The Reactions of Planarians to Light 131 



Moreover, light of any intensity or direction frequently proved 

 ineffective in arousing a quiescent worm into any state of apparent 

 activity, particularly if the worm had but recently passed into the 

 state of rest after a prolonged period of exercise. 



Effect of Excessive Light. In the experiments with non-direc- 

 tive light it appeared that Planaria gonocephala, when subjected 

 to an intensity of 431 cm., showed somewhat less activity than at 

 lower intensities, both with respect to rate of locomotion (Table 

 III, p. 57) and to the number of turnings made (Table VI, p. 59); 

 yet, so high a degree of intensity of the light stimulus was appar- 

 ently not sufficient to cause a change into the physiological state 

 of light rigor. It was comparatively easy, on the other hand, to 

 transform Bdelloura Candida by means of excessive light from 

 the state of normal activity into that of light rigor. 



Effect of Sudden Change in Light Conditions. A sudden 

 change in light intensity either by increase or decrease is more effec- 

 tive in producing a new physiological state than an equal grad- 

 ual change. The sudden withdrawal of the lamp to a consider- 

 able distance, for example, is usually sufficient to throw a worm 

 from a normal state into violent activity, that is, from a gliding 

 movement into a disturbed state in which the anterior end is 

 waved actively about. But if the light is gradually withdrawn 

 the same distance the worm will usually not pass into a different 

 physiological condition. 



The sudden introduction of complete darkness was never 

 found sufficient to reduce an active worm more than temporarily 

 to the resting position. Sudden dark might temporarily halt a 

 moving worm, but it would not cause it to come to rest and assume 

 the relaxed contour. In Bdelloura Candida sudden dark, instead 

 of checking the animal's movements, threw it into violent activity. 



Effect of Continued Exposure to Light. Continuous exposure 

 to light results in fatigue, which finally causes planarians to change 

 from the state of normal activity to that of relaxation. The tend- 

 ency toward such a change is shown in Table XXXV, where the 

 responses of a number of worms newly subjected to light stimu- 

 lation are contrasted with the responses made by the same worms 

 after they had been moving about for several hours in the light. 



