Figure 107. Stentor coeruleus in the process of orientation. Curved line, spiral course; 

 arrows m and n, direction of light from two sources ; a-j, different positions of Stentor on 

 its course ; o, oral surface ; ab, aboral surface. At a the Stentor is oriented in light from 

 m, n being shaded. If n is exposed and ?« shaded simultaneously when the Stentor is 

 in position b, there is usually no reaction until it reaches c and the oral side faces the 

 light; then the organism may respond by suddenly stopping, backing, and turning sharply 

 toward the aboral side (dotted outline), and become oriented at once; or it may merely 

 swerve toward the aboral side without stopping. At e the oral side is again exposed, 

 and the organism is again stimulated and it again swerves from the source of light. 

 This process continues until the oral side is approximately equally exposed to the light 

 in all positions on the spiral course. If the Stentor is at c when n is exposed, it responds 

 at once and orients as described above. If the light from n is more intense than that 

 from m, or if the organism is very sensitive when n is exposed and w shaded, it re- 

 sponds at once, no matter in which position it is. If it is at b, it turns toward the source 

 of light, but now repeats the reaction, successively turning in various directions until 

 it becomes oriented. (After Mast, 1911.) 



