MOVEMENT TOWARD LIGHT IN COELENTERATES 157 



has something to do with this movement. It is not due 

 entirely to internal changes. The organism must be af- 

 fected differently when the anterior end is illuminated than 

 it is when this end is shaded. A decrease in illumination 

 ordinarily causes increase in activity, but this fact cannot 

 be the cause of locomotion after orientation, for if it were, 

 we should expect the greatest tendency to move when the 

 anterior end is directed from the source of light in place of 

 toward it. 



One thing is clear from our results stated above, that is, 

 that the light condition which tends to inhibit turning in 

 various directions also tends to cause locomotion. From 

 this it may be concluded that orientation and locomotion 

 are phenomena w^hich are regulated by different processes. 

 It may be that the former is dependent largely on difference 

 of intensity on opposite sides of the organism and the latter 

 upon an action of light similar to that of heat. 



e. General conclusions. — It can be definitely stated 

 then that Hydra in the positive state reaches a position in 

 which the anterior end faces the light by random move- 

 ments; that it remains in this position longer than in any 

 other, and that it ordinarily starts to travel only when it is 

 in this position. The last two statements prove that light 

 affects it differently when the oral end is exposed than when 

 it is shaded. 



It is impossible to say whether Hydra tends to retain 

 the position in which the light strikes the oral end because 

 of an inhibition due to an increase in effective light inten- 

 sity, when the body Is turned from a position in which the 

 oral end is shaded to one in which It Is Illuminated; or 

 whether it tends to remain oriented because of Increase in 

 motion due to a reduction in effective intensity when the 

 anterior end is turned away from the light; or whether the 

 tendency to retain the oriented position is due not to a 

 change of Intensity, but to the fact that In this position 

 the anterior end is most highly illuminated and that the 

 inhibition is due to the effect of constant intensity. 



