56 THE BEHAVIOR OF I,OWER ORGANISMS. 



revolves on its long axis, bringing the dorsal side down. Since it can 

 not swing downward, owing to the narrow space, this has little effect 

 on the reaction, save to stop the movement to the left. Now, by con- 

 tinued rotation the dorsal side has come to lie to the (observer's) 

 right ; the Euglena may then be seen to swing far to the right. In each 

 case under these conditions it is at once evident by observing the 

 larger lip at the anterior end that the organism is swinging toward 

 the dorsal side. 



This method of reaction is very effective in preventing Euglena from 

 passing from an illuminated region to a shaded one. As soon as the 

 anterior end enters the shadow, the animal swings far toward the dor- 

 sal side till the anterior end is brought again into the light, repeating 

 the I'eaction if necessary. There is then no further cause for reaction. 

 The reaction to a very strong increase of illumination is, as we have 

 seen, identical with that to a decrease in illumination. 



In our experiments thus far we have directed attention primarily to 

 the effects of changes in the intensity of illumination, and have found 

 that such changes produce a motor reaction independently of the direc- 

 tion of the light rays. But it is of course well known that Euglena 

 does react with reference to the direction of the light rays. Euglenae 

 swim toward the source of light when weakly illuminated, away from 

 the source of light when strongly illuminated. If Euglenas are swim- 

 ming at random in a diffuse light they soon become oriented when the 

 light is allowed to act on them from one side, even if the intensity of 

 illumination remains the same. Or, if EuglenaB are swimming toward 

 a source of very weak light and a stronger light is allowed to act upon 

 them from the opposite side, they become oriented, in time, with 

 anterior ends toward the stronger light. In examining this dependence 

 of the direction of swimming on the direction of the rays of light, we 



swerves strongly toward the dorsal side, at the same time continuing to revolve 

 on the long axis. It thus occupies successively the positions 2, 3, 4, 5, 6. In 

 passing from 3 to 6 the illumination of the anterior end is increased; hence the 

 reaction nearly or quite ceases. In the next phase of the spiral, therefore, the 

 organism swerves but a little toward the dorsal side from 7 to 8. But this 

 movement causes a decrease in the illumination of the anterior end, and this 

 change induces again the strong swerving toward the dorsal side. Hence in 

 the next phase of the spiral the organism swings through 9 and 10 to n. In 

 this movement again the illumination of the anterior end is increased; hence 

 the reaction ceases, so that from 12 the organism swerves only as far as 13. 

 Then owing to the decrease in illumination caused by this movement, the 

 swerving increases, so that the Euglena swings from 13 through 14 and 15 to 16. 

 Now it is directed toward the source of light, and such swerving as takes place 

 in the spiral course neither increases nor decreases the illumination of the 

 anterior end. Hence there is no further reaction; the Euglena continues to 

 swim forward in the direction 16-17. 



