134 S. J. HOLMES 



are somewhat elevated. The spines on the posterior side of the 

 body may be supposed to have their backward movement em- 

 phasized, while those in front, being less affected by the light, 

 will act less vigorously or be somewhat raised because the 

 light stimulates most the region lying above them. Light also 

 tends to cause the tube feet to become more retracted on the 

 stimulated side while they are extended freely in the shade. 

 These reactions make for the locomotion of the animal away 

 from the light, but by themselves they are insufficient to 

 accomplish this result. 



This circumstance does not I think afford a serious diffi- 

 culty in interpreting the phototaxis of the sea urchin as belong- 

 ing to the reflex type of behavior. Stimuli affecting one side 

 of the body are responded to by movements of organs on the 

 opposite side. It is probable that the sea urchin crawls away 

 from the side stimulated by a beam of light for the same reason 

 that it escapes from mechanical irritation. I have found that 

 sea urchins in a dark room kept as free as possible from other 

 sources of light except a small spot of intense light focussed on 

 one side by a lens would crawl away from the source of stimulus. 

 The light was passed through several inches of water to filter 

 out the heat rays, and was screened away so that it had little 

 chance to affect directly the parts which were active in carrying 

 the sea urchin away. Since the animal had just shown itself 

 irresponsive to light considerably stronger than that which 

 reached any other than the spot most stimulated, we are justi- 

 fied in regarding its negative reaction as essentially like its 

 crawling away from a point scratched by a needle. In these 

 reactions the tube feet let go their hold near the stimulated 

 spot, and those over the lower surface bend away from the 

 stimulated side, extend, attach and pull the animal away. 

 Along with this there is a parallel co-operative movement of 

 the spines. 



In these reactions there is no direct response to a stimulus 

 on one side of the body by a reaction of organs on the opposite 

 side as in the crossed reflexes of higher animals. The first 

 response is near the stimulated point. From here the movement 

 spreads until all the locomotor organs of the animal are in- 

 volved. The stimulus to the activity of the distant organs is 

 not so much the primary stimulus received from the outside 



