Abstract
The locomotor activity of 28 Helice crassa was studied, individually, in constant conditions. The rhythms displayed were the most complicated ever described for a crab. Two basic cyclic patterns were found: 25% of the crabs displayed a unimodal form (the group period averaging 26.5 + 1.4 hours); while a bimodal pattern was shown by 68% of the individuals (mean period = 25.7 + 0.5 hours). These rhythms persisted for varying lengths of time in the laboratory. Period lengths sometimes increased or shortened spontaneously, and sometimes one or both peaks of a rhythm disappeared and reappeared again later.
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