Abstract
It is well known that the smooth adductors of lamellibranch molluscs can hold the shells closed against the tension exerted by the elastic hinge ligament for prolonged periods without visible signs of fatigue. Two opposing hypotheses have been put forward to explain this phenomenon. One postulates that tonic contraction is a tetanic phenomenon and that the economy of lamellibranch smooth muscles is due to their slow speed of relaxation (Ritchie, 1928); whilst the other proposes the existence of a ‘catch mechanism’ which enables tension to be maintained without expenditure of energy, so that no excitation is needed during tonic contraction (Jordan, 1938).
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