690 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



At Banyuls-sur-Mer Bohn found that the examples of Antedon mediterranea 

 in the tanks of the aquarium were only very slightly sensitive to light, in this 

 respect acting much like the ophiurans which inhabit sandy bottoms. 



Dr. H. L. Clark says that almost the first fact noted with reference to the 

 habits of the comatulids at Macr was that they showed an evident tendency to 

 withdraw from brightly illuminated areas. On overturning a rock fragment on 

 the lower surface of which crinoids were living they at once began to move. Some- 

 times they withdrew into dark holes or crannies in the rock, but generally the 

 movement was over the surface of the rock to its margin and thence down into the 

 shade of its now lower side. The movement was never spasmodic or hurried, but 

 unless interfered with was very steady and soon accomplished its purpose. 



He remarks that it is difficult to see how this invariable habit can be interpreted 

 as anything else than a response to light as a stimulus. Since the rock might be 

 3 feet under water, no change of temperature could possibly have caused the move- 

 ment. It is conceivable, however, that the change of position due to the overturning 

 of the rock was the effective stimulus. But the theory that change of position is 

 the determining factor seems to be eliminated by the facts that among the living 

 corals comatulids occur with the oral surface either up, down, or lateral, and that 

 when the coral around them is broken away with a hammer, exposing them to strong 

 light but not disturbing their position, their movements are also away from the 

 lighted area into the shadow. 



Experiments in the laboratory were not altogether satisfactory, so much indi- 

 vidual difference was shown. As a rule, if a comatulid was placed in the middle 

 of the tank, one end of which was covered over while the other end was in sunlight, 

 movement to the shaded end almost always was the first result. Often, however, 

 the animal did not remain there, but moved about restlessly, even to the brightly 

 illuminated end. In such cases it is a fair inference that the absence of suitable 

 surface for attachment was the cause of the restlessness. 



More consistent results followed when direct sunlight was allowed to fall on 

 comatulids which had been in the shade. With scarcely an exception they moved 

 out of the brightly lighted area. Experiments with artificial light were entirely 

 negative, presumably because of the weakness of the stimulus, nothing better than 

 an ordinary kerosene lamp being available. 



No specific differences were noted with reference to light reactions, all the 

 species watched giving the same results. 



Bright sunlight thrown abruptly on a comatulid lying in shadow often pro- 

 duced immediate and sometimes more or less spasmodic closing of the arms over 

 the oral surface of the disk, as though that part of the animal were most sensitive 

 to the stimulus. In confirmation of the view that the disk is more sensitive to 

 light than the arms is the fact that in their natural habitat the crinoids often 

 extended their arms out into the light while the body itself was in heavy shadow. 

 Indeed, all along the southwestern reef, on the brightest days, they were to be seen 

 in such a position. 



