NEW EXPERIMENTS ON LIGHT REACTIONS 3 



possess. Our observations teach anew how greatly physiologi- 

 cal experiment can aid us in the interpretation of histological 

 discoveries. 



II 



Among the lightreactions of Echinodermata which I have 

 newly discovered and upon which I shall make only a brief 

 report tonight, a certain interest attaches to those of the star- 

 fish, if for no other reason but that until now almost nothing of 

 their sensitiveness to light was known. On the ground of 

 anatomical research it was taken for granted that the familiar 

 red points at the ends of their five arms were light receiving 

 apparatus. Attempts to elucidate the question experimentally 

 led to contradictory results. Some authors assert that those 

 starfish which have an inclination to move toward the light 

 cease showing this impulse after the tips of the arms with the 

 " eyes " are cut off; according to other writers individuals thus 

 mutilated still crawl to the light. 



In the course of systematic experiments I discovered the 

 surprising fact that the feet of the Astropectinids are highly sensi- 

 tive to light. If light is flashed on them their little feet, relaxed 

 in the dark, are instantaneously jerked in and the widely opened 

 ambulacral groove is closed along the whole of the lighted area, 

 the flanking white spines shutting over the incurled little feet. 

 This startling phenomenon, which I was able to record in a 

 number of snapshots, gave me the opportunity of examining 

 the differing effects of colored lights. As with all the hitherto 

 thoroughly examined invertebrates, it was found that colored 

 lights have similar or identical relative values for our starfish 

 as for the totally color-blind human eye; red lights remain 

 almost or quite without effect even when very strong, while 

 green and blue lights have a much stronger effect than the red 

 lights, even when the latter seem to our normal eyesight much 

 darker than the former. I was able also to prove adaptive 

 changes in these starfish and to carry out exact measurements 

 during my observations. 



New and most remarkable light reactions in sea urchins were 

 also disclosed. So far it had been known from experiments of 

 Sarasin and Uexkull that some sea urchins raise their spine 

 slightly when shaded from the light. More exact observations 

 of their qualities of sight had not yet been made. I discovered 



