Fig. 13. Schematic of the neuromuscular organization of the subumbrella of Aequorea. The arrows represent excitatory chemical connections and the bars, inhibitory connections. The resistor symbols represent electrical coupling, as do the dotted lines within the radial and circular muscle boxes. FMRF, FMRFamide-immunoreactive nerve net of the subumbrella; MNN, motor nerve net of the subumbrella;???, unknown pathway responsible for activation of the FMRFamide-immunoreactive nerve net following mechanical stimulation of the margin. The circular and radial muscle sheets refer to the musculature from a single subumbrellar segment.