PHOTIC KEACTIONS OF TADPOLES 261 



visible response is dependent on other conditional factors. Again 

 the failure of the animals to produce a reflex when the eyes are 

 illuminated may be due to the inaccuracy of the visual apparatus. 

 An example of this is found in the mollusk Pecten. In Pecten 

 along the free edge of the mantle are situated a number of eyes, 

 each one histologically representing an elaborate visual apparatus. 

 Patten ('87) found in the eyes of Pecten a true cornea, lens, 

 ciliary muscles, retinal structures, and nerve fibers, all occupying 

 the same position as in the eye of the higher vertebrates. Uex- 

 kiill ('12) reports that even the image of an enemy, such as a 

 starfish, is not sufficient to initiate a response in Pecten, unless 

 the image formed in the retina is reinforced by a movement of 

 the object. Similar observations on Pecten have been recorded 

 by Wenrich ('16). 



The experimental evidence, therefore, under the three condi- 

 tions: 1) eye and skin, 2) skin alone, and 3) eye alone, in terms 

 of the amount of energy necessary to act as a stimulus as ex- 

 hibited by the intensity-reaction-time products, shows that this 

 amount of energy is constant. Repeated stimulations of the 

 eye with intensities which were effective when the photoreceptors 

 of the skin were illuminated, led to no responses. It appears 

 therefore, that, under the conditions of the experiments, the 

 tadpoles of Rana clamitans are stimulated through the skin 

 and that the eyes take no part in the recorded responses to light. 



VI. VELOCITY OF CHANGES IN THE RECEPTORS WITH HIGHER 



LIGHT INTENSITIES AND THE DURATION OF SECONDARY 



PROCESSES FOLLOWING SENSITIZATION 



The steady increase in the rate of changes in the photoreceptors 

 with the gradual increase in the intensity of illumination was 

 demonstrated graphically when the logarithm of the intensity of 

 light was plotted as function of the corresponding reaction-times 

 This relation was found to be linear (figs. 2 and 4). The more 

 exact relation between the velocity of change in the sense organs 

 and the dependence of this on the intensity factor, as well as 

 the exact range within which this relation holds true, can be 

 studied by an employment of the reciprocity law as derived 

 from the Bunsen-Roscoe law formula. 



