34 THE BEHAVIOR OF LOWER ORGANISMS. 



in the manner above described. In such cases the nature of the reac- 

 tion is seen with especial clearness, the entire animal being projected 

 on the screen and the differentiations of bodily structure (mouth, oral 

 and aboral sides, etc.) being conspicuous. Specimens which swim 

 completely into the lighted area are usually compelled to react two or 

 more times before they escape from the lighted region. 



When the light is cut off entirely the Stentors distribute themselves 

 throughout the dish. If the light is now admitted from below, the 

 unattached Stentors in the lighted area react by swimming backwards 

 a certain distance, turning toward the right aboral side, then swimming 

 forward again. This reaction is repeated frequently until after an 

 interval the Stentors are carried by these movements outside the lighted 

 area. They then cease to give the reaction. The reaction, under these 

 conditions, is thus the same as that produced when Stentors or Para- 

 mecia are subjected to other adequate stimuli, as when they are placed 

 in a chemical or dropped into very warm or very cold water. The 

 result of the reaction is, in every case, to remove the organism from 

 the sphere of action of the stimulus. When the stimulus is light this 

 result is produced in exactly the same way as when the stimulus is 

 heat or cold or a chemical. 



The same results may be obtained by lighting the vessel containing 

 the Stentors directly from above and shading one portion with a screen. 

 The Stentors remain in the shaded region, responding by the motor 

 reaction above described when they come to the lighted area. With a 

 favorable culture the experiment succeeds even when the source of 

 light is comparatively feeble, as when an ordinary incandescent electric 

 light is used as the source of illumination. 



The results so far show that a sudden increase in the intensity of 

 illumination induces in Stentor a reaction which is of the same 

 character as the reaction to other strong stimuli. Such a sudden 

 increase may be due either to the passage of the Stentor from a dark 

 to a light region, or to a sudden increase in the brightness of the light 

 which falls upon the animal. The general effect of the reaction is to 

 prevent the Stentor from entering a brightly illuminated area, or to 

 remove it from such an area. 



We may now arrange the conditions so that the light shall come 

 from one side, while at the same time differences in illumination shall 

 exist in different regions. This may be done by illuminating the vessel 

 containing the Stentors from the side, then covering one portion of the 

 vessel with a screen. 



The organisms are placed before a lighted window, or an incandes- 

 cent electric light, in a vessel with a plane front (Fig. 12). One-half 



