104 Papers from the Marine Biological Laboratory at Tortugas. 



V 



While Echinaster is very sensitive to light and usually reacts posi- 

 tively to it, such is not always the case. I have described above the 

 reaction of this starfish when placed in a black-lined box with bright 

 light entering one end. Under these conditions the specimen usually 

 moves almost directly toward the lighted end of the apparatus. Making 

 a few tests of this sort, one might draw the conclusion that the starfish's 

 behavior is invariable, that it is stereotyped; but if enough series of 

 tests are made it is found that occasionally the starfish does not react 

 positively to the light. Furthermore, it frequently occurs that when a 

 starfish reaches the lighted end of the dish, instead of staying there it 



returns again to the dark end, especially when 

 it follows the base of the vertical walls of the 

 dish. Bohn (1908) and, above all, Jennings 

 (1907) lay stress on the importance of not 

 neglecting the internal factors while studying 

 the behavior of the starfish and they recog- 

 nize the variability of the reactions. 



REACTION TO BRIGHT LIGHT. 



Using the apparatus shown in fig. 2, 

 many echinasters were tested to determine 

 their reaction to the intense light reflected 

 from a white sand-bank. The starfish was 

 placed in the experimenting dish, with a single 

 ray or with an interradius directed toward 

 the lighted end. The method of handling was 

 varied. Many series of tests were made and 

 in the large majority of cases the starfish re- 

 acted positively and moved to the brightly 

 lighted end of the dish. Under the influence 

 of the intensely lighted region at the open end 

 of the apparatus the specimens seldom failed to react positively, but 

 the reaction was not invariable. 



The results of these experiments show that Echinaster, as a rule, 

 reacts positively to regions of increased light intensity, that it does this 

 irrespective of the ray or rays directed toward the bright light, and that 

 its locomotion is somewhat more accurate when a ray is pointed toward 

 the light than when an interradius is directed in that direction. Fig. 5 is 

 a plan of the apparatus as used in tables 6 and 7. Table 6 records a series 



TABLE 6. 



FIG. 5. Plan of apparatus used 

 in test i, table 6. The ar- 

 row indicates the direction 

 of light, a, wooden box; c, 

 glass dish. 



of tests with a single ray directed toward the bright end. Table 7 

 records tests with an interradius directed toward the bright end. 



