lo8 LIGHT AND THE BEHAVIOR OF ORGANISMS 



is undoubtedly more sensitive than the posterior. Speci- 

 mens were also repeatedly seen to react as soon as the 

 anterior end in front of the eye-spot came into the shadow, 

 but many were seen to turn about before the anterior end 

 reached the shadow at all, presumably owing to causes 

 other than changes in light intensity. I was therefore 

 at no time certain that those which reacted when only 

 the tip of the anterior end touched the shadow would not 

 have reacted had they not come in contact with the shadow. 



But suppose that those which did react before the eye- 

 spot was shaded were stimulated by the shadow on the 

 anterior end in front of the eye-spot, would this prove that 

 the eye-spot is not a light recipient organ or that there is 

 no highly sensitive structure back of the part stimulated? 

 It evidently would not, for as soon as the anterior end 

 touches the shadow, the light which is reflected from it 

 onto the structures in the interior of the body before it 

 reaches the shadow is cut off. The light intensity on 

 structures which are not in the shadow at all is therefore 

 reduced as well as that on those which are in the shadow, 

 and it may be that the decrease of intensity on the former 

 causes the reaction. 



The possible effect on structures in Euglena near the eye- 

 spot, due to shading merely the tip of the anterior end, can 

 readily be illustrated by noting the effect if one looks into 

 the mouth of a test tube full of translucent jelly and throws 

 a shadow on the closed end. The reduction of light will 

 of course affect the eye at once, although it may be a con- 

 siderable distance from the shadow. 



If there were a differentiated bit of protoplasm highly 

 sensitive to variation in light intensity, located in close 

 proximity to the eye-spot on the side facing the interior 

 of the body, one might even expect the organism to react 

 before the anterior end reaches the shadow at all, for, since 

 there is no light reflected from the shaded area, it is evident 

 that merely turning the anterior end toward it would result 

 in a decrease of light intensity on the postulated sensitive 



