1. Repetition-produced modifications in the behaviour of the swimming sea anemone, Stomphia coccinea, are described. Lowered threshold to number of electrical shocks on successive trials indicates a kind of ‘learning‘ called here longterm facilitation.
2. Dissection of the behaviour into its components, both by experimental techniques and observation of atypical cases, shows the swimming reaction not to be simply a chain of reflexes, but to be ‘centrally’; co-ordinated.
3. The conditions for electrical elicitation of swimming are shocks sufficient in intensity to activate the through-conducting nerve net repeated eight times in the frequency range of 1/2 sec. to 4/sec. Fewer than 8 shocks constitute a subthreshold stimulus in fresh animals; more than 8 are rarely required.
4. Repeated subthreshold stimulation by starfish or by 7 electric shocks result in a long-lasting facilitated state in which the same stimulus repeated hours later may produce a full response. The facilitated condition has been observed to last 7 days. Controls kept without stimulation do not show facilitation.
5. The probable site of this long-term facilitation is discussed. It is suggested that this site is at the point of convergence of the two types of stimulation used and between the through-conducting nerve net and the responding muscles.