202 LIGHT AND THE BEHAVIOR OF ORGANISMS 



to be directed toward the source of light, it was thrown 

 equally strongly in the opposite direction. These results, 

 together with some statistical tabulations of direction of 

 head movements with different portions of the body back 

 of the sixth segment exposed to unilateral illumination, indi- 

 cate that, while these portions are no doubt sensitive to 

 light, only the anterior end is immediately functional in 

 regulating orientation. Jennings (1906a, p. 442) arrived 

 at practically the same conclusion with reference to other 

 stimuli. 



Parker and Arkin in their experiments used a Welsbach 

 gas burner as a source of light. Owing to the width of the 

 luminous part of the burner it is evident that a shadow of an 

 object in light from such a source w411 not have sharp edges; 

 and likewise a beam produced by means of a screen con- 

 taining an opening will not have well-defined edges. In 

 such a beam there is a region of uniform highest inten- 

 sity in the middle and a region of graded intensity on 

 either side. By calculations based on the data furnished 

 by Parker and Arkin it was found that in the beam of light 

 at the place where they exposed the earthworms the region 

 of uniform highest intensity was 10 mm. wide and the 

 region of graded intensity on either side was 16 mm. wide. 

 Outside of this on either side there was another region 15 

 mm. wide faintly illuminated by light reflected from the 

 water screen. It is evident that when the middle or pos- 

 terior end of a specimen of Allolobophora foetida, usually 

 only about 4 cm. long, is exposed in the region of greatest 

 intensity in such a field, the anterior end will be exposed to 

 the weaker light in the adjoining region. It may be then 

 that the preponderance of negative head movements found 

 by Parker and Arkin with the posterior portion of Allolo- 

 bophora exposed to relatively strong lateral illumination, 

 was due to the effect of the weak illumination on the an- 

 terior end. Since the light in the beam becomes gradually 

 weaker as one proceeds outward, it is clear that the anterior 

 end of the worm will be in higher light intensity when the 



