ABSTRACT The nature and timing of septal movements during gentle crawling have been examined in relation to the coelomic pressure (Pc) in and between adjacent segments. Between neighbouring anterior segments the phase lag was 0·05−0·1 s and the maximum Pc difference 3·3 cm water. The mean Po in a leading segment exceeded that in a trailing segment by 0·9 ± 0·06 cm water. In forward crawling, the septa bulged maximally backward and forward when segments were half elongated and half shortened respectively, and were flat when segments were longest and shortest: the area of a septum in an elongated segment was one quarter that in a shortened one and thus a septum is strongest (small and thick) when the Pc to be longitudinally confined is highest. Bulging of septa is closely related to the intersegmental Pc difference. Bulging forces acting on a septum range from o to about 1 g wt. The pressure drop along a worm is not linear and the ventral foramina may act as ‘septal valves’ to equalize coelomic volume and pressure when required. The influence of gut, septum and body wall movements on each other is discussed.