THK BEHAVIOR OF LOWER ORGANISMS. 



action of the light on the two sides of the animal to a precise test, and 

 found it to be incorrect. 



The same result is brought out in perhaps a still more striking 

 manner by the following method of experimentation : A vessel con- 

 taining Stentors is placed on a dark background near a source of light 

 (a window or an incandescent electric lamp). The light thus comes 

 from one side and a little from above. An opaque screen is placed 

 between the window and the vessel containing the Stentors, of such a 

 size and in such a position that the top of the shadow of the screen falls 

 across the middle of the vessel on the line x-y (Fig. 13 ; see also Fig. 

 14). Thus the half of the vessel next to the window (A) is darker 

 than the farther half (-B), and the Stentors collect in this shaded half 

 After some time scarcely a specimen is found in the lighted part of the 

 vessel away from the window. The conditions in this case are illus- 

 trated in the side view (Fig. 14). 



The exact behavior of the Stentors in 

 the darkened portion of the vessel is then 

 studied by focusing upon them the Braus- 

 Driiner microscope. The Stentors within 

 the shaded area are not oriented nor gath- 

 ered in any particular region, 

 but swim about at random. 

 When one of the specimens 

 comes in its course to the 

 line x-y (Fig. 13), separating 

 the darkened area from the 

 light, it responds to the sudden light which falls upon it from the 

 window by giving the motor reaction, turning to the right aboral side 

 and swimming back into the shaded region. Often the reaction occurs 

 as soon as the anterior end of the Stentor has crossed the line x-y, so 

 that the entire Stentor does not pass out into the lighted area. In other 

 cases the specimen crosses the line x-y completely before the reaction 

 occurs, so that the entire body is illuminated. It then reacts in the 

 usual manner, turning toward the right aboral side, so that it is headed 

 toward the shaded region ; thus swimming back across the line (Fig. 

 13, c) . After returning into the shaded region the animals swim about 

 at random as before. 



What is the reason for the return of the Stentor into the darkened 

 area after it has crossed the line into the light region? 



*Fio. 14. Sectional view, from the side, of the conditions shown in Fig. 13. 

 The arrows show the direction of the light rays. The region from 5 to x is 

 shaded by the screen s. 



X 

 FIG. 14.* 



