PHOTOTAXIS IN THE SEA URCHIN 129 



several minutes of rest the action may be again induced. Keep- 

 ing individuals in the laboratory for some days diminishes their 

 sensitiveness to shadows, and indeed their reactions to photic 

 stimuli in general. 



The reaction to shadows is doubtless of service as a protec- 

 tive device, as it has commonly been regarded. There seems, 

 however, to be no manifest tendency for the spines to turn toward 

 any particular region which is shaded. Small shadows were 

 thrown upon various parts of the sea urchin. The nearest 

 spines would react by becoming more nearly perpendicular to 

 the surface. If the spine happens to point away from the shaded 

 spot it is brought so as to point more nearly toward the object 

 casting the shadow, but if it is pointing toward the shaded 

 spot it reacts by straightening up as before. Horizontal light 

 was thrown on several sea urchins and my hand was passed be- 

 tween them and the light. The spines pointed directly outward in- 

 stead of toward my hand. To a sudden diminution of light the sea 

 urchin responds by bristling up, regardless of the position of 

 the source of the shadow. 



Von Uexkiill 2 found that spines on isolated parts of the test 

 provided with a radial nerve would still show the shadow reflex, 

 but that this reflex would no longer occur when the radial nerve 

 was destroyed. I have repeated von Uexkull's experiments and 

 obtained the same results. 



LOCAL REACTIONS OF PARTICULAR PARTS 



As we have seen, local shadows produce simply an erection 

 of the neighboring spines. Local illumination, however, evokes 

 a somewhat different reaction. Throw a spot of light upon the 

 surface of a sea urchin and the neighboring spines will bend 

 over toward the illuminated spot, regardless of their previous 

 position. The nearest spines are the first affected, but the 

 reaction soon spreads, if the stimulus is sufficiently strong, to 

 the outlying spines, so that one may observe quite a cluster of 

 spines pointing toward the stimulated region. The reaction is 

 practically the same as that given to a mechanical or chemical 

 stimulus. After bending toward the stimulated point for a 

 while the spines subsequently sway about in other directions 

 and finally come to rest. The effect of a beam of light thrown 



2 Zeit. f. Biol., 40, 447. 1900. 



