1907] Jennings. — Behavior of the Starfish. 105 



Thus the starfish tends to come to rest in darkened regions. 

 It likewise tends to move from a lighter to a darker region. If, 

 in a 7'egion lighted mainly from above, one-half or one-third the 

 starfish is shaded b}' a screen, the animal as a rule moves toward 

 the shaded side, and gets behind the screen. 



4. Direction of Light Rays versus Differences in Illumination. 

 — We find then that light determines locomotion in two ways. 

 (1) The starfish tends to move away from the source of light 

 (thus in the direction of the light rays). (2) It tends to move 

 from a lighter to a darker region. 



AVhat will happen when the two directions of movement thus 

 defined do not coincide? If the dark region lies to one side, or 

 nearer the source of light, will the starfish move into it, or will 

 it move away from the source of light, even though this takes 

 it into lighter regions! 



This is essentially the problem which has been so much dis- 

 cussed for many organisms, and which is commonly put as fol- 

 lows : What is the relative efficiency of the direction of the rays 

 of light, as compared with differences in intensity of illumi^iation 

 on different parts of the body, in determining the direction of 

 movement ? An animal so large and so peculiarly formed as the 

 starfish offers exceptional advantages for the study of this prob- 

 lem, and it should be attacked here with such precise means as 

 have recently enabled Mast (1906, 1907) to settle the question 

 for certain lower forms. 



But it may be pointed out that the form in which this general 

 question is stated above is perhaps a misleading one. When the 

 starfish is placed in a field lighted from one side, the body of the 

 animal of course itself casts a shadow ; that side or surface of the 

 body away from the light is not illuminated. Thus there are in 

 all cases differences in the intensity of illumination of different 

 parts of the body. Further, when the starfish moves in the direc- 

 tion of the rays of light, away from the source, it is of course 

 moving toward the imilluminated side of its own body. It ap- 

 pears to be impossible to so arrange conditions that the effect of 

 the dii'cction of the rays can be tested independently of differ- 

 ences of illumination. 



Thus the problem is perhaps merely this: When there are 



