138 BEHAVIOR OF THE LOWER ORGANISMS 



completely into the dark; it then turns and passes back into the light. 



At the boundary of the lighted area the 

 organism is, of course, subjected to a sud- 

 den decrease in illumination, and this, our 

 previous experiments have shown us, is the 

 cause of the avoiding reaction. Whenever 

 lighted or shaded areas are open to Eu- 

 glenae, the organisms gather in the lighted 

 areas in the way just described. 



If the entire area containing the Eu- 

 glenae is illuminated from one side, the 

 organisms swim toward the side from 

 which the light comes. That is, they be- 

 come oriented with anterior end toward 

 the source of light. If we watch them as 

 they become oriented, we find that the 

 orientation takes place, as in Stentor, 

 through the avoiding reaction. The course 

 of events is about as follows : The Eu- 

 glenae are swimming about at random in 

 a diffuse light, when a stronger light is 

 allowed to fall upon them from one side. 

 Thereupon the forward movement be- 

 comes slower and the Euglenae begin to 

 swerve farther than usual toward the 

 dorsal side. Thus the spiral path be- 

 comes wider and the anterior end swings 

 about in a larger circle and is pointed 

 successively in many different directions. 

 In some part of its swinging in a circle 

 the anterior end of course becomes directed 

 more nearly toward the light; thereupon 

 FIG. 94 . - Spiral path of EU- the amount of swinging decreases, so that 



glena. a, b, c, d, successive positions the Euglena tends to retain a certain posi- 



taken. The arrows at the right in- , , , T ,1 f .-, 



dkate the direction of an incoming tlon so reached. In other parts of the 

 force, as light, showing how the reia- swinging in a circle the anterior end be- 



tion of the body axis and the anterior i , . ,1 ,. i , .1 



end to such a force changes con- comes less exposed to the light ; thereupon 

 tinuaiiy. At d the body axis is the swaying increases, so that the organism 



nearly parallel to the lines of force, , , . , u ... , 



and the anterior end is directly illu- d es not retam tms Potion, but SWingS 



minated. At b the axis is nearly to another. The result is that in its spiral 



transverse, and the sensitive anterior ., i ! 



end is largely shaded, so as to re- COUrse it SUCCCSSlVely SWerVCS Strongly 



ceive but little light. toward the source of light, then slightly 



