Spatially Determined Reactions 169 



In Volvox, also, orientation is held by Oltmanns (298) and 

 Mast (262) to be an affair of intensity differences rather than 

 of light direction. The reaction of a Volvox colony, which in 

 moderate light is positively phototropic, occurs in consequence 

 of a response by each individual in the colony given when, 

 as the colony rotates, that individual passes from a higher 

 to a lower intensity of light. 



In Hydra, the effect of light is photopathic rather than 

 phototactic. We have seen that these animals, when sub- 

 jected to light either above or below a certain " optimum" 

 intensity, wander about until they reach a region of the right 

 degree of illumination ; their movements manifest no definite 

 orientation (444). One sea-anemone, Actinia cereus, ob- 

 served by Bohn, does show an oriented response to light. 

 Weak light causes expansion of its tentacles perpendicularly 

 to the light rays. If the light is increased, the tentacles "tend 

 to orient themselves in the direction of the rays, and finally 

 converge in a bundle parallel to that direction," a response 

 which has the effect of protecting them from the intense 

 light (62). 



The medusa Gonionemus offers an instance of opposition 

 between photopathy and phototaxis, the former being nega- 

 tive, the latter positive, in daylight. That is, it moves toward 

 the light when swimming, but being less active in darkness 

 than in light, it comes to rest, and hence tends to collect, 

 in darkened regions. Intense light gives a negative photo- 

 taxis. Sudden decrease and sudden increase of light intensity 

 have alike the effect of temporarily inhibiting activity. On 

 swimming either from shadow into sunlight, or from sunlight 

 into shadow, the medusa stops, turns over, and sinks to the 

 bottom. But when this effect has been produced by entering 

 shadow, the animal, on again becoming active, may move 

 in any direction ; when it has been produced by entering sun- 



