Harper, Reactions of PerichcBta. 583 



It was shown that weak light may be rendered inconstant as a 

 stimulus bj the movements of the earthworm, since in projecting the 

 anterior end forward the photoreceptor cells become more exposed 

 and the sensibility to light is consequently increased. There is an 

 alternation therefore of varying degrees of sensibility to light in 

 locomotion and this may account for the random character of many 

 movements in weak light and the slowness of the process of orienta- 

 tion. On the other hand, immediate and continuous orienting effects 

 may be observed in light of sufficient strength ('05). 



Torrey cited this instance as an example of two fundamentally 

 different types of response occurring in the same animal for the 

 same stimulus, light. In the weak light, the inconstant stimuli 

 (made so by the movements of the animal) give rise to reactions of 

 unterschiedsempfindlichkeit. On the other hand in stronger light the 

 tropic or orienting effects are produced. The view that the reactions 

 in weak light are unterschiedsempfindlich is then based on the propo- 

 sition that the anterior end becomes more sensitive when extended 

 and that the worm tends to turn away more strongly after an exten- 

 sion of the head. It is a natural inference that it reacts to the 

 change of intensity felt in extension. The character of the reaction 

 ought however to be considered as well as the nature of the stimulus. 

 Retraction into the burrow on sudden illuminatioli is a clear example 

 of a shock reaction. It was pointed out above that the threshold 

 for tropic responses was lower than for shock reactions. It is easy 

 to convince oneself that the same rule holds for light. As described 

 above one may cause turning movements in response to either 

 sudden or steady illumination and rarely a shrinking movement. 

 If a worm which has extended its anterior end, then makes a 

 turn away from the light, is that to be interpreted as an unter- 

 schiedsempfindlich response because it followed a change of intensity 

 of the light upon the cells ? If so turning responses would have to 

 be separated into two classes on a merely logical basis, corresponding 

 to no real difference in the reactions. The other supposition that the 

 threshold for tropic, i. e., tonic responses is lower than for shock 

 effects, corresponds to observed differences in the reactions. Tropic 

 effects of brief duration might be produced by stimuli too weak to 

 produce any shock effect. 



