

The Reactions of Planarians to Light 1 45 



are (1) the physiological state of the organism at the time of obser- 

 vation, and( 2) the simultaneous effect of other stimuli. 



A physiological state may be directly traceable to known causes, 

 such as previous exposure to other stimuli or the condition of meta- 

 bolic balance in which the animal chances to be at the time of 

 observation, or, again, it may be the result of factors at present 

 unknown, which consequently, although in active operation, are 

 not susceptible of analysis. In any case it is certain that the 

 uncontrolled factors comprehended under the term " physio- 

 logical state" prove individual planarians to be not identical 

 mechanisms, but organisms possessing a more or less definite 

 individuality. Moreover, it has been shown that differences in 

 physiological state play a greater part in the determination of 

 behavior than do intensity differences in the light stimulus. When 

 a planarian is approaching a state of fatigue, for example, it 

 becomes indifferent to differences of intensity. 



With regard to the simultaneous effect of other stimuli acting in 

 conjunction with light, it has already been pointed out that behav- 

 ior is the resultant of all the factors, external as well as internal, 

 which may be acting upon an organism at a given time, and that 

 consequently the effect of any one of the operating factors, such 

 as that of light, for example, cannot be determined unless the 

 value of the other factors involved is also taken into account. 

 In support of this view, which is so self-evident, it will be recalled 

 that some of the ways in which the responses of planarians to light 

 may be modified by geotaxis, thigmotaxis, goniotaxis and chem- 

 otaxis, were touched upon. 



Summary. Direction and intensity are separable qualities of 

 light. Direction is dependent upon the relative positions of the 

 light and the organism, whereas intensity depends upon the dis- 

 tance between the light and the organism as well as the initial 

 intensity of the light. 



When applied to living organisms intensity may act independ- 

 ently of direction, or in conjunction with it. Direction cannot 

 act independently of intensity upon organisms, since the latter 

 possess definite form and consequently cannot receive the light 

 at a single point. 



