A heart-accelerating principle for crustaceans which had been found in extracts of their eye-stalks is shown not to be identical with the chromatophorotropic hormone, and is present in nerve tissues other than those of the eye. Extracts of muscle have an inhibitory action on the various hearts. Excess potassium stimulates the action of nerve extracts but it is not present in them in sufficient amounts to be responsible for their effects. Adrenalin excites the crustacean heart but it is probably not the nerve substance in question. Acetylcholine excites this heart and in various concentrations produces a series of effects very similar to those produced by nerve extracts. By appropriate extraction methods and methods of bioassay relatively large amounts of acetylcholine were found in crustacean nerve tissues; more being present in ganglia than in nerve fibres. Hence the active principle in question is, with little doubt, acetylcholine.

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