1. The propagated massed potentials which follow stimulation of the radial nerve in Arbacia, Diadema, Echinus and Paracentrotus are described.
2. Approximate values for the averaged absolute and relative refractory periods and the conduction velocity were obtained.
3. The response of Diadema has a double peak which is shown to represent responses of nerves differing in excitability and conduction velocity. The fast potential is concerned with spine movement. The slow potential is related to inhibition of spine movements excited photically.