An analysis of bailer movements responsible for gill ventilation in the crab,cancer novae‐zelandiae

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Abstract
The movements of the bailer during normal ventilation can be resolved into two components, a cycle of pronation and supination being superimposed on a cycle of protraction and retraction. Pronation leads protraction with a phase angle of about 90° in a normal cycle. Pronation is accompanied by flexion of the bailer. The skeletal anatomy of the bailer is such as to restrict movements of the bailer to those described above. Further the pronated and supinated positions of the limb represent the two stable positions of a skeletal click mechanism, the operation of which may help to resolve the functional duality of the promotor and remotor muscles. This functional duality arises because the muscles are positioned so as to produce either protraction or supination of the limb. Other muscles in the limb are monofunctional. The bulk of muscle tissue responsible for protraction and supination seems to be greater than that responsible for pronation and retraction. The sequence of muscular activity during the ventilation cycle follows that expected for a sequence, of protraction, supination, retraction and pronation. Overlap in the periods of activity of the bifunctional muscles and muscles responsible for pronation may also help to resolve the functional duality of the former. The amplitude of bailer excursion (protraction‐retraction) is not greatly affected by changes in frequency. An advance in the onset of activity in some muscles at higher ventilation • frequencies suggests that the system is tailored to produce a constant beat amplitude at all frequencies. Pauses in ventilation occur with the bailer in the retracted position, and it is maintained in this position by tonic activity in the appropriate muscle. During normal ventilation the relative contraction duration of this muscle is positively correlated with cycle period, so that pauses apparently represent a prolongation of the normal retracted phase. The relative contraction durations of some other muscles are negatively correlated with cycle period. The different signs of these correlations may be related to the type of endogenous oscillator present in the central nervous system.