REACTIONS TO LIGHT IN THE EARTHWORM. 25 



crawling on a moist paper it may be shaded by the hand or 

 otherwise. When the worm crawls to the edge of the shadow 

 and thrusts out its anterior end into the light it is jerked back 

 suddenly. But if the light be thrown upon the worm when con- 

 tracted, there is no sudden response, but only a gradual awaken- 

 ing to stimulation, as evidenced by subsequent movements. The 

 bearing of this observation upon the movements of the worm would 

 seem to be as follows : The worm contracted is like an animal with 

 its eyes partly closed. It extends its head at random, thus grad- 

 ually receiving the full stimulation upon its surface. If the 

 movement is toward the light, this causes it to contract more or 

 less and so check stimulation. If the movement is away from the 

 light, the oblique illumination produces less stimulation and the 

 movement is more prolonged. An animal with eyes, as a crusta- 

 cean, or an insect, is of course so organized that movements 

 toward the light may be checked, as it were, at the outset, in the 

 case of negatively phototactic animals. 



It is to be observed that the earthworm begins these random 

 movements while in the contracted state. After extension it 

 draws up its body by means of the longitudinal muscles and is 

 therefore in the contracted state. It then advances again, and at 

 each advance there may be a random change in direction. Thus 

 the worm begins these random movements when in the con- 

 tracted state and under minimum stimulation. The nature of 

 its locomotion and of the sensitive elements in its skin necessitates 

 the alternation of states of low and high sensitiveness. The 

 random movements of an earthworm under light stimulation are 

 consequently of an entirely special character, due to causes in- 

 herent in its structure. 



To recapitulate, three situations in regard to light have been 

 described, with their characteristic reactions. First, in weak 

 light, second in strong light and third in a situation involving 

 change of light intensity. 



The stimulus of a change in intensity causes the animal to 

 draw back its anterior end slightly and it then usually alters its 

 course. When crawling under the influence of sufficiently strong 

 light, it bends its head away from the light at each successive 

 advance, until it gets into the oriented position. In light not 



