Tentacles retractile into an introvert ("tentacle-sheath").
(Lankester.) A, Simple introvert completely introverted.
- Zooecia with soft uncalci fled' walls, the external part of the introvert being closed during retraction by a membranous collar.
At the tip of the introvert the mouth opens, and is surrounded in Sipunculus by a funnel-shaped, ciliated lophophore (figs.
D, E, A similar simple introvert in course of eversion by the forward movement, not of its sides, but of its apex, as in the proboscidean Rhabdocoels =acrecbolic.