Phascolion Théel

Diagnosis: Introvert 0.5–4 times trunk length, with or without hooks. Trunk usually with modified holdfast papillae. Body wall with continuous muscle layers. Tentacles arranged around mouth. Introvert retractor muscle system modified by fusion of dorsal and ventral pairs; relative size and degree of fusion define the subgenera. Contractile vessel generally without villi. Gut coiling generally loose, without axial spindle muscle. One nephridium (usually right). Small to medium­sized worms (<50 mm long) that commonly inhabit empty mollusc shells.