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PHOTIC SENSITIVITY OF CIONA INTESTINALIS 



2. Before this study had proceeded very far it became clear that the 

 reaction time could not be reduced below approximately 1.5 seconds. 

 No matter how intense the stimulating light, this interval was always 

 maintained. To explain this, I adopted the provisional hypothesis 

 that the reaction time was not a simple interval but that it contained 

 at least two components. This indeed, was soon found to be true, 

 when experiments were made to determine how much of the reaction 

 time was actually devoted to the reception of the photic stimulus and 

 how much was concerned with a real latent period. 



By exposing an animal for a shorter time than its reaction time, an 

 interval was found during which it was not necessary for Ciona to be 

 illuminated. At the end of this interval it reacted as usual although 

 at the time of reaction, like the hydroids used by Loeb and Waste- 

 neys (1917), they were in the dark. The reaction time is therefore 

 composed of two parts. The first is a sensitization period during 

 which Ciona must be exposed to light. The second is a latent 

 period during which Ciona need not be illuminated in order to react 

 at the end of the period. With a little practice the sensitization period 

 could be regulated to within 0.2 second. An example is given by 

 Experiment 8.6. 



Experiment 8.6. Light Intensity, 743 Candle Meters. 



Result: Sensitization period = 2.0. Latent period = 1.5. 



With the duration of the sensitization period we shall be concerned 

 later, because it varied with the intensity of the light. The latent 

 period, however, showed itself to be a constant quantity under con- 

 siderable variations in intensity. A series of 53 determinations of 



