Priapulida

Priapulida (priapulid worms or penis worms, from Gr. πριάπος, priāpos ‘Priapus’ + Lat. -ul-, diminutive) is a phylum of marine worms. The name of the phylum relates to the Greek god of fertility, because their general shape and their extensible spiny introvert (eversible proboscis) may recall the shape of a penis. They live in the mud (which they eat) and in comparatively shallow waters up to deep. Some species show a remarkable tolerance for hydrogen sulfide and anoxia. Together with Echiura and Sipuncula, they were once placed in the taxon Gephyrea, but consistent morphological and molecular evidence supports their belonging to Ecdysozoa, which also includes arthropods and nematodes. Among Ecdysozoa, their nearest relatives are Kinorhyncha and Loricifera, with which they constitute the Scalidophora clade named after the spines covering the introvert (scalids). They feed on slow-moving invertebrates, such as polychaete worms. Priapulid-like fossils are known at least as far back as the Middle Cambrian. They were likely major predators of the Cambrian period. However, crown-group priapulids cannot be recognized until the Carboniferous. Only 16 extant species of priapulid worms are known.