102 University of California Publications in Zoology. [Vol. 4 



Reaction to Light. 



The reaction by movement in a certain direction may be in- 

 duced, as we have seen, by mechanical, thermal and chemical 

 stinnili of many kinds, — these including most of the agents which 

 act on the starfish under natural conditions. The locomotor 

 reaction may also be induced by light. We must describe the 

 main features of the reaction to light, since an understanding of 

 this reaction is necessary for comprehending many important 

 points in the animal's behavior. An exhaustive investigation of 

 the reactions of this animal to light would be of great value; 

 such an investigation I was unable to make in the time at my 

 disposal. But the main outlines of the behavior may be given. 



It should first be noted that while the reaction of this starfish 

 to light is very striking, it is likewise extremely variable, and is 

 often modified or quite prevented by various internal conditions, 

 as well as by external influences. I give first the characteristic 

 reactions to light when no interfering influences are at work. 



Unlike most starfish whose reactions to light have been de- 

 scribed, this species is decidedly negative to light of ordinary 

 intensity. This is shown in the following different ways : 



(1) When the vessel containing the animal is placed near a 

 window or other source of light, the starfish crawls directly away 

 from the lighted side. 



(2) When transferred without other disturbance from a dark 

 region to a light one, the starfish becomes much disturbed and 

 begins to creep about actively. 



(3) If part of the body of the starfish is shaded, the animal 

 moves toward the shaded side, and after exploring a vessel in 

 which it is confined, it takes up a position in the region sheltered 

 from the light, and there comes to rest. 



(4) If confined in a field uniformly lighted from one side, 

 so that the animal is unable to get out of the light, it takes a 

 peculiar position, such that what are apparently the more sensitive 

 parts of the body are sheltered from the light. 



(5) When turned on its back, it tends, in righting itself, to 

 turn toward the side away from the light. 



We will take up in order certain facts concerning each of 

 these methods of action. 



