REACTIONS TO LIGHT IN CILIATES AND FLAGELLATES. 39 



By so doing it swims toward the window, thus in the direction from 

 which the strongest light is coming. According to the theory of photo- 

 taxis as due to the direct action of the light on the motor organs of the 

 animal, this movement is inexplicable. Thus, in the analysis of this 

 theory given by Holt & Lee (1901), it is shown that in the case of a 

 negative organism, such as Stentor, light of supraoptimal intensity, 

 like that coming from the window, must be assumed to cause increased 

 contraction of the cilia. After the organism has passed across the line 

 x-y, or while it is passing across this line, it has the anterior end directed 

 away from the source of light ; according to the tropism theory this 

 is a stable position and should not be changed. For, supposing the 

 organism swerves a little toward either side, the cilia on that side will 

 be more strongly affected by the light, so that the animal will at once 

 be turned back into the position of equilibrium with anterior end directed 

 away from the light. 



Nevertheless, under these circumstances the organism does turn and 

 swim back into the darkened area. An explanation for the apparent 

 movement of a negative organism against the direction of the light rays 

 is sometimes given in the following form : The light from the window 

 is said to fall upon the side or end of the dish farthest from the window 

 and is reflected back, so that the chief source of light for the Stentors 

 is not the window, but the side of the dish opposite the window. The 

 animal therefore becomes oriented with relation to this source of light 

 and swims away from it. 



Comparison of the movements of the Stentors in the darkened area 

 A with those in the lighted area B shows that this explanation can not 

 possibly be correct. Consider an individual at the point <5, Fig. 13, 

 which turns and swims toward the window into the dark region. It 

 is affected by light from two sources, (i) from the window, (2) reflected 

 from the side opposite the window. According to the above theory 

 the turning is due to the fact that the light from the opposite side is of 

 greater strength than that from the window (in itself a most improbable 

 suggestion). Compare this Stentor b with an individual at a, in the 

 darker region. This animal receives no direct rays from the window, 

 yet does receive the reflected rays from the opposite side. If these 

 reflected rays are sufficient to cause b to become oriented in spite of 

 the opposing rays from the window, they must produce the same effect, 

 a fortiori, on the individual 0, since they are the only rays which 

 reach it. Yet individuals in the position a do not become oriented at 

 all. The individuals in the shaded portion of the vessel swim about 

 in all directions, without relation to the direction of the light rays. It 

 is only when they come to the line x-y, where they would pass into the 



