150 LIGHT AND THE BEHAVIOR OF ORGANISMS 



hold good. ... It seems to me that Trembley's experi- 

 ments cannot be interpreted unless we assume that the 

 progressive movements of Hydra are determined by the 

 direction of the rays of light." 



Wilson (1891) found that while Hydra viridis usually 

 collects on the bright side of the dish, it collects in shaded 

 regions if the light is very intense ; that it aggregates more 

 freely in a blue field than in a yellow or white field ; and that 

 it collects more abundantly in the blue field even if it con- 

 tains no more blue rays than the white field and is therefore 

 of a much lower intensity. He found that a change from 

 light to darkness or from blue or white light to light of 

 other colors causes the animals to wander about more. 

 When they pass into blue or white light they tend to come 

 to rest. This explains their collection in blue and white 

 light. Wilson thinks that the Hydras may also go toward 

 the source of light directly; that the collection at the more 

 highly illuminated side of the aquarium is not entirely due 

 to random wandering. Washburn (1908, p. 123) however 

 says, "Hydra shows no response to light other than a tend- 

 ency to come to rest in the more illuminated parts of the 

 vessel containing it." 



The following experiments were undertaken with these 

 questions in mind: i. Do Hydras wander about more in 

 darkness than in light? 2. Do they move directly toward 

 a source of light? 3. Do they orient? 4. What factors 

 are involved in orientation ? 



b. Effect of light intensity on activity. — Experimental 

 results recorded in literature show that Hydra is in general 

 more active in sub and supra optimum intensities than in 

 optimum intensity. The following observation shows, how- 

 ever, that total darkness seems to inhibit movement. On 

 April II at 10 a.m. six green Hydras were taken from the 

 culture which was in strong diffuse light, put into some 

 water in a small rectangular aquai;ium and placed in total 

 darkness without the temperature's being changed. They 

 soon became attached to the bottom of the vessel in posi- 



