SELIG HECHT 165 



given here it seems clear that the entire process takes place in the 

 sense organs themselves. The attainment of a condition of sensory 

 equilibrium of Ciona depends upon the existence of a "stationary con- 

 dition" of the reversible reaction system responsible for its photo- 

 sensitivity. Dark adaptation represents such a sensory equilibrium 

 when the "light reaction" is completely removed. Rapid adapta- 

 tion to light is essentially the reverse process, in which the "dark" re- 

 action is practically absent, and the "light" reaction is in full swing. 

 In these experiments there is no need either for the postulation of 

 a learning process or for the presence of a "higher behavior" in order 

 to rationalize the results obtained. 



SUMMARY. 



1. Ciona possesses two means of responding to an increase in the 

 intensity of illumination. One is by means of a local reaction; the 

 other is by a retraction reflex of the body as a whole. 



2. The "ocelli" are not photoreceptors. The photosensitive area 

 is in the intersiphonal region containing the neural mass. This area 

 contains no pigment. 



3. The reaction time to light is composed of a sensitization period 

 during which Ciona must be exposed to the light, and of a latent period 

 during which it need not be illuminated in order to react to the 

 stimulus received during the sensitization period. 



4. The duration of the reaction time varies inversely as the inten- 

 sity. Analysis shows the latent period to be constant. The relation 

 between the sensitization period and the intensity follows the Bunsen- 

 Roscoe rule. 



5. During dark adaptation the reaction time is at first large, then it 

 decreases until a constant minimum is reached. 



6. A photochemical system consisting of a reversible reaction is 

 suggested in order to account for the phenomena observed. This 

 system includes a photosensitive substance and its precursor, the 

 dynamics of the reaction following closely the peculiarities of the 

 photosensitivity of Ciona. 



7. It is shown that in order to produce a reaction, a constant ratio 

 must be reached between the amount of sensitive substance broken 

 down by the stimulus and the amount previously broken down. 



8. From the chemical system suggested certain experimental pre- 



