The introvert is everted by raising pressure in the fluid-filled body cavity, which forces it outward like a finger of a glove being pushed inside out. Retractor muscles running from the trunk wall to the introvert tip pull it back in. The tentacles at the tip gather sediment and organic particles, so feeding requires repeated cycles of extension and withdrawal.
Anatomy inside is unusual too. The gut is U-shaped and coiled, with the anus opening on the trunk near the front rather than at the rear, an arrangement that suits an animal living head-down in a burrow. The body is unsegmented, which for a long time was taken as evidence that sipunculans were not annelids at all.
That classification has changed. Molecular phylogenetics now places Sipuncula firmly within the annelids, meaning segmentation was lost in the lineage rather than never present. Some support for this comes from developmental work showing transient segmental patterns in sipunculan larvae. Their exact position among annelid groups is still being refined.