3O E. H. HARPER. 



shade rather quickly, but usually not completely. The com- 

 monest way in which the worm gets under the shade is as fol- 

 lows : It makes all sorts of random movements in every direction, 

 and tries to burrow into the thin layer of dirt, until it acci- 

 dentally gets the tip of the anterior end under the shadow of the 

 screen. It then at once is oriented, so to speak, and crawls com- 

 pletely under the glass. It may crawl under as if circling around 

 a post. The imaginary post may be exposed to the light so that 

 the posterior part has to crawl forward into the light to get 

 around the post. Usually, however, the anterior end travels 

 faster so as to jerk the middle part under the screen at once. 



These so-called " photopathic >: reactions are consequently 

 very definite and predictable because they are adaptations im- 

 portant in the normal life of the worm. As compared with the 

 random lateral movements we see that they are controlled by 

 weaker stimuli and are more definite. The anterior and posterior 

 ends are more sensitive than the middle for the obvious reason 

 that the ends alone come into contact to any great extent with 

 light stimulation. 



The lateral movements, which are typical of life outside the 

 burrow, are as we have seen of a random nature and less defi- 

 nitely controlled. The worm " dashes back like a rabbit into its 

 burrow," to use Darwin's expression, under a weak stimulus. 

 But when crawling on the surface the same strength of stimulus 

 produces only a general irritation and swaying random move- 

 ments occur which lead to orientation away from the light only 

 after many trials. With a higher intensity of light the worm is 

 oriented more quickly. Thus we see that a very high stimulus 

 is required to produce a direct sidewise movement away from the 

 light while a very weak stimulus will cause it to move back into 

 its burrow away from the light. The random lateral movements 

 are aptly described by Holmes as " inconsequential vermicula- 

 tions." But this description does not apply to the movements 

 which are typical of its burrow life. The worm is as definitely 

 adapted to the burrow and as little adapted for life in the open 

 as some other burrowing animals of higher rank that could be 

 mentioned. However this statement must be modified when we 

 consider that a worm exposed to the light on the ground does not 



