The Sipuncula, also known as peanut or star worms, include roughly 150 exclusively marine and benthic species. Sipunculans generally lead a cryptic lifestyle. They burrow into sediment or soft rocks, hide in cracks and crevices or inhabit abandoned shells, tests and tubes of molluscs, foraminiferans and polychaetes. They are known from all oceans and span depths from the intertidal to the deep sea. They show the highest diversity is in tropical shallow waters.

The adult body consists of two major regions: an unsegmented trunk region and an introvert with an anterior crown of tentacles. The introvert can be completely retracted into the trunk by a set of 1-4 retractor muscles which originate in the trunk wall and insert at the anterior introvert. The introvert is often adorned with recurved hooks which may be scattered or organized in rings. The presence or absence of hooks and their microscopic structure are important taxonomic characters. The epidermis may be smooth or covered in more or less prominent papillae. The circular and longitudinal body wall musculature may be split up into in bands or form smooth sheaths. If split up into bands, these are often visible through the body wall.

The mouth lies at the anterior end of the introvert. The esophagus extends through the introvert and transitions into an intestinal coil that dominates the internal anatomy of the trunk. The intestine descends towards the posterior end and ascends again to the anus at the dorsal anterior area of the trunk. The descending and ascending branches coil around each other in a double helix. Other conspicuous internal structures are the metanephridia (paired except in Phascolion) and the ventral nerve cord.

Sipucula are split into two major classes, based on the arrangement of the tentacles and the nuchal organ (probably a chemosensory organ). In the Sipunculidea, the tentacles are arranged in a circle around the mouth while the nuchal organ lies outside of the circle. In the Phascolosomatidea, the tentacles surround the nuchal organ in a horseshoe shape with the mouth in a separate position.