Two types of electrical potentials were recorded from competent echinopluteus larvae of Lytechinus pictus. The first was associated with reversal of cilia that line the ridges of larval epaulettes. Ciliary reversal potentials were recorded from all parts of the larvae and from isolated body parts. Estimated conduction velocity of ciliary reversal potentials was 10–20 cm/s. The second type of potential was largest when the electrode was placed over the urchin rudiment. Rudiment potentials could not be recorded from isolated epaulettes and could not be correlated with any obvious behavior of the larva.
Both potentials increased in frequency when a larva was brought in contact with a “conditioned” substrate. Ciliary reversal potential frequency increased six- to seven fold within a minute or two of contact and remained elevated until the point of partial tissue collapse during metamorphosis. Rudiment potential frequency also increased, but only three- to five fold and with a longer latency (3–6 min to peak frequency). Rudiment potentials maintained an elevated level of firing through total tissue collapse, at which time the larvae were electrically silent. Attempts to record from juvenile urchins, to determine the fate of the rudiment potential system in the final stage of metamorphosis, were unsuccessful.