546 



SENSORY EQUILIBRIUM AND DARK ADAPTATION 



Sejtsitivity of My a. 



1. My a retracts its double siphon when it is suddenly illumi- 

 nated. The reaction time of any individual to a given intensity is 

 constant. In addition, animals of the same size give about the same 

 reaction time under similar conditions (see Table I). The figures 

 in Table I represent three measurements of the reaction time of 

 each of six animals to one intensity. 



Table I shows that it is a safe procedure to consider all the values 

 together in order to express the mean reaction time of the group of 

 animals. Because of this it was possible to perform a single ex- 

 periment on several individuals, rather than to repeat the same de- 



TABLE I. 



Light Intensity, 85 Meter Candles. Animals in Dark Room for 24 Hours. 



termination several times on one animal. This possesses the advan- 

 tages of convenience and greater numbers. 



2. The reaction time of My a is similar to that of Ciona in respect 

 to its composition. It may be divided intx) two periods. The first 

 is a sensitization period during which Mya must be exposed to the 

 stimulating light in order to respond with the usual reaction time. 

 The second interval is a latent period during which the presence or 

 absence of the stimulating light in no way influences the duration of 

 the reaction time. During this latent period Mya may be in the 

 dark, but will respond as usual to the effect initiated by the light 

 during the sensitization period. 



The composition of the reaction time in Mya is striking because 

 of the disparity in the duration of its two phases. The latent period 

 may vary between 1 and 4 seconds depending on the temperature. 



