136 University of California Puhlications in Zoology. [Vol. 4 



away, five at right angles, one toward the window, and one in 

 which the reaction was so complicated that its relation to the 

 light could not be determined. 



Thus, in sixty righting reactions, the turning was in thirty- 

 nine eases almost directly away from the source of light, in seven 

 cases obliquely away, in nine at right angles to the light, one 

 intermediate, and in only four of the sixty cases toward the 

 source of light. 



It is therefore evident that direction of the source of light 

 is an important factor in determining the impulse to turn. 

 Often from the very beginning the two rays on the side away 

 from the window bend down and twist over, preparatory to at- 

 taching, while those directed toward the window at once rise 

 up and begin to go over. Light coming from one side may pro- 

 duce an immediate unified impulse and determine its direction. 

 (After the impulse is established, as we shall see later, the direc- 

 tion of the light has little or no efi^ect on the further course of 

 the reaction.) 



To test other factors, it is of course necessary to experiment 

 under conditions in which the determining effect of light is 

 excluded. This is readily done by placing the vessels containing 

 the starfish in the bottom of a high cask or box, the light coming 

 from above. Experiments now show that no particular direction 

 of turning prevails. 



(2) Structural differences in the rays. — Where certain rays 

 are comparatively undeveloped, as when they are regenerating, 

 these rays are little used in righting. The starfish attaches itself 

 by the fully developed rays and turns toward these. In most 

 starfish this factor plays little part, — though possibly invisible 

 structural differentiations may at times play a role. 



(3) Rays that first come in contact with the bottom. — When 

 the starfish is placed on its back, it usually rests at first on its 

 disk, the dorsal surface being convex. The rays, as we have seen, 

 move up and down and feel about. It thus at times happens 

 that certain rays come in contact (by their dorsal surfaces) with 

 the bottom, before others do so. These rays are then likely to 

 attach themselves by their tube feet, and to determine the direc- 

 tion of turning. A unified impulse appears, — the tube feet of 

 the unattached rays inclining toward the attached ones. 



