REACTIONS TO LIGHT IN CILIATES AND FLAGELLATES. 



43 



which results in orientation? The experiments directed precisely 

 upon this point show that the stimulus producing the motor reaction 

 is an increase in the intensity of light upon the sensitive anterior end. 

 Now, in the reaction to a continuous light coming from one side, the 

 conditions are present for exactly such changes in the intensity of light 

 at the anterior end as would induce the observed reactions. In the 

 spiral course the animal swerves successively in many directions. In 

 certain directions the swerving subjects the anterior end to a more 

 intense illumination. This change acts as a stimulus to produce the 

 motor reaction, which carries the anterior end elsewhere. In other 

 directions the swerving leads to a decrease in the intensity of light 

 affecting the anterior end. In this case no reaction is produced, and the 

 organism continues to swim in that general direction. The details of 

 this method of reacting 

 will be given in the ac- 

 count of the reactions of 

 Euglena, where the mat- 

 ter was subjected to care- 

 ful analytical experimen- 

 tation. The evidence all 

 indicates that the condi- 

 tions in Stentor are ex- 

 actly parallel to those in 

 Euglena. 



We may sum up our 

 results on Stentor as fol- 

 lows : A change from 



dark to light, such as is caused by swimming from a shaded into an 

 illuminated region, acts as a stimulus to produce a typical motor reaction ; 

 the Stentor backs and turns toward the right aboral side, so that it 

 returns into the shaded region. A change in the illumination of the 

 anterior end produces the same effect as a change in the illumination 

 of the entire organism. The direction from which the light comes has 

 no observable effect on this reaction. But when the illumination is 

 uniform and the light comes from a definite direction, then light fall- 

 ing on the anterior end of the Stentor causes the reaction, while light 

 falling upon the posterior end causes none. The result is that the 

 animal turns (toward the right aboral side) until its anterior end is 



FIG. 16.* 



*FiG. 16. Method by which Stentor becomes oriented to light, when the light 

 falls on the aboral side of the animal. Stentor turns, as shown by the arrows, 

 at first toward the light, but the turning is repeated or continued until the 

 anterior end is directed away from the light 



