Spatially Determined Reactions 201 



actions, transforming negatively phototropic into positively 

 phototropic animals, and vice versa. That such reversal 

 should occur in response to increase or decrease of the 

 intensity of the light is what one would naturally expect; 

 if a certain intensity of illumination is favorable to the life 

 processes of an animal, it would seem appropriate for it to 

 seek light of that intensity but avoid light of greater in- 

 tensity. Many animals, like Gonionemus, are positive to 

 light of moderate intensity and negative to strong light 

 (802). The females of the crustacean Labidocera migrate 

 to the surface of the water at nightfall because, like the 

 earthworm, they react positively to faint light ; and move 

 downward at sunrise because they are negative in their 

 response to intenser light (534). On the other hand, 

 Holmes observed that Orchestia agilis, an amphipod crusta- 

 cean, would, if brought from strong to weaker light, be- 

 come negative for a short time; the meaning of such a 

 change it is difficult to conjecture (330). Sudden reduction 

 of light causes a temporary negative phase also in Con- 

 voluta roscojfensis (253). 



Prolonged action of light may alter phototropism : the 

 " depth migrations," that is, the periodical movements 

 toward and away from the surface of the water, in the free- 

 swimming larvae of the barnacle, Balanus, are due ap- 

 parently to the fact that an exposure of several hours of 

 light will make positive animals negative, even though the 

 light at the end of the period of exposure is decidedly fainter 

 than it was at the beginning (269). The positive reactions 

 of the water insect Ranatra increase in violence the longer 

 the light acts; on the other hand, after being kept in 

 darkness for several hours, Ranatra is negative on first 

 being taken out (335). Daphnias kept in darkness for a 

 time become decidedly negative to diffused daylight, 



