. Behavior of the lower organisms. Animal behavior; Invertebrates. REACTIONS OF INFUSORIA TO LIGHT AND TO GRAVITY 141 We can thus distinguish an optimum intensity of light, in which Euglena tends to remain. Movement toward either a greater or a less intensity of light causes the avoiding reaction, with its trial of different positions and directions of movements, till a position or direction is found which leads toward the optimum, or retains the optimum intensity undi- minished. Or, in other words, after Euglena receives an amount of light which we might call "enough," it avoids mor
. Behavior of the lower organisms. Animal behavior; Invertebrates. REACTIONS OF INFUSORIA TO LIGHT AND TO GRAVITY 141 We can thus distinguish an optimum intensity of light, in which Euglena tends to remain. Movement toward either a greater or a less intensity of light causes the avoiding reaction, with its trial of different positions and directions of movements, till a position or direction is found which leads toward the optimum, or retains the optimum intensity undi- minished. Or, in other words, after Euglena receives an amount of light which we might call "enough," it avoids more light, and also less That degree of light in which it tends Fig. 96. — Diagram to illustrate the results of Famintzin's experiment. The light comes from the direction indicated by the arrows, while the opposite side of the vessel is shaded, as indicated by the dots. The Euglenae gather in the inter- mediate region, across the middle. to remain seems to be about the amount which is most favorable to its life activities. Euglena requires light for assimilating carbon dioxide by the aid of its chlorophyll, just as do higher plants. If confined to dark- ness, it soon ceases activity, contracts into a sphere, and becomes en- cysted. On the other hand, direct sunlight is very injurious to it; if long continued it causes the organism to fall to the bottom and die. Euglena avoids both the higher and the lower intensities that are injurious to it. C. Negative and Positive Reactions compared Thus in both negative organisms (Stentor) and positive organ- isms (Euglena), the determining cause of the reaction is a change in the intensity of light, and the reaction takes place by the usual method of the performance of varied movements, subjecting the animal succes- sively to different conditions. When the sensitive anterior end is sub- jected alternately to light and shade, the organism "tries" other direc- tions of movement till it finds one where such changes are not p
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