PHOTOTAXIS IN THE SEA URCHIN, 

 ARBACIA PUNCTULATA 



S. J. HOLMES 

 From the Zoological Laboratory of the University of Wisconsin 



GENERAL FEATURES OF THE REACTION 



Arbacia punctulata, like many other species of sea urchins, is 

 quite responsive to light and usually tends to go toward the 

 darkest part of its environment, although more rarely certain 

 individuals may show a pronounced positive photo taxis. This 

 species may be quite fitful, however, in its reactions, and there 

 is a very large amount of individual variability in the photo- 

 taxis of different specimens. When the sea urchins are brought 

 near a window from a comparatively dark region they slowly 

 wander away from the light approximately in the direction of 

 the rays. When the dish in which they are contained is reversed 

 they may again go away from the light, but after a few excur- 

 sions of this kind they settle down and remain quiet. Even 

 strong sunlight fails to disturb them for more than a short 

 time. In fact, specimens remain phototactic in weak light 

 longer than in light of high intensity, which quickly dulls their 

 sensitivity. Specimens which no longer react to weak light 

 may show a negative response of short duration when brought 

 into strong light. Whatever the direction and strength of the 

 light, provided all parts of their enclosure are illuminated to 

 nearly the same intensity, the sea urchins do not remain for 

 more than a limited period in the negative end, but become 

 scattered in entire disregard of the direction from which light 

 falls upon them. 



After sea urchins have ceased to respond to the light they 

 may be induced to resume their phototaxis by mechanical 

 stimulation. Poking the spines about or picking up an indi- 

 vidual and setting it down again usually starts the creature 

 crawling toward the negative end of the dish. Stimulation by 

 chemicals has the same effect. Since locomotor movements are 

 somewhat unwonted activities under the natural conditions of 

 the sea urchin's life and involve a condition of more or less 

 general excitement, they tend to exhaust themselves rather 



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