Burrowing of Priapulus caudatus (Vermes) and the significance of the direct peristaltic wave

Priapulus caudatus is an active predatory animal found burrowing in soft mud. It may employ relatively high body fluid pressures during escape reactions (up to 6 kPa recorded) but normal burrowing probably involves much lower pressures (< 2 kPa). Three points d'appui are formed during the burrowing cycle. A characteristic direct peristaltic wave (DPW) occurs at a low pressure stage in each cycle (<0–5 kPa). The DPW is regarded as an adaptation to burrowing in soft mud. Its function appears to be to transfer body wall anteriorily, providing “slack”, which is used to allow the trunk to extend forward into the cavity vacated by retraction of the large praesoma at the end of each cycle. The circular and longitudinal muscles of the posterior trunk act synergically through most of the burrowing cycle.