212 LIGHT AND THE BEHAVIOR OF ORGANISMS 



rian Thyone briareus as follows: " In a series of twenty-four 

 reactions the locomotion in every case carried the ani- 

 mal away from the light to the end of the dish, but there 

 was no definite orientation of the body in relation to the 

 light. In ten of these negative responses the anterior end 

 was ahead as the individual moved; in nine instances the 

 posterior end preceded the anterior; and in five the loco- 

 motion was straight toward the right or left. Not one of 

 the eight individuals [used in this experiment] moved in 

 every case with the anterior or posterior end in front." 



Very little is known about the actual mechanism involved 

 in these reactions. It seems clear however that it is the 

 shading of part of the body that produces the stimuli which 

 regulate the direction of motion, for if a shadow is thrown 

 on one side of a specimen of Asterias forreri, e.g., in such a 

 way as not to produce any change of intensity on the 

 exposed side, it moves toward the shaded side; and the 

 continued movement toward the shaded side seems to be 

 due to a constant difference in absolute light intensity on 

 opposite sides. If this is true the direction of movement is 

 regulated by light acting constantly as a directive stimulus. 

 It must however be borne in mind that there is no orienta- 

 tion in these organisms. They move with any side ahead 

 much like an amoeba. In Amoeba, it will be remembered 

 that the formation of pseudopods on the illuminated side 

 is probably checked by the action of the light, and that this 

 results in movement toward the shaded side of the organism. 

 It may be that in the echinoderms light has a similar effect 

 on the extension of the tube feet. If it has, direction of 

 movement is of course regulated by changes of intensity on 

 these structures. This idea is supported by the fact dis- 

 covered by von Uexkiill (1897) that a single spine or pedi- 

 cellaria connected with a piece of shell responds to stimuli 

 practically as it does in the entire animal, showing that the 

 parts of this organism are capable of independent action, 

 and the same is probably true of other organs in this form 

 and also in the other echinoderms. 



