Slow contraction and its relation to spontaneous activity in the sea-anemone Metridium senile

It has been known for some time that the most rapid reflex contractions of sea-anemones show a characteristic relation to the stimulus. Thus in CalUactisparasitica a short series of adequate electric shocks, given at a frequency of about 1 per sec, is regularly followed by a facilitated contraction of the marginal sphincter (Pantin, 1935a, b). That is, each successive shock sets up a nervous impulse which facilitates the response of the muscle to the next; so that the contraction increases by steps corresponding to the shocks. The facilitating effect of each impulse decays so that as the frequency of stimulation is lowered the response becomes smaller and finally vanishes. But in Calliactis stimulation at frequencies which are too low to cause the sphincter response nevertheless succeeds in calling certain other very slow systems of muscles into action. The mesenteric retractors, the longitudinal parietal muscles, and the circular muscle of the body wall seem to be excited by successively lower frequencies (Pantin, 1935 a). Hall & Pantin (1937) found a rather similar facilitation sequence in Metridium senile, though in this case it was the mesenteric retractor