Abstract
1. Acetylcholine produces in Daphnia magna vigorous intestinal contractions which persist for some time after they are established. 2. The period which elapses between the addition of the acetylcholine and the onset of the characteristic effect is definitely dependent on the concentration of the drug employed. 3. Atropine blocks the action of acetylcholine. 4. Physostigmine causes intensification and prolongation of the effects of acetylcholine. 5. Acetylcholine, when it is preceded by physostigmine, causes in Daphnia magna a considerable reduction in the time which elapses between the administration of the drug and the appearance of the vigorous intestinal contractions.

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