(1) The musculoskeletal system and innervation of the scaphognathite (SG) of the second maxilla inCarcinus maenas are described. Rhythmic beating of the SG is effected by ten muscles subdivided into two main sets, five levators and five depressors, and further, into four groups designated D1 and D2 (depressors) and L1 and L2 (levators). Sequential activation of the groups of muscles results in trapezoidal movements of the anterior and posterior tips of the SG, about 180° out of phase, and roughly sinusoidal movement of the middle (hinge) region, resulting in efficient pumping of water (Fig. 11). (2) The SG can pump water either in the forward or the reversed direction. During forward pumping the sequence of recruitment of the groups is D1, D2, L1, L2... During reversed pumping, the muscles remain grouped as in forward pumping but the recruitment sequence is inverted: L2, L1, D2, D1, L2... At the onset of reversal the group D2 forward mode units are suppressed and replaced during the subsequent reversed cycles by a different set of reversed mode units to the D2 muscles (Fig. 14). (3) Peak bursting frequencies in muscles from the same group are roughly synchronous in both beat modes, while peaks for muscles from different groups alternate in such a way as to suggest synchronizing or positive interactions between units in the same group and negative interactions between units from temporally adjacent groups. (4) Spike train analyses suggest direct interactions between units both from the same group and from different groups among the depressor muscles, but these perhaps, do not contribute towards organizing the main features of the output pattern. (5) The durations of the whole levator and depressor sessions, durations of the bursts in the various muscles, and latencies between the starts of bursts in muscles from temporally adjacent groups all show strong, positive, linear correlation with period length. The motor output pattern is essentially phase constant but latencies between the starts of bursts in muscles from the same group are very weakly or not at all correlated with period length, suggesting that within group timing is less directly related than between group timing, to activity in the pacemaking oscillator (Mendelson, 1971) which drives the reciprocating motor activity of the SG. (6) The present findings, along with the other available data on the SG motor system suggest a tentative, hierarchical model for the SG “motor programme”, in which the main features of the motor output pattern, including reversals, are proposed to arise through interactions at premotoneuronal levels (Fig. 16).