LIGHT REACTIONS OF THE MEDUSA GON1ONEMUS. 357 



in nearly straight lines when coming to the surface, in the so- 

 called surface reaction. In fact, the only stimulus to which it 

 seems to respond in pretty straight lines is gravity, though the 

 question of the influence of light in the "surface reaction " is 

 difficult to decide. 5 Yerkes's statement" " that the direction of 

 its movement is definitely determined by light " is based on analogy 

 and not on experiment. In this paper" such expressions as 

 "movement toward the source of light," and "strong light . . . 

 soon repels the animals"; again, "an animal passes from the 

 shadow into the sunlight " (page 305), I do not take to mean that 

 the animals swam directly toward or from the light. However, 

 if this is meant, the strongest argument in favor of the medusae's 

 swimming in a direct line not vertical is (page 282) where a 

 medusa inhibited by strong light, starting up again, " usually 

 turns in such a way as to move back into the shaded region." 

 Morse 8 reports a similar experiment differing in that he observed 

 the movement of medusae in the sunlit half of the dish, saying, 

 " the medusae begin to swim in all directions." With this my ob- 

 servations agree. Indeed, in Yerkes's answer to this criticism of 

 Morse's, having repeated the experiment, he says : " I found that 

 when the animals swam so far into the sunlit region before turn- 

 ing over that they were entirely in the sunlight when they came 

 to rest on the bottom of the dish, they moved away from the 

 region of shadow about as often as toward it." In this, then, 

 he agrees with Morse's contention that the medusae do not turn 

 directly toward the shadow and swim into it more often than away 

 from it. In fact he adds (page 462) " with the light perpendicu- 

 lar to the bottom of the vessel I obtained the same results as 

 Mr. Morse. There was no evidence of the directive influence of 

 light." 10 But Yerkes (page 461) also points out that his infer- 



5 This "surface reaction" has been observed in three species of the genus, geo- 

 graphically far enough removed from each other that it seems to indicate an ancestral 

 character. 



6 Amur. Jour. Physio!., 1903, Vol. IX., page 285. 

 1 Amer. Jour. Physiol., 1903, Vol. IX., p. 284. 



% Jour. of Comp. Neural. Psychol., 1906, Vol. XVI., p. 454. 



9 Jour, of Comp. Neural. Psychol., 1906, Vol. XVI., p. 461. 



10 Page 462 Yerkes suggests that the contradiction between himself and Morse in 

 regard to oblique light might disappear if Morse's meaning of the term were more 

 fully explained. Turning to Morse's experiment we find (pp. 453-454) that he used 



