lee. It is not the direct action of the light on 

 the motor organs of the side on which it impinges that determines 

 the direction of turning, but the latter is due to an internal factor. 

 This becomes still more evident when the conditions are so arranged 

 that the direction of turning demanded by the internal factor is the 

 opposite of that required by the tropism theory. 



These conditions can be fulfilled in the following manner : The 

 light to be turned on (Fig. 15) is so moved beforehand that its rays 

 shall fall, not directly on the anterior end of the Stentor, but obliquely 

 at an angle to the path they are following. The animals then react 

 as before, by turning toward the right aboral side. It often happens 

 that this involves first a direct turning toward the light, as illustrated 

 in Fig. 16. In such a case the turning is continued or repeated until 

 the anterior end is directetfaway from the source of light. We have 

 seen the same result produced under similar conditions in the experi- 

 ments illustrated in Fig. 13. 



What is the real stimulus to the production of the motor reaction 



