Summary
The heart of Daphnia magna can be arrested by applying a mechanical stimulus to a region in the digestive tube where the stomach enters the intestine. This induced inhibition and likewise the recovery after inhibition, has enabled us to make a study of the action of acetylcholine and physostigmine on the possible control and coördination of the heart beat. If the animal is treated with acetylcholine when the heart beat, after escape from inhibition, is still feeble, slow and irregular, there is observed an immediate acceleration in the heart beat and the cardiac activity becomes more powerful and rhythmic in character. Animals treated with physostigmine previous to the arrest of the heart, show complete restoration of the regular beat soon after the escape from inhibition.
