VERMES, FLY LARVAE, AND ECHINODERMS 211 



5. Echinoderms 



The Echinoderms are peculiar in that they can move 

 with any side ahead. In reversing the direction of move- 

 ment they ordinarily do not turn around but merely move 

 with the opposite side ahead. Some appear to be perma- 

 nently positive and others permanently negative, while still 

 others may be either positive or negative, depending upon 

 circumstances. 



Washburn (1908, p. 131) says that the starfish and sea 

 urchins depend for their response to light upon pigment or 

 eye-spots on the arms. This conclusion is based largely 

 upon the observations of Romanes, who obtained no reac- 

 tions after the tips of the arms bearing these structures 

 were amputated. In some species however the response 

 appears to be independent of the eye-spots, for Cowles 

 found that in Echinaster crassispina the reactions to Hght 

 were normal three hours after one centimeter had been cut 

 from the tip of each arm. 



Jennings (1907), working on Asterias forreri, a negative 

 starfish, found that it moves toward the shaded side no 

 matter whether the side is shaded by the substance in the 

 starfish itself or by some other object, showing that the 

 direction of motion is regulated by difference of intensity 

 on the surface regardless of the direction of the rays. If 

 illuminated from one side it therefore moves from the source 

 of light because the side opposite the light is shaded. If 

 the position of the source of light is changed it alters its 

 direction of motion at once, ordinarily by simply proceeding 

 with the side ahead which has become shaded. Bohn (1908), 

 however, working on several different species, found that 

 there, is some tendency to turn after the direction of the 

 light is changed so that a given ray will be ahead. 



The lack of orientation in moving from a source of light 

 is much more striking in the holothurians, which are superfi- 

 cially at least much more definitely bilaterally symmetrical. 

 Pearse (1908, p. 278) describes the process in the holothu- 



