1. The neuromuscular activities of isolated marginal sphincter preparations of the sea anemone, Calliactis parasitica, have been studied. They showed almost no rhythmical or spontaneous activity.
2. In addition to the facilitated step-like quick contractions in response to stimuli at frequencies between 0·2 and 3·0 sec., the preparations gave smooth slow contractions in response to stimuli (usually not less than 6 stimuli were necessary) at frequencies up to about 15 sec. (at 17-18°C.). These contractions are similar to those given by the so-called ‘slow’ muscles which do not take part in the quick closing movements of the animal.
3. A study was made of the relations between the size and latent period of the slow contraction and the number and frequency of stimuli delivered. At each frequency there is a threshold number of stimuli which only just gives a response; with additional stimuli the response gets bigger until a maximum is reached. There is an optimal frequency (stimuli about 6-8 sec. apart at ordinary temperatures) at which the biggest responses are obtained with fewest stimuli.
4. Certain mechanical and temporal features of the quick and slow contractions are compared and the observations are discussed in relation to current views on neuromuscular transmission and innervation in these and other invertebrates.