PHOTOTAXIS IN THE SEA URCHIN 127 



quickly. The effect of disturbance upon the creature's photo- 

 taxis is apparently that of producing a condition of activity 

 which is directed and controlled to a certain extent by the 

 rays of light. 



When sea urchins are brought near a source of light there is 

 produced a general movement of the spines and tube feet over 

 the whole surface of the body. The points of the spines move 

 back and forth or describe a more or less circular course, each 

 spine moving in a manner different from its neighbors. The 

 tube feet are extended, retracted and bent in various directions. 

 Soon order begins to emerge out of this chaos of actions. The 

 spines on the side toward the light become depressed and rela- 

 tively quiet. Those on the opposite side tend to become some- 

 what raised. The spines on the sides, and especially those that 

 reach the bottom move back and forth, and tend, when they 

 are pulled back, to force the animal ahead. The movement 

 resembles that of a galley rowed by numerous oars. The spines 

 can frequently be seen to be raised somewhat when they are carried 

 forward as an oar is in rowing. There is considerable irregu- 

 larity in their movements, but there seems to be a predominant 

 tendency to combine a downward movement with the back- 

 ward phase of the stroke, a circumstance which would easily 

 explain the role of the spines in locomotion. As the sea urchin 

 travels the test is raised and lowered at intervals owing to the 

 irregular movements of the spines. 



The movement away from the light is aided by the action 

 of the tube feet. Those on the under side of the body are pro- 

 truded and bent in the direction of locomotion. As they touch 

 bottom they adhere by thejr terminal disk, contract, and thus 

 pull the body forward. The direction of locomotion is not 

 determined by a particular set of tube feet which lie on the 

 anterior side, because all of these organs capable of reaching 

 the bottom are brought into play. Anteriorly, posteriorly, and 

 at the sides the tube feet as they extend are bent forward and 

 co-operate to pull the animal along very much as they do in 

 the locomotion of the starfish described by Jennings. 



Generally also, there is an extreme extension of several of 

 the tube feet from the equatorial or upper region of the corona 

 on the side away from the light. Those on the more illuminated 

 side are more retracted. The extended tube feet swav back 



