1. The swimming response of Stomphia coccinea evoked by electrical stimuli has been re-examined.

2. Contrary to earlier accounts, definite relationships have been discovered between the occurrence of swimming and the number, frequency and intensity of electrical stimuli applied. At higher frequencies (1 stimulus/0.4 sec), 4-6 stimuli cause swimming; at lower frequencies (1 stimulus/3.0 sec), 24-40 stimuli are needed to produce the same effect.

3. Stronger stimuli are required to evoke the swimming response than to cause the retraction response. At constant durations, the threshold for swimming is at least 50% above the voltage required for retraction; at constant voltage, the threshold duration of pulses that give swimming is at least three times the duration of pulses that cause retraction.

4. It is concluded that retraction and swimming are controlled by different excitation systems. The system controlling swimming has a higher threshold and it is not an all-or-nothing process.

5. The results are discussed in relation to facilitation of the swimming response and some behaviour patterns in another sea anemone, Calliactis parasitica.

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