248 BEHAVIOR OF THE LOWER ORGANISMS 



rect manner by a mode of procedure which is in some respects similar 

 to the method of trial and error followed by higher forms" (I.e., p. 99). 

 The precise behavior of the earthworm in becoming oriented to light is 

 described as follows : "As the worm crawls it frequently moves the head 

 from side to side as if feehng its way along. If a strong light is held in 

 front of the worm, it at first responds by a vigorous contraction of the 

 anterior part of the body ; it then swings the head from side to side, or 

 draws il: back and forth several times, and extends again. If in so doing 

 it encounters a strong stimulus from the light a second time, it draws back 

 and tries once more. If it turns away from the light and then extends 

 the head, it may follow this up by the regular movements of locomotion. 

 As the worm extends the head in crawhng it moves it about from side to 

 side, and if it happens to turn it toward the hght it usually withdraws it 

 and bends in a different direction. If it bends away from the Hght and 

 extends, movements of locomotion follow which bring the animal farther 

 away from the source of stimulus" (I.e., p. 100). 



Other observers — Parker and Arkin (1901), Adams (1903) — had 

 observed that when the earthworm is hghted from one side, it by no means 

 always turns directly away from that side ; Adams, however, showed that 

 it turns more frequently away from the Hght than toward it, thus indicat- 

 ing that the animal has some direct locaHzing power. This is confirmed 

 by Harper (1905), who shows that in a strong Hght the earthworm 

 Perichseta commonly turns directly away from the source of Hght, 

 though if the Hght is weak, the "trial movements" are seen. Harper 

 gives many other examples of the performance of varied movements 

 under the action of stimuH in this animal, and brings out some of the 

 internal factors on which some of these depend. 



Holmes (1905) found that the leech and the larva of the blowfly 

 react to Hght in essentially the manner which he had found in the earth- 

 worm. For the leech the following account is given: "In its progress 

 the leech frequently raises the anterior part of the body and waves it 

 from side to side as if feeHng its way. If the animal turns it in the direc- 

 tion of a strong Hght, it is quickly withdrawn and extended again, usu- 

 ally in another direction. If the Hght is less strong, it waves its head 

 back and forth several times and sets it down away from the Hght ; then 

 the caudal end is brought forward, the anterior end extended and swayed 

 about and set down stiU farther away from the Hght than before. When 

 the leech becomes negatively oriented, it may crawl away from the Hght, 

 like the earthworm, in a nearly straight Hne. The extension, withdrawal, 

 and swaying about of the anterior part of the body enable the animal to 

 locate the direction of least stimulation, and when that is found it begins 

 its regular movements of locomotion. Of a number of random move- 



