130 



BEHAVIOR OF THE LOWER ORGANISMS 



Thus an area lighted from below acts in the same manner as a region 

 containing a strong chemical. The animals keep out of both by the 

 avoiding reaction. 



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 placing the glass 

 vessel containing the Sten- 

 tors near a source of light 

 which falls obliquely from 

 one side, then shading a 

 portion of the vessel with a 

 screen. We may first so 

 place the screen that the 

 vessel is divided into right 

 and left halves, at equal 

 distances from the source of 

 light, but one shaded, the 

 other illuminated (Fig. 87). 

 The Stentors are at the be- 

 ginning scattered through- 

 out the dish and are 

 moving in all directions. 

 Stentors in the illuminated 

 half whose path lies in the 

 proper direction pass into 



FIG. 87. Reaction to light in Stentor. The light the shaded region without 



comes from the left, as indicated by the arrows. S-Sisa reaction Since nearlv all 

 screen shading one half the vessel, so that the line x-y is . . , 



the boundary of the shadow. At b, 1-4, is shown the keep in motion for a long 



reaction of a Stentor on reaching this boundary line, time after an interval 

 (The dotted outline a, 1-4, shows the reaction that would 



occur if the light caused increased activity in the cilia of nearly all Will have passed 



the side which it strikes.) j nto tne shaded half. 



Stentors in the shaded half respond by the avoiding reaction as soon 

 as they come to the boundary of the lighted area. That is, they 

 swim backward and turn toward the right aboral side (Fig. 87, b). Thus 

 they remain within the shaded area, and after a short time most of the 

 Stentors in the vessel are to be found in the shaded half. 



It is evident that the Stentors do not simply turn and swim parallel 

 with the light rays from the source of light. If this were the method 

 of reaction, a Stentor coming to the boundary x-y, Fig. 87, would turn 

 and swim directly toward the side y. This it does not do. The direc- 

 tion of turning depends upon the position of the right aboral side ; the 



