134 Herbert Eugene Walter 



Planarians kept for some time in darkness pass into a state in 

 which they are more responsive to Hght than individuals exposed 

 for a similar length of time to light. 



C Psychological Basis of Behavior 



Among the first questions that naturally arise concerning the 

 behavior of planarians in light are those which approach the mat- 

 ter from a psychological point of view. How much can planarians 

 actually see, and can they, by repeated experience, "learn" to 

 adapt themselves to changes in the light surrounding them ? 



To this kind of inquiry it is most difficult to give a satisfactory 

 answer, for the reason that it is impossible to go beyond conjecture 

 and inference in judging what any animal, aside from man, can 

 see or know or experience. It is only possible to state, in more 

 or less definite terms, the responses which animals make to light, 

 since it is beyond man's power ever to experience how animals 

 "feel" under anv circumstances. 



a. How Much Can Planarians See ? 



Broadly speaking it may be said that planarians can distinguish 

 light from darkness. The experiments described on pp. 84, 

 et seq.j relating to planarians placed in aquaria so surrounded by 

 backgrounds as to produce regions of different light intensity, 

 point to this conclusion, since when subjected to such differential 

 environments the worms come to rest in the darkened areas. 



Again, the numerous responses made at the critical line separa- 

 ting two light intensities may be regarded as evidence of some 

 power of discrimination on the part of the worm between dif- 

 erent intensities of light. 



It is probable, furthermore, that planarians can distinguish a 

 moving object when that object is of sufficient size and contrasts 

 with its surroundings in its degree of illumination, for the reason 

 that a moving object from which light is reflected, means the same 

 to a worm coming into the vicinity of the object as any other change 

 in the direction of light, such as might be caused by moving a 



