252 LIGHT AND THE BEHAVIOR OF ORGANISMS 



4. Reactions to Light Caused hy the Effect of Con- 

 tinued Illumination 



In all of the animals referred to above, the reaction is 

 clearly a response to change of intensity, except in the case 

 where Gonionemus becomes active after having been sub- 

 jected to low light intensity for some time. In the actin- 

 ians Aiptasia, Cerianthus and Eloactis the response appears 

 to be due primarily to the action of light owing to its abso- 

 lute intensity. 



" Aiptasia annulata," Jennings says (1905, p. 459), " is 

 very sensitive to light, expanding in darkness, but con- 

 tracting after a few seconds when exposed to strong light. 

 In ordinary daylight the animal remains contracted for 

 some hours, but after such a period most specimens extend 

 in spite of the light. In comparative darkness the animals 

 direct the disk toward the source of light, through a con- 

 traction on the side of the column exposed to the light. 

 After remaining undisturbed for a long time in an aquarium 

 that is fairly well lighted, the animals give up their orienta- 

 tion with respect to the strongest source of light; with less 

 light they retain it." 



Regarding Eloactis, Hargitt says (1907, p. 277) that they 

 begin to retract almost immediatelyafter exposure to diffuse 

 daylight. " This reaction is not sudden or general at once, 

 as in such creatures as the earthworm, but begins in a some- 

 what indefinite movement of the body, accompanied by 

 similar movements of the tentacles, followed very soon by 

 a slow but definite retraction of the entire body within the 

 tube, often including likewise the tentacles as well." In 

 direct sunlight the reaction is more striking. " In some 

 cases the reaction was so definite and prompt as to leave the 

 impression on the observer that the creature was possessed 

 of something akin to visual sensation." 



While, as stated above, these reactions appear to be pro- 

 duced by the action of light owing to constant intensity, it 

 may even here be due to the effect of the changes of inten- 



