Phascolosoma Leuckart

Diagnosis: Introvert variable in length, often longer than or equal to trunk, with numerous rings of recurved hooks. Fewer than 30 small tentacles in crescent around nuchal organ (peripheral tentacles lacking). Contractile vessel without true villi. Four introvert retractor muscles, lateral pairs sometimes partially fused. Two unilobed nephridia. Body wall musculature in separate bands.