454 HOVEY JORDAN 



constant position, and that, in Lyon's movable-environment experi- 

 ments, their total stimulation was much greater. It can not be said 

 whether the tactile corpuscles were subjected to a proportionate stim- 

 ulus or not. It may be that the hamlet, too, would orient to a movable 

 environment regardless of. contemporaneous tactile stimulations 6 by 

 the current; but the facts remain that the tactile-corpuscles do, of them- 

 selves, effect orientation, and that this orientation under the above cir- 

 cumstances is unaltered by the presence of eyes. This orientation, 

 it seems to me, is caused by a direct stimulation of the integument 

 by the water currents as such, and to it we should apply the term 

 rheotropism. The response so well described by Lyon as due to optic 

 reflex might then be called a rheoscopic response in view of the fact 

 that it is due to the optical effect of a flowing or moving environment. 

 How the current stimulates these tactile end organs is still a matter 

 of speculation. It may be that differences of velocity in different por- 

 tions of a current provide slight local variations of force sufficient to 

 cause a definite response on the part of the fish. How the stimulation 

 results in orientation is a further question, for the mechanism and 

 sensation may or may not be the same for rheotropism that they are 

 for stereotropism. 



The author wishes to express his appreciation to Dr. E. L. Mark 

 for the privilege of working at the Bermuda Biological Station, and 

 to Dr. Mark and Dr. W. J. Crozier for valuable assistance and ad- 

 vice in the work. 



BIBLIOGRAPHY 



(1) VERWORN: Allg. Physiol., 2te Aufl., 1897, xi, 606. 



(2) LYON: This Journal, 1904, xii, 149. 



(3) SCHULZE: Arch. f. mikr. Anat., 1870, Bd. vi, 62. 



(4) PARKER: Bull. U. S. Fish Com. for 1902, 45. 



(5) TULLBERG : Bihang K. Svenska Vet. Akad. Handlingar, Stockholm, 1903, 



Bd. xxviii, No. 15. 



(6) PARKER: Amer. Nat., 1903, xxxvii, 185. 



6 It would be interesting to determine the relative importance of the eyes and 

 cutaneous elements (tactile corpuscles) as sense organs in the orientation and 

 rheotropic motions of many other fishes. 



