NEUROMUSCULAR PHYSIOLOGY OF A SESSILE SCYPHOZOAN

¹ Department of Zoology, University of California, Berkeley 4, Calif., and Friday Harbor Laboratories, University of Washington, Seattle 5, Wash.

1. Previous work on the neuromuscular systems of coelenterates has been confined to Anthozoa and free-swimming medusae, and certain differences in the response to electrical stimulation have been noted. It was thus of interest to investigate the responses of a sessile scyphozoan, Haliclystus auricula (Rathke), to determine the characteristics of the response mechanism.

2. Spontaneous activity of H. auricula is arrhythmic, and there is no demonstrable "pacemaker." The exumbrellar surface of the organism is relatively insensitive to mechanical stimuli, but the tentacles and subumbrellar surfaces are very sensitive. The conducting system possesses properties of a "nerve net." Conduction is slow, diffuse, and with an apparent decrement.

3. H. auricula does not respond to single-impulse electrical stimulation of the exumbrella. A pair of stimuli elicits stalk contraction at an interval of 0.2 second, and calyx contraction at 0.5 second. On the other hand, single-impulse stimulation of the subumbrella will elicit a response, but this response is local, involving only a part of the calyx near the position of the electrodes. A possible mechanism of the graded response is suggested.

4. Physiological evidence indicates that the conducting system is diffuse, with some evidence of a through-conducting pathway at the bell margin that operates at relatively high-frequency stimulation. Conduction speed in the subumbrellar nerve net is calculated at 7-15 cm. per second at 11-13° C.

5. The physiological properties of the neuromuscular system of Haliclystus appear to be intermediate between Calliactis and Cyanea. There is a demonstrable difference in the response of stalk and calyx which shows emphasis on sequential elicitation by mechanical stimulation of progressively greater intensity of tentacle, local marginal, essentially whole calyx, and finally symmetrical stalk involvement. The significance of this series of events in relation to the normal behavior of the animal is pointed out.