Behavioural Physiology of the Colonial Hydroid Obelia

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Abstract
1. The bioluminescent system of Obelia responds to stimulation by producing a burst of excitatory potentials (luminescent potentials, LPs) with each stimulus except the first. These LPs initiate flashes from the coupled luminescent effectors. 2. There is temporal facilitation between the applied stimulus and the LP-system; the LP-system shows adaptation to stimulation by a rise in threshold as well as fatigue. 3. There is facilitation within the LP-system that is temporal at a given point and spatial (in the distance of spread) between two or more points. The result is a limited spread of the first potentials and decreasing numbers of potentials with increasing distance but often a through-conduction of the final potentials. 4. The conduction velocity of the LPs is about22 cm/s at 12 °C; the refractory period is about 60 ms at 20 °C and 150 ms at 12 °C; the latency from stimulation to the first LP in a stimulus train first decreases from 130-600 ms to 20-80 ms and then increases again to 150-600 ms at 20 °C. The latency between LP and the flash is about 20±9 ms. 5. The intervals between LPs in bursts evoked by stimuli applied at 1/s gradually increase from about 65 ms to about 135 ms at 20 °C. The absolute LP intervals are temperature-dependent (the relative ratios are not) but independent of the stimulus frequency. The number of LPs in a burst is frequency dependent. 6. The facilitation curve of the LP-system-luminescent effector junctions, as determined from flash intensity, shows a rapid rise between 60 and 80 ms, a fairly rapid decay between 80 and 200 ms, and a long residual decay which lasts for several hundred milliseconds.

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