Priapulida, sometimes referred to as penis worms, is a phylum of unsegmented 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 resemble the shape of a human penis. They live in the mud and in comparatively shallow waters up to 90 metres (295 ft) deep. Some species show a remarkable tolerance for hydrogen sulfide, anoxia and low salinity. Halicryptus spinulosus appears to prefer brackish shallow waters. They can be quite abundant in some areas. In an Alaskan bay as many as 85 adult individuals of Priapulus caudatus per square meter has been recorded, while the density of its larvae can be as high as 58,000 per square meter.

In Greek mythology, Priapus is a minor rustic fertility god, protector of livestock, fruit plants, gardens, and male genitalia. Priapus is marked by his oversized, permanent erection, which gave rise to the medical term priapism. He became a popular figure in Roman erotic art and Latin literature, and is the subject of the often humorously obscene collection of verse called the Priapeia.