1. A substance which is extractable with acetone from the marginal ganglia of the jellyfish Aurelia causes acceleration of the rhythm of intact ganglia used as test preparations in concentrations of under 50 ganglia/ml., while higher concentrations cause inhibition. As comparable extracts of non-nervous tissue had no effect, the active substance probably originates from nervous tissue. It is distinct from substances extractable from mesogloea.
2. The RF lies near 0·85 in tert.-butanol-methanol-water (40:50:10 by vol.). The chemical nature of the acceleratory substance is unknown though it is certainly none of the known or suspected neurohumours of other organisms.
3. The active substance has no effect on crustacean or clam hearts or upon crustacean hindgut assay preparations but it does accelerate the rhythm of a hydromedusan Phialidium.