THE NATURE OF THE LATENT PERIOD IN THE PHOTIC 

 RESPONSE OF MYA ARENARIA. 



By SELIG HECHT. 



{From the Physiological Laboratory, College of Medicine, Creighton University, 



Omaha) 



(Received for publication, May 9, 1919.) 



The response of Mya to light involves the existence of a latent 

 period. The stimulus consists of the exposure of the animal to the 

 influence of light for a short interval. Following the exposure period, 

 Mya may remain in the dark for a length of time which is usually- 

 greater than the exposure. At the end of this latent period it retracts 

 its double siphon in response to the stimulus. The demonstration of 

 the latent period may be easily accomplished by the use of a photo- 

 graphic shutter. By this means the animal is exposed for very short 

 intervals. At intensities above 3,000 meter candles, an exposure of 

 0.01 second is sufficient to produce a response after a latent period of 

 about 1.3 seconds. 



The sensitization process occurring during the exposure period 

 has already been shown to depend on a reversible photochemical re- 

 action (Hecht, 1919, a). This consists of the decomposition of a 

 photosensitive substance into its two precursors. The dynamics 

 of the decomposition and regeneration of the sensitive material fol- 

 lows closely the peculiarities of the sensory behavior of the animal. 



The significance of the latent period, however, is still unknown. 

 Of course it must include the time for the transmission of impulses, 

 and for the latent period of the contraction of the muscles used in 

 the siphon response. These are, however, notoriously short inter- 

 vals of time, and may be almost ignored in the face of a latent period 

 which lasts at least 1.3 seconds, and may be as long as 3 or 4 seconds 

 under certain conditions. In order, therefore, that the photosen- 

 sory process in Mya be understood in its details, it is necessary to in- 

 vestigate the probable nature of the latent period by a consideration 

 of some of its properties. 



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