PHOTOTAXIS IN THE SEA URCHIN 135 



as the stimulus afforded by the activities of the intermediate 

 organs, as in the chain reflexes of higher forms. This spreading 

 of a certain consensus of action, if I may use the expression, 

 is the means by which the sea urchin reacts as a whole to outer 

 stimulation. It is beautifully shown in the righting movements 

 of starfishes where there may be at first antagonistic efforts of 

 various arms, which are sooner or later overcome by a series 

 of co-operative activities which extends from one attached arm 

 until all the others take part in the unified impulse to turn the 

 body over. It is also shown in the capture of food and its trans- 

 fer to the mouth. The performance of a certain act by any of 

 the numerous organs of the body of a sea urchin or starfish 

 seems to stimulate neighboring organs to a like activity. Upon 

 this characteristic many of the co-ordinations to be observed 

 in echinoderm behavior apparently rest. 



The* foregoing experiments were carried on at the Bureau of 

 Fisheries at Woods Hole, Mass., and I wish to thank Dr. F. B. 

 Sumner and Mr. Pope for many courtesies received during my 

 brief stay at that place. 



SUMMARY 



Arbacia punctulata generally reacts negatively to light of a 

 great range of intensity, although it is occasionally positive in 

 weak light. 



After it has ceased to respond it may be made to resume 

 phototactic movements by mechanical disturbance. 



Movements away from light are generally effected by the 

 combined action of the spines and tube feet, although photo- 

 taxis may occur when either one of these sets of organs has 

 been removed. 



Arbacia will move against the rays of light to get into a 

 shaded region. 



Arbacia responds to shadows by an erection of the spines 

 and not necessarily by a movement of the spines toward a 

 shaded area. The response occurs only a few times, but is 

 resumed after a short period of rest. The reaction is dependent 

 upon the radial nerves. 



Local stimulation by light causes the spines to move toward 

 the stimulated part. This reaction occurs in isolated pieces of 

 the test which have no part of a radial nerve. 



