Excitatory and inhibitory control of inherent contractions in the sea anemone Calliactis parasitica.

The shape of the column of a sea anemone depends partly on the relative state of contraction of two antagonistic muscle groups in the body wall endoderm. These muscles, the circulars and the longitudinals (parietals), are attached to the mesogloea and act against the hydrostatic pressure in the coelenteron. The circulars form a continuous muscle sheet whereas the parietals lie only where mesenteries insert onto the body wall. Both muscles give slow contractions that can be either symmetrical or local. Column extension and column shortening result from symmetrical contractions; peristaltic waves and column bending result from local contractions, although in some species column bending may be due to contraction of parieto-basilar muscles. Although within wide limits the muscles have no fixed resting length, the body shape may remain constant for long periods. The circular and parietal muscles are not, however, at rest. In Metridium senile continual activity of the column muscles maintains the average state of tone that balances the hydrostatic pressure (Batham & Pantin, 1950a). Both M. senile and Calliactis parasitica show occasional slow spontaneous contractions involving several muscle groups (Batham & Pantin, 1950 6; Needier & Ross, 1958); the contraction cycles differ in detail between the two species but each involves parietal and circular muscle activity. Batham & Pantin (1954) proposed, from kymograph studies of Metridium, that spontaneous contractions follow low-frequency bursts of pulses in the through-conducting nerve net. Such bursts can be directly recorded from isolated preparations of Calliactis parasitica and are followed by parietal and circular muscle contractions (McFarlane, 1973 a, b). Batham and Pantin also suggested, however, that the nerve net is not necessarily directly responsible for exciting the observed contractions but that it may act to co-ordinate the natural inherent activity of the separate muscle groups involved. The nature of the control of parietal and circular muscle contractions is here reviewed in the light of the recent demonstration of multiple conduction systems in sea anemones (McFarlane, 1969 a, 1973 c). There are three known conduction systems in C. parasitica; these are the nerve net and two slow-conducting systems, the SSi and SS2, possibly non-nervous and apparently located in the ectoderm and endoderm respectively (McFarlane, 1969 a). Spontaneous SS2 activity has been recorded from isolated preparations and intact animals (McFarlane, 1973 a, b) but no behavioural correlate was described. The present work relates circular and parietal muscle contractions to electrical activity in