1 28 Herbert Eugene Walter 



No marked difference in response to light is shown between 

 worms stimulated on the ventral surface and those equally stim- 

 ulated on the dorsal surface. 



Aside from the eyes, which form at least a part of the photo- 

 receptive apparatus, no definite light-receiving organs were recog- 

 nized. 



Planarians possessing dark-colored pigment distributed over 

 the body show in general greater activity when subjected to light 

 than forms in which there is no dark pigment except in the eyes. 



The central nervous system, as well as the more exterior parts 

 of the planarian, may possibly be stimulated directly by such light 

 as passes through the translucent body. 



B The Physiological Basis of Behavior 



The continually changing adjustment in any organism between 

 the incoming and the outgoing energy gives rise to varying phases 

 of metabolic balance, which may be designated as different "physi- 

 ological states." Such physiological states form a noticeable fac- 

 tor in the behavior of any animal, a fact to which Jennings ('04b, 

 p. 109) in particular has called attention. 



That the difference between such states is great may be readily 

 demonstrated. A planarian's response to directive light when it 

 is in a relaxed, quiescent condition is plainly different from that 

 exhibited after it has been vigorously disturbed by a brush. In 

 fact, it is extremely difficult to get two animals that are in precisely 

 the same physiological condition, or the same animal in precisely 

 the same state at two different times, since the exact adjustment 

 of physiological states is too delicate a matter to be controlled by 

 the present gross experimental methods. 



The attempt is ordinarily made to eliminate from experiments, 

 so far as possible, the disturbing element of changing physiological 

 conditions, that is, to keep constant all the factors except the one 

 which is being subjected to test, and those results are counted as 

 most successful in which such disturbance is reduced to a minimum. 



It is the purpose of this section first, to give a possible classifica- 

 tion of the different physiological states in which a planarian may 



