112 Studies in Animal 'Behavior 



A at all. We are warranted in assuming that the 

 ability to respond to light depends upon the exist- 

 ence of substances that undergo photochemical 

 changes. The effect of light would be to exhaust 

 certain substances and to cause the accumulation of 

 their products. These products when they exist in 

 a certain degree of concentration may stimulate di- 

 rectly or indirectly (possibly through their influence 

 on oxydative processes) certain locomotor mechan- 

 isms, while at a lower degree of concentration other 

 mechanisms may be more stimulated. It may not 

 be necessary to make any particular assumptions 

 concerning the role of reversible chemical reactions, 

 although the fact that such reversibility is a general 

 property of chemical changes is naturally involved 

 in any explanation of recovery from the effects of 

 stimulation, and indeed in the interpretation of many 

 other vital phenomena. 



To give a concrete illustration of how reversal 

 may occur according to the preceding interpretation 

 let us consider the reactions of Planaria to weak 

 and strong mechanical stimuli. It has been shown 

 by Pearl that Planaria maculata reacts positively to 

 very weak mechanical stimuli, and in fact to very 

 weak stimuli of many kinds, while to strong stim- 

 uli it gives the negative reaction. The positive re- 

 sponse is brought about by the contraction of the 

 longitudinal muscles near the stimulated point. In 

 the negative response the planarian turns away, not 

 like a higher animal by the contraction of the muscles 



