Fine structure of the neuromuscular system of Polyorchis penicillatus (Hydromedusae, Cnidaria).

Polyorchis penicillatus exhibits outer, inner and endodermal nerve rings. The inner ring contains a number of giant axons with infolded plasma membranes and annular gap junctions. The existence of an innervation supplying the velar radial muscle strengthens the view that the steering mechanism is under nervous control. The basal portions of the cells of the endoderm canals form a muscle band which might enable the animal to regulate the flow of materials or could perform peristalsis.