SENSORY EQUILIBRIUM AND DARK ADAPTATION IN 

 MYA ARENARIA. 



By SELIG HECHT. 



{From the Physiological Laboratory, College of Medicine, Creighton Universily, 



Omaha.) 



(Received for publication, March 31, 1919.) 

 INTRODUCTION. 



Animals which are sensitive to a sudden increase in illumination 

 possess two outstanding characteristics. One is the ability of the 

 organism to come into sensory equilibrium on continued exposure to 

 a given intensity of light. The other is the restoration of its sensi- 

 bility to this intensity by subjection to a lower intensity for a definite 

 interval. The latter represents a process of dark adaptation; the 

 former, by analogy, a process of light adaptation. The terms are 

 not happy ones. However, since the "light-adapted" eye, and the 

 "dark-adapted" eye are commonplaces of sensory physiology, they 

 are retained. 



The mollusk, My a arenaria Linne, presents these two phases of its 

 sensitivity in measurable form. It has therefore been possible to 

 investigate them carefully and to suggest a photochemical system 

 which underlies them. In a measure, this has already been sketched 

 in for Ciona intestinalis (Hecht, 1918, a, h). The present investiga- 

 tion of Mya, however, serves as an analysis of the photochemical 

 system in such quantitative detail as to increase materially its scien- 

 tific value and its probable reality. 



The purpose of this paper is threefold: first, to describe the general 

 properties of the photic sensitivity of Mya; second, to analyze the 

 data relative to dark adaptation and sensory equilibrium; and third, 

 to record some experiments designed to test the validity of the photo- 

 chemical system which is suggested as an explanation of these phe- 

 nomena. The experimental work was done at Woods Hole, Mass., 

 during July and August, 1918. 



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