In a recent study (Ross, 1957) isolated marginal sphincter preparations of the sea anemone, Calliactis parasitica, showed two distinct responses to stimulation: (1) a quick contraction in response to stimuli at frequencies from 0–2 to 3–0 sec, the facilitated response of Pantin (1935 a) beginning only on the second stimulus of a series, with a latent period of about 0 1 sec; (2) a smooth slow contraction in response to stimuli at lower frequencies, with a latent period of not less than 30 sec, and beginning only after several stimuli. This slow movement of the marginal region had not been detected in work on whole animals, but it is similar to the contractions of the ‘slow’ muscles of Calliactis (Pantin, 1935 b) and Metridium (Batham & Pantin, 1950 a). However, Calliactis sphincter shows almost no spontaneous activity.

Both quick and slow contractions had previously been observed in excised sphincters and mesenteric retractors of Metridium (Batham & Pantin, 1954), the chief muscles utilized in the closure of that animal. In all these cases, the quick contraction would seem to be a specialized mechanism for sudden withdrawal superimposed upon, and perhaps developed from, a more primitive and general slow contractile mechanism. As part of a programme to investigate and compare the properties of quick and slow contractions in these muscles it seemed desirable to see what part, if any, slow contractions of the marginal sphincter play in the life of Calliactis. This involved us in a wider study of the neuromuscular activity of this anemone which normally lives on shells inhabited by the hermit crab, Eupagurus bernhardus.