52 THE BEHAVIOR OF LOWER ORGANISMS. 



time after the decrease in illumination takes place the organisms behave 

 just as they did before, swimming in the same direction. Further, 

 the illumination may be decreased very slowly to the same extent 

 without causing a reaction. If the screen is at first far away from the 

 preparation and is then slowly moved to the position it occupied in 

 the experiment just described no reaction is produced. It is only the 

 sudden change that has caused the reaction. The change, however, 

 need not be a very marked one in order to be effective. 



Our experiments thus far have shown that in a moderate light Eu- 

 glena reacts to a decrease in illumination. But the absolute amount of 

 light present has an effect on the reaction. If the light is very strongly 

 increased the same reaction is produced as when the light is decreased. 

 If while the organisms are swimming toward a moderately lighted 

 window direct sunlight is allowed to fall upon them, they respond in 

 the same way as to a sudden decrease in illumination ; that is, they 

 turn strongly toward the dorsal side, continuing or repeating the re- 

 action till the anterior end is directed away from the source of light. 

 They now continue to swim in that direction, the positive reaction 

 having been transformed into a negative one. Thus under intense 

 light the conditions of stimulation are the opposite of those under 

 moderate light. This is paralleled in the reactions of the infusoria to 

 chemicals ; often a strong solution of a certain chemical produces a re- 

 action under opposite conditions from those in which a weak solution 

 of the same chemical is effective. 



Let us now proceed to a more careful study of the reaction itself. 

 The reaction which occurs when the illumination is changed is really 

 an accentuation of a certain feature of the usual movements. Euglena, 

 as we know, revolves on its long axis as it swims forward, and at the 

 same time it swerves toward the dorsal side. The resulting path is 

 therefore a spiral one (Fig. 20). The usual reaction to a stimulus is 

 an accentuation of this normal swerving toward the dorsal side, as com- 

 pared with the other factors in the swimming; the organism suddenly 

 swerves so much farther than usual in this direction that the path may 

 be completely changed. If the reaction is a very decided one the revo- 

 lution on the long axis and the movement forward may cease during 

 the swerving toward the dorsal side ; the anterior end then describes 

 the arc of a circle about the posterior end as a center. In a less pro- 

 nounced reaction the revolution on the long axis continues. The circle 

 described by the anterior end is then less and the whole body describes 

 the surface of a cone, or a frustum of a cone, as illustrated in Fig. 21. 

 Every gradation exists between the normal spiral course and the strong 

 reaction in which the anterior end swings in a circle about the 

 posterior end as a center. 



