In decapod crustacea, gill ventilation is produced by the rhythmic dorso-ventral movements of the scaphognathite (SG) or gill bailer of the second maxilla. The beating of the SG pumps water through the bronchial chamber and over the gills. The SG may pump water in either the forward or reverse direction, although forward ventilation is the prevalent mode in Carcinus maenas. The SG is controlled by five depressor and five levator muscles, with each set classified into two groups termed D1 and D2 (depressors) and L1 and L2 (levators). The temporal sequence in forward ventilatory beating is D1, D2, L1, L2, while in reverse ventilation, the groups are the same, but the recruitment sequence is reversed: L2, L1, D2, D1. During periods of reverse ventilation, the motor neurons driving the D2 and L2 muscle groups during forward ventilation are inhibited and replaced by a discrete set of "reversal" motor neurons that drive these muscle groups.

The ventilatory CPG of the crab is comprised of several functional classes of nonspiking neurons: frequency modulating interneurons (FMi) that start, stop, or alter the frequency of the ventilatory rhythm, nonspiking central pattern generator interneurons (CPGi) that form the basis of the CPG circuit, a single reversal switch interneuron (RSi) that changeon (RSi) that changes the ventilatory motor pattern from forward to reverse mode, the nonspiking oval organ afferents (OOA) that provide sensory feedback to the CPG, and the motor neurons that drive the ventilatory muscles and also make feedback connections to the CPG. G.