132 BEHAVIOR OF THE LOWER ORGANISMS 



vessel are equally illuminated and the light comes from one side. This 

 may be done by placing the Stentors in a glass vessel with plane sides, at 

 one side of the source of light, as a window or an electric lamp. Move- 

 ment from one part of the vessel to another cannot cause a change from 

 darkness to light, for all parts are equally lighted. 1 Yet the Stentors 





FIG. 90. Method of observing the reaction of Stentor to light. A and B are two electric 

 lights, which can be extinguished or illuminated separately. 



usually, after a short interval, turn and swim away from the source of 

 light, after a time reaching that side of the vessel farthest from the lamp 

 or window. If the animals are observed as they turn, it is found that 

 the turning is brought about through the avoiding reaction. A short 

 time after the light is directed upon them, they swim more slowly or 

 cease the forward movement, and begin to swerve more strongly toward 

 the right aboral side, thus swinging the anterior end about in a circle. 

 The direction of movement thus becomes changed; in the new direc- 

 tion the animal swims forward. If its anterior end is still not directed 

 away from the source of light, the avoiding reaction is repeated; the 

 animal continues to try new directions till the anterior end is directed 

 away from the lighted side. In that direction it continues to move, so 

 that it finally comes to the side opposite the window or lamp. 2 



1 There is of course an infinitesimal difference in the illumination of different parts 

 of the vessel, due to the fact that one part is nearer the source of light than another. 

 The experiment succeeds equally well when the sun is employed as the source of light, 

 in which case the difference of illumination in different regions is practically infinitely 

 minute. The reaction cannot be therefore conceived as due to these differences. 

 Experiments show that the differences in illumination necessary to produce reaction 

 are much greater than those obtaining in different parts of a vessel thus lighted from 

 one side. 



2 The reaction may be obtained by focussing the Braus-Druner binocular micro- 

 scope on a shallow vessel of Stentors swimming about at random in a diffuse light, then 

 allowing a strong light from an electric lamp or a brightly lighted window to fall upon 

 them from one side. In order to have the reaction repeated many times, so as to give 

 opportunity for careful study, the vessel containing the Stentors may be placed between 

 two electric lights, as in Fig. 90. One of these lights can be extinguished at the same 

 instant that the other is brought into action ; by repeating this process the direction of 

 the light rays is repeatedly reversed. At each reversal the Stentors react in the way 

 described in the text. 



