Isolated elytra of polynoid worms emit a flash of bioluminescence when stimulated by an electric shock. With repeated stimulation, hundreds of flashes can be elicited which, in typical series, exhibit large and progressive variations. The amount of luminescence emitted by each flash first increases during a period of facilitation and then decreases exponentially during a longer period of decay. Through a microscope and image intensifier, the activity of individual microsources or photosomes was observed, using their fluorescence as a natural probe, in that its intensity is a function of the amount of luminescence previously emitted. Sequential observation showed a progressive and basically intracellular recruitment that correlated with facilitation. Facilitation and/or recruitment depended on the frequency of the stimulation. Recruitment proceeded among the photosomes of each photocyte, beginning with those of the cell periphery and progressing to those of the center. When the repetitive stimulation was interrupted and then resumed, the refacilitation was a function of the duration of the pause, and the pathway of recruitment duplicated that of the preceding sequence. It therefore appears that, within a given cell, individual photosomes can be either coupled and respond to stimulation or uncoupled and quiescent, that the coupled state has a basic lifetime of about 1 s which can be lengthened by reinforcement, and that this state must be established in a matter of milliseconds as a result of the stimulation. In preparing an increased response to a forthcoming stimulation, coupling acts as a short-term memory.

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