STEREOTROPIC REACTIONS OF THE SHOVEL-NOSED RAY, RHINOBATUS PRODUCTUS

I t has been pointed out by Loeb 1 that our orientation in space is determined mainly by three groups of tropistic influences; namely, light, gravitation, and contact. Light and gravitation cause the orientation of organisms through effects upon muscle tonus. When the lines of force strike the animal obliquely, as for example, when light rays fall unequally on 1:he two eyes, the unequal stimulation causes differences of tonus on the two sides, and the symmetrically placed muscle groups acting with unequal strength, bring about forced changes in the direction of locomotion. When the lines of force coincide with the axis of symmetry, or the plane of symmetry of the body, the effects are equal on the two sides and movement can go forward in a straight line. For the contact, or stereotropic, reactions, quantitative relations of this nature have not heretofore been described. In my studies on the physiology of the labyrinth I have found it necessary to distinguish carefully between those eye and fin movements which result from excitations of end-organs in the ear, and movements which arise from other sources. In this way I have come to make observations on the contact reactions of the shovel-nosed ray, or guitar fish, Rhinobatus productus, which will, I believe, throw important light on the nature of stereotropic reactions in general. Rhinobatus is not so broadly expanded as most of the other rays. The pectoral fins, however, have the characteristic fleshy thickened