RHEOTROPIC RESPONSES OF EPINEPHELUS STRIATUS 441 



In the same year Tullberg (5) (p. 20) carried out experiments in which 

 he eliminated the ear of certain fishes and found the operated animals 

 to be insensitive to water currents. He therefore concluded that the 

 ear is the chief receptor of this stimulus, which in his opinion affects 

 principally the cristae acusticae of the ampullae. Parker (6) (pp. 

 202-203) contended, on the other hand, that the failure of fishes to 

 orient in a normal fashion to the current when their ears had been de- 

 stroyed is caused by an interference with the ear, "though the primary 

 stimulus for this form of response might be received by the skin." 1 



In confirmation of this view i.e., primary cutaneous sensitivity to 

 currents he (4) (p. 61) showed that specimens of Fundulus hetero- 

 clitus in which the lateral-line nerves had been severed responded 

 normally, i.e., swam against the current in a glass tube, and he (6) 

 (p. 202) also succeeded, after cutting of the lateral-line nerves and the 

 spinal cord (in an anterior region), in inducing normal responses to a 

 current of water directed against the sides of the body posterior to the 

 cuts. Moreover, he excluded the possibility of stimulation through 

 the ear or lateral-line senses by severing the appropriate nerves and 

 draws this conclusion (4) (p. 63): "Surface waves and current action" 

 . . . "must stimulate the general cutaneous nerves (touch)." 



However, his attempts to inhibit the action of the cutaneous nerves 

 by the application of cocaine, and thus to show directly what the in- 

 direct method (the elimination of ear and lateral-line organs) had ren- 

 dered probable, were unsuccessful. 



In 1904 Lyon, basing his conclusions upon the results of ingenious 

 experiments with Fundulus, scup, stickleback and butterfish sur- 

 rounded by movable environments, put forward another and totally 

 new explanation of rheotropism. He contended that "the primary 

 cause of orientation [and locomotion of fishes] in streams of some 

 uniformity of motion is an optical reflex." "The essential element of 

 stimulation is the environment [apparently moving, but in reality 

 stationary], not the currrent;" the latter "does not directly stimulate." 

 He, however, adds that cutaneous sensations [touch?] contact of 

 the body with stationary solid objects and even "the sliding contact 

 between fish and [rushing] water"- may sometimes be the cause of 

 such orientation and locomotion. But even these responses, like those 

 of a strictly optical reflex nature, are to be regarded, he thinks, as 



1 i.e., stimulation of the skin (tactile corpuscles) directly by small currents. 

 This, it should be noted, is different from the indirect (optical reflex or thig- 

 motropic by solid object) stimulation, which is described below. 



