CHAPTER VI 



OBSERVATIONS ON UNICELLULAR FORMS IN THE PROCESS 

 OF ATTAINING AND RETAINING A DEFINITE AXIAL 

 POSITION WITH REFERENCE TO THE 

 SOURCE OF LIGHT (continued) 



I. Stentor coeruleus 



a. Introduction. — Davenport (1897) and Holt and Lee 

 (1901) worked out the general features in the light reactions 

 of Stentor coeruleus. They found that these animals are 

 negative and that they orient rather accurately. Holt and 

 Lee concluded that orientation takes place in accord with 

 Verworn's theory, that light acts constantly as a directive 

 stimulation. If one side is more highly illuminated than 

 ^e other the cilia beat more effectively on the illuminated 

 side. This causes the animal to turn directly from the 

 source of light until it is oriented and both sides are equally 

 illuminated. Jennings (1904) found that an increase in the 

 light intensity of the field causes a definite reaction in Sten- 

 tor regardless of the direction of the rays or the surface 

 exposed at the time the change is made. This reaction was 

 designated the avoiding reaction. It consists of turning 

 toward the right aboral side. The organisms may stop 

 and turn very sharply or they may simply swerve farther 

 towards this side as they proceed on their spiral course. 

 By means of this turning the anterior end is directed toward 

 various points in space. Sooner or later it becomes directed 

 away from the source of light and the organism is oriented. 

 This direction is retained because when the anterior end is 

 turned from the light it is not subjected to changes in light 

 intensity as the animal rotates on its axis and continues 

 on its spiral course. 



Orientation, therefore, according to Jennings, is brought 



113 



