8o 



Herbert Eucrene Walter 



o 



By reference to Fig. 7, in which the relative size of the eyes is 

 somewhat exaggerated and made diagrammatic for sake of clear- 

 ness, it will be seen that no more light reaches the sensory cells of 

 either eye from position A, the diagonal position, than from posi- 

 tion B, and that it is only when the light comes from some source 

 more lateral than A that the left eye receives more illumination 

 than the right. 



This view may furnish a possible explanation of the diagonal 

 paths representing imperfect orientation among planarians, but it 

 can in nowise apply to the case of earthworms since in them direc- 

 tion eyes are absent. 



/ / ^ '1' 



II l'' I. 



II I ," \ 



Fig. 7 A, diagonal direction of light; "^j posterior direction of light; C, location of sensory cells; 

 D, pigment shield. 



Degree of Wandering. The degree of wandering decreases with 

 an increase of intensity. It may be found approximately through 

 the degree of error in orientation in a unit space under different 

 intensities of light, for perfect orientation signifies the minimum 

 of random wandering and, conversely, the greater the error of 

 orientation the greater the probable wandering. 



The error of orientation expressed in percentages was computed 

 as follows. With a negative worm emergence from the circle at 



