The dependence of proboscis eversion on the behaviour of the trunk coelom and the effect of increasing the external resistance to eversion have been investigated in Arenicola marina (L.). Two types of proboscis eversion are distinguished; Type I, in which there is an increase of 50–100% in the volume of the head region and where the high pressures recorded in the trunk coelom are needed, it is suggested, to force fluid into the head coelom; Type II, in which the volume change in the head region is small and where simultaneous recordings of head and trunk coelomic pressures indicate that the head coelom can be isolated from the rest of the coelom. Pressures in the trunk are only related to the extent of proboscis eversion when there is a high external resistance to eversion.