152 Herbert Eugene Walter 



the worm into a different physiological state. Exploring move- 

 ments, such as these seem to be, may bring about asymmetrical 

 stimulation, in which* case the worm makes a tropic response. 



It was particularly noticed that when planarians received light 

 from below, the anterior end of the body was frequently tilted 

 back and forth as if to make it possible for the light when coming 

 from such an unusual direction to enter the pigment cups of the 

 eyes. The phenomenon suggested the craning of necks and bob- 

 bing of heads among a crowd of people who are all trying to see 

 the same object at once. 



Wigwag movements seem to be oftener connected with changes 

 in the intensity of light than with changes in its direction. When 

 the latter occur, tropic response is immediately the result. 



In the course of the experiments previously described wherein 

 the worms glided from an area of one intensity of non-directive 

 light into another it was noticed that in a majority of cases when 

 the critical line was not crossed at right angles, no change in course 

 occurred, even when the worm halted and made wigwag move- 

 ments. Of course at a certain instant of any diagonal crossing 

 of the critical line one eye must receive more stimulus than the 

 other, in which case according to an inflexible tropism theory 

 asymmetrical response ought to occur. But such a response does 

 not frequently appear and the reason for this becomes clear when 

 it is remembered that a considerable number of responses were 

 shown to occur which were called "latent wigwags" (Fig. 4, E), be- 

 cause they failed to make their appearance until in some instances 

 the worm had passed more than the length of its body beyond 

 the critical line. Since, therefore, latency of response to intensity 

 is by no means uncommon, it is evident that the brief interval of 

 asymmetrical stimulation occurring when a worm glides diagonally 

 into an area of different intensity is not sufficient to result in an 

 asymmetrical response. 



Two conclusions, then, seem reasonable, namely, that phototaxis 

 as related to planarians is primarily due to asymmetrical response 

 resulting from asymmetrical stimulation, and that wigwag move- 

 ments, together with similar apparent trial and error forms of 

 behavior, contribute chiefly to this end, i. e., to phototaxis. 



