The Reactions of Planarians to Light 133 



able time are more responsive when subjected to it, for the reason 

 that they are in a physiological state farther removed from fatigue 

 than those worms which have remained a long period in the light. 

 This point is brought out in Table XXXVI. 



TABLE XXXVI 



Percentage of reactions of two worms, Planaria gonocephala, to a sudden change in light intensity both 

 when previously kept several hours in the dark and also when previously exposed for several 

 hours to light 



Percentage of responses 



Number of observations 



After several hours in the light. 

 After 48 hours in the dark 



54 

 66 



100 

 100 



Summary. Physiological states grade imperceptibly into each 

 other, but may be tentatively divided into: 1, relaxation; 2, slight 

 activity; 3, normal activity; 4, violent activity; 5, rigor; 6, exhaus- 

 tion. 



Various stimuli besides light may induce a change from one 

 physiological state to another. 



No light intensity lower than 431 cm. is sufficient to throw a 

 worm into a higher state than that expressing normal activity, 

 nor is the absence of light sufficient to bring a planarian to rest. 



Excessive light intensity shows a tendency to carry Planaria gono- 

 cephala from a state of normal activity to one of rigor. Bdelloura 

 Candida is easily changed into a condition of rigor by light. 



A sudden change of light intensity acts more immediately than 

 a gradual change in causing planarians to pass from one physio- 

 logical state to another. 



Continuous exposure to light induces fatigue, finally resulting 

 in the passage of the worm into a state of continuous relaxation, 

 in which condition it becomes practically indifferent to light- 

 Repeated trials of the time required in constant light to come to 

 rest show that a progressively shorter interval occurs between the 

 state of normal activity and that of relaxation until a point of 

 complete inactivity is reached, the worm finally remaining in the 

 latter state for a prolonged period. 



