1. 1.

    Voluntary and reflexive antennal movements of an Australian fresh-water crayfish are described.

  2. 2.

    An anatomical and physiological study showed that the flagellar flexor muscle is innervated by three motor neurones (a common inhibitor and two exciters) and the flagellar extensor muscle receives four motor neurones (a common inhibitor and three exciters).

  3. 3.

    The discharges of three of the motor neurones to the flexor and extensor muscles (tonic flexor, tonic extensor, common inhibitor) were recorded simultaneously with the movements of the flagellum in intact but restrained animals. Phase relationships between flexors and extensors are not simply antagonistic for all voluntary movements. The common inhibitor is more often in phase with, or leads, tonic extensor activity, than tonic flexor activity.

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