REACTIONS TO LIGHT IN THE EARTHWORM. 29 



the worm or portions of it while crawling on a moist surface, 

 preferably covered with a thin layer of dirt. 



If a worm which has been kept in the dark is placed on the 

 moist surface, and the screen is suddenly moved so as to expose 

 the anterior end to light, it contracts the anterior segments slightly, 

 sometimes so slightly as to be barely noticeable, and crawls back- 

 ward into the shadow. If the posterior end be illuminated in the 

 same way the worm crawls forward into the shade, but after a 

 noticeably longer interval. A slight twitching of the posterior 

 end may be noticed at first, if the light is suddenly turned on. 

 The worm always crawls forward when stimulated posteriorly, 

 If a worm is crawling backward it can always be reversed by 

 stimulating the posterior end. Crawling backward is of course 

 the method by which the worm comes to the surface to eject 

 castings. The two sorts of responses described are of the kind 

 called " photopathic " as distinct from phototactic, and they serve 

 of course to inhibit the worm from leaving its burrow in the light. 

 These " photopathic " responses are very definite and the stimulus 

 calling them forth may be quite weak light. Adams has shown 

 that in very weak light AllolobopJwra fcetida is positive and he 

 suggests that the worm leaves its burrow in response to the 

 stimulus of very weak light upon its anterior end. These ob- 

 servations show that the movements of the worm in its burrow 

 are very definitely controlled by the light, so far as they may 

 come in contact with it by their more"sensitive anterior and pos- 

 terior ends. The middle of the worm is less sensitive to light 

 but its sensitiveness may be shown in the following way. 



If the worm is placed on the moist surface exposed to full 

 light from overhead (a 32-candle-power incandescent was used, 

 at a distance of 15 inches) and a screen is then brought over the 

 posterior part of the body leaving the anterior end exposed, the 

 worm does not draw back as when the anterior end was suddenly 

 illuminated. Instead it begins to make random movements in 

 various directions. It may crawl farther out into the light, thus 

 bringing the middle portion into stimulation. This movement is 

 however checked before the more sensitive posterior end is ex- 

 posed. After a noticeable latent period showing the lower sensi- 

 tiveness of the middle portion, the worm crawls back under the 



