The Reactions of Plananans to Light 93 



lently in the water. It is uncertain how far this behavior is attrib- 

 utable to light alone or to some combination of light and thigmo- 

 taxis. 



This phenomenon of compound stimulation occurs in a less pro- 

 nounced way whenever a change of light intensity results in the 

 "wigwagging" response common to planarians. The same uncer- 

 tainty prevails as to how far the subsequent behavior of the worm 

 may be due to the direct stimulation of light and how far to thig- 

 motactic stimulation primarily and to light stimulation second- 

 arily. It is evident, then, that under any circumstances there is 

 such a close interrelation of stimuli that an accurate analysis of 

 the consequent behavior is difficult. 



Further evidence of the close relation between different kinds 

 of stimuli is afforded by the fact that planarians are more respon- 

 sive to the mechanical stimulus of a slight jar when the entire ven- 

 tral surface of the body is in contact with the substratum than 

 when the anterior end is lifted up and waving about. Apparently 

 the greater the degree of contact the greater is the effect of a jar- 

 ring mechanical stimulus. 



This point was demonstrated by means of a small aquarium 

 mounted on a turntable, such as is used in "ringing" micro- 

 scopic slides, in such a way that it could be rotated with great 

 ease and delicacy. A light from one direction only was projected 

 upon the single planarian placed in the aquarium. Any attempt 

 to change the angle of light by rotating the aquarium ever so slightly 

 resulted instantaneously in a momentary halt on the part of the 

 worm, provided it happened to be gliding with its ventral surface 

 entirely in contact with the floor of the dish. If, however, the 

 rotation was made when the anterior end of the worm was lifted, 

 the halt did not so readily occur. This response was of such deli- 

 cacy that with a little practice it was possible to halt the anterior 

 end of a worm without disturbing the continuous progress of the 

 posterior end! That this halting was due to thigmotaxis rather 

 than to any rheotaxis induced by the movement of the animal 

 against the relatively stationary water particles, is shown by the 

 fact that the reaction was more pronounced when the anterior 

 end of the body was held flat than when it was raised and so brought 

 more under the possible influence of a water current. 



