Sipunculans use their fluid filled bodies as a hydrostatic skeleton. The animals are basically a muscular bag of incompressible fluid and by controlling where fluid in their coelom flows, they can control body movement.
Sipunculids have much more control than this however. They have muscles called longitudinal muscles (muscles that run the length of the body from anterior to posterior)within the body and the body wall just under the skin which they can contract to bring the end of the body up to have the body curl. The longitudinal muscles assist in retraction, body curling, and moving the body from side to side.
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