^^ 



Fig. 14. Stentor coeruleus in the process of orientation. The curved line 

 represents the spiral course; the arrows m and n the direction of light from two 

 sources; a-f, dififerent positions of Stentor on its course; 0, the oral surface; ab, the 

 aboral surface. At a the Stentor is oriented in Hght from m, n being shaded. If 

 n is exposed and tn shaded simultaneously when the Stentor is in position b, there 

 is usually no reaction, if the intensity has not been changed, until it reaches c and 

 the oral side faces the light; then the organism may respond bj^ suddenly stopping, 

 backing and turning sharply toward the aboral side, as indicated by the dotted 

 outline, and become oriented at once; or it may merely swerve more or less 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 is continued 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 « is more 

 intense than that from m, or if the organism is very sensitive when n is exposed and 

 m shaded, it responds 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, 117 



