The functions of the five fibres innervating the flexor muscle of the carpopodite was investigated in Panulirus interruptus. The four thicker fibres were found to be motor axons, each eliciting a contraction with different characteristics. These four contractions were accompanied by four different types of action currents.

The thinnest fibre when stimulated simultaneously with any of the four motor fibres caused inhibition of the contraction.

It is concluded that all four contractions take place in all the muscle fibres and that the conception of the mechanism of crustacean nerve muscle system developed before is enlarged to include the new results. The possible biological significance of the quintuple innervation is discussed.

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