1. The discharge of tentacular nematocysts in response to food was studied in Stomphia coccinea before and during swimming, and during the period of resettling.
2. The tendency of Stomphia's nematocysts to discharge on contact with food virtually disappeared during the swimming response. The disappearance of the nematocyst response coincided with the release of the pedal disk; its reappearance coincided with the re-attachment of the pedal disk.
3. Objects rubbed on the aboral surface of Dermasterias can cause swimming if brought into contact with a single tentacle of Stomphia.
4. A general inhibition of the swimming response to objects rubbed on Dermasterias was set up by flooding the tentacular crown with a food extract. A local inhibition was set up when food was applied to a single tentacle just before the same tentacle was touched with material from Dermasterias.
5. The results are discussed in relation to the view that nematocysts function as independent effectors.