860 PROCEEDINGS OF THE AMERICAN ACADEMY. 



left, the small at the right. The number of responses falling in each 

 class are recorded on the corresponding ordinates. In the first poly- 

 gon (Figure G, A) are represented the experiments with the large 

 light alone (as recorded in Table II), where, it will be noticed, the 

 polygon lies mainly on the side away from the light ; similarly the 

 second polygon (Figure 6, B) lies chiefly on the side away from 

 the small light, which was alone used in this case. The third figure 

 (Figure 6, C) represents the data when both lights were used, and is 

 remarkably symmetrical, with its mode at 0. This indicates a con- 

 dition as nearly balanced as could well be expected. 



These results on the earthworm would appear to admit of only one 

 interpretation, namely, that the intensity of the light is the controlling 

 factor in its reactions and not the size of the field from which the 

 light is derived. The latter, in fact, appears to have no influence 

 whatever. This is in exact accordance with what would be expected 

 in the case of an eyeless animal. It possesses no organs so difieren- 

 tiated as to be able to discriminate space relations in objects beyond 

 its touch ; there is no mechanism for preserving these relations in 

 space, as the light from the difierent parts of the object falls upon the 

 sensitive portion of the animal, and consequently there is no sug- 

 gestion of image formation. The light stimulates according to its in- 

 tensity only, and since the intensity is the same on each side, the 

 animal responds by approximately an equal number of deviations in 

 each direction.^ It may be asked why, if the amount of stimulation 

 on the two sides is balanced, the worm does not always crawl in a 

 straight line, deviating neither one way nor the other, and always 

 cross the circle so as to fall in the class. This is probably for 

 the same reasons that with unilateral illumination the worms do not 

 uniformly turn away from the light. Similarly Parker and Arkin 

 (:0l) pointed out that in their experiments only 30.2 per cent of the 

 "head movements " were away from the light, 65.6 per cent being 

 straight ahead and 4.2 per cent toward the light. These authors at- 

 tributed the movements toward the light to the uneven surface of 

 the paper on which the worms were allowed to crawl, and to other 

 disturbing influences. In view of Harper's recent work (:05), it 

 would seem that conditions of uneven or one-sided expansion or con- 

 traction of the animal might have some influence in producing these 



^ Loeb (: 05, p. 2, footnote) maintains that "if two sources of light of equal 

 intensity and distance act simultaneously upon a heliotropic animal, tlie animal 

 puts its median plane at riyht anjiles to the line connecting the two sources of 

 light." This statement needs a number of qualifications. 



