RHEOTROPIC RESPONSES OF EPINEPHELUS STRIATUS 453 



between fish and water, although he admits the possibility of another 

 interpretation involving the idea of unequal pressures on different parts 

 of the body of the fish. Whether the rheotropism induced by this 

 "sliding contact" is the equivalent of stereotropism, with which Lyon 

 believes it is closely related, is a question needing further investi- 

 gation. 8 



The comparative importance, in the behavior of Fundulus, of these 

 two types of impressions optic and cutaneous is, I think, suggested 

 by some of Lyon's interesting experiments. An example of this is his 

 experiment 3 (p. 153). Here a normal fish, surrounded by a rapidly 

 moving artificial environment, is immersed in a current of water flow- 

 ing in the same direction as the environment, but less rapidly. The 

 fish swims in the direction of, but faster than, the current flow, follow- 

 ing the moving environment in rate as well as direction. This is un- 

 questionably -a case of optical response. When the environment 

 stops moving, the current still flows on in the same direction, but with 

 a gradual decrease of speed due to friction; but the fish, having been 

 carried passively by the current, turns, without a reference point, and 

 faces [swims against?] the current. This may be due, as Lyon says, 

 to an apparent reversal of the visual field; but, since blinded fishes 

 (experiment 9), without touching a reference point, also orient against 

 the current, it seems equally logical to interpret the turning of fishes 

 with eyes (experiment 3) as the result of the normal rheotropic re- 

 sponse to direct tactile stimulation of the integument, which had, 

 during the movement of the optical field, been subordinated to the 

 sight-reflex. 



If this be a proper interpretation of the results with Fundulus, we 

 have in the hamlet a reversal of the relative importance of the two kinds 

 of stimuli. Here, under the experimental conditions described, all 

 optic stimuli were, apparently, subordinated to tactile impressions; 

 the direct effect of the current being predominant and able to cause 

 entirely normal reactions in the absence of eyes. It is, however, not 

 quite satisfactory to compare the two sets of experiments (those by 

 Lyon and by myself) from this point of view, because the relative 

 amounts of cutaneous and optic stimuli in each are indefinite and vari- 

 able. It is certain that, in my experiments, there 'was relatively little 

 stimulation of the eyes, because the fish remained in an approximately 



5 It seems probable that the currents of a narrow trough would not be suffi- 

 ciently strong nor distinct from one another to simulate solid objects, but that 

 there would be an almost insensible gradation between them. 



