Holmes, Random Movements. 105 



other direction, or it may be swung back without being with- 

 drawn. If a strong Hght is thrown upon a larva from one side 

 it may swing the head either towards or away from the Hght. 

 If the head is swung towards the Hght, it may be withdrawn or 

 flexed in the opposite direction, or, more rarely, moved towards 

 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 move- 

 ment which brings it away from the stimulus, its gen- 

 eral direction of locomotion is away from the light. 

 In very strong illumination 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 movement. Occa- 

 sionally the larva may crawl for some distance directly towards 

 the light, but after a time its movements carry it in the oppo- 

 site direction. When once oriented the direction of locomotion 

 of the larvae is comparatively straight. 



In the animals here described there is, so far I can discov- 

 er, no forced orientation brought about by the unequal stimula- 

 tion of the two sides of the body, but an orientation is produc- 

 ed indirectly by following up those chance movements which 

 bring respite from the stimulus. I do not deny that there may 

 be an orientating tendency of the usual kind, but if there is it 

 plays only a subordinate role in directing the movements of the 

 animal. The orientation of these forms is essentially a selec- 

 tion of favorable chance variations of action and following them 

 up. It is a type of reaction differing from photataxis in its typ- 

 ical form. It does not come under the head of photopathy and 

 it differs from Jennings' "motor reflex" by which many of the 

 so-called tropic reactions arc producted in the Protozoa, al- 

 though in common with the two latter modes of response it may 

 be considered as a form of "selection of over-produced move- 

 ments. " The light reactions of the forms studied may be inter- 



