Neural redundancy and regularity of swimming beats in scyphozoan medusae

Despite the apparent symmetry of jellyfish the eight ganglia do not all have equal levels of activity in initiating swimming beats. Differences in activity level persist over several hours, both in the intact jellyfish and after isolation of the individual ganglia. Horridge has proposed a specific model of how the regularity of the swimming beat should be increased by the redundancy of pacemakers (one in each ganglion). Our data are quantitatively incompatible with this model. However, this incompatibility may be due more to the impossibility of maintaining comparable levels of sensory input at different stages of the test than to deficiencies in the model. Regular beating in the whole jellyfish is probably due to both redundancy of pacemakers and more regular functioning of individual pacemakers when they receive greater excitatory input.