158 PHOTIC SENSITIVITY OF CIONA INTESTINALIS 



the curve represent the averages of the reaction time of two indi- 

 viduals which were used in this particular experiment. 



At any given point, the duration of the reaction time (really of the 

 sensitization period) measures the quantity of Hght energy required 

 by the photochemical system to produce a response of Ciona. In 

 terms of the Bunsen-Roscoe law this, in addition, measures the pho- 

 tochemical effect, and consequently the amount of photosensitive sub- 

 stance which must be changed before the retraction reflex can be 

 ehcited. The curve in Fig. 2 shows this quantity to be at first large, 

 then smaller and smaller. At the end of the process of dark adapta- 

 tion it becomes constant at its minimal value. The interpretation of 

 these changes will be considered in the photochemical system now 

 to be presented. 



VI. Nature of Photochemical System. 



1. The decomposition of a photosensitive material by light presup- 

 poses the formation of such a substance and its deposition within the 

 sense organ. The nature and dynamics of the formation and decom- 

 position of the photosensitive material must be consistent with the 

 four phases of the photic behavior of Ciona. First, it must explain 

 the ability of Ciona to come to sensory equilibrium in any intensity 

 of light. Second, it must be clear why under given conditions the 

 reaction time to a definite increase of illumination is constant. Third, 

 the process must be consistent with the Bunsen-Roscoe rule for the 

 relation between intensity and sensitization period. Fourth, it must 

 indicate the basis of the regular change in the reaction time that 

 occurs during dark adaptation. 



2. The simplest assumption which will account for all the phe- 

 nomena observed is that we are dealing here with a reversible chemical 

 reaction. A photosensitive material {S) is formed from its precursor 

 (P), the reaction being of the first order and following the ordinary 

 laws of such processes. It may be of a higher order, but the assump- 

 tion is made for the sake of simphcity. The action of hght upon the 

 photosensitive material {S) is to change it back into its precursor (P). 

 In accordance with current photochemical knowledge such a reaction 

 will follow the Bunsen-Roscoe rule. A reaction which in many re- 

 spects resembles the system suggested here, is the conversion of 



