The most recognizable part of Sipunculan worms is their mouth, which is surrounded by a mass of 18–24 ciliated tentacles, all of which may be inverted into the body. The tentacles are used to gather organic detritus from the water or substrate, and are probably also function as gills. The mouth and tentacles lie at the tip of the introvert, which is otherwise decorated with numerous spines or nodules. In contrast, the trunk tends to be relatively smooth.