Male Carcinus collected in summer and given several days to recover from eyestalk ablation showed a continuously high level of activity. Injection of eyestalk extract inhibited activity. Unlike control animals from which retinae were removed, experimental animals did not show a rhythm after chilling. Chilling the eyestalks alone elicited a rhythm similar to that appearing when the whole animal was chilled.

The results are consistent with the view that locomotor rhythmicity is mediated via an inhibitory hormonal mechanism in the neurosecretory cell groups of the eyestalk. There is no direct evidence that the neuroendocrine mechanism alone constitutes an autonomous clock, however, and it is in any case probably co-ordinated by one or more other rhythmic systems within the crab.

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