SELIG HECHT 



157 



V. Dark Adaptation. 



1. Under ordinary circumstances, Ciona remains expanded in diffuse 

 light. The stream of sea water flows steadily through its branchial 

 sac except for certain rhythmic interruptions presented in detail 

 elsewhere (Hecht, 1918, a). Similarly, the animal becomes non- 

 responsive after a time when placed in a lower or a higher intensity 

 of illumination. After such adaptation it fails to react to a decrease 

 in the intensity of the illumination but responds vigorously to an 

 increase. On remaining for some time in the dark — -becoming "dark 

 adapted" — it is sensitive to the light of an ordinary Mazda lamp. 



1 /o 



< 8 



I- 



o — — o-. 



SCO 



/ooo // oo 



Time 



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/'OO 



Fig. 2. Course of dark adaptation of Ciona represented by reaction time at 

 different periods of sojourn in dark room. 



The process of dark adaptation was followed in some detail. Ani- 

 mals which had been in the light for a few hours were removed to the 

 dark room and their responses to light noted at regular intervals. A 

 constant intensity was used, so that any variations in the reaction 

 time would serve to indicate corresponding variations in the quantity 

 of stimulating agent required to cause a reaction. 



The results were regular and fairly uniform. The process of dark 

 adaptation required at least 2 hours; usually more. At first the reac- 

 tion time was long, then it shortened rapidly, then more slowly, and 

 finally at the end of 2 or more hours it reached a constant minimum. 

 A good example of what occurred is given by Fig. 2. The points on 



