An account is given of the way in which Priapulus caudatus burrows in the muddy sea-bed in which it lives. Three phases are distinguishable in the muscular activity which is responsible for locomotion. During the first phase the animal is able to feed and defaecate, during the second the proboscis becomes invaginated, and during the third the animal moves forward. The power for locomotion is provided by contraction of the longitudinal and circular muscles of the body wall, not, as has been suggested previously, by the retractor muscles of the praesoma. Invagination of the proboscis is apparently stimulated by the arrival of a wave of contraction in the body wall musculature, propagated from the trunk.
In general the animal burrows in a way common for soft-bodied animals; the anterior and posterior extremities acting in turn as “terminal” and “penetration” anchors in the substratum. The muscular activities of the larva are limited by the presence of a lorica which encases the trunk, and the animal's powers of movement at this stage are very restricted.