PHOTOTAXIS 43 



turned away from the light and pull the body around. If 

 the head in moving to and fro comes into strong light it is 

 often retracted and extended again in some other direction, 

 or it may be swung back without being withdrawn. If a 

 strong light is thrown upon a larva from one side it may 

 swing the head either toward or away from the light. If 

 the head is swung toward the light, it may be withdrawn or 

 flexed in the opposite direction, or, more rarely, moved to- 

 ward the light still more. If it is turned away from the light 

 the larva usually follows up the movement by locomotion. 

 Frequently the larva deviates considerably from a straight 

 path, but as it continually throws the anterior end of the 

 body about and most frequently follows up the movement 

 which brings it away from the stimulus, its general direction 

 of locomotion is away from the light. In very strong illum- 

 ination the extension of the anterior part of the body away 

 from the light is followed by a retraction, since in whatever 

 direction it may extend it receives a strong stimulus and the 

 larva writhes about helplessly for some time. Sooner or 

 later, however, it follows up the right movements." 



The earthworm was found to orient itself in a similar 

 manner. " If a strong light is held in front of the worm it at 

 first responds by a vigorous contraction of the anterior part 

 of the body; it then swings the head from side to side, or 

 draws it back and forth several times, and extends again. 

 If in doing so it encounters a strong stimulus from the light 

 a second time it draws back and tries once more. If it 

 turns away from the light and then extends the head, it 

 may follow this up by the regular movements of loco- 

 motion. As the worm extends the head in crawling it 

 moves about from side to side, and if it happens to turn it 

 toward the light it usually withdraws it and bends in a 

 different direction. If it bends away from the light and 



