The Nature of the Latent Period in the Photic Response of Mya Arenaria. by Selig Hecht

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, M y a 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 photographic shutter. By this means the animal is exposed for very short intervals. At intensities above 3,000 meter eandles, 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 reversibl~ photochemical reaction (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 follows 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 intervals 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 photosensory process in Mya be understood in its details, it is necessary to investigate the probable nature of the latent period by a consideration of some of its properties. 657