The Priapulida are a small group of marine worm-like animals, whose affinities remain unclear.
The worm-like animals are unsegmented and covered with a chitinous cuticle. A small surface bears spines and papillae. The body cavity is large and displays a near-radial symmetry, except for the paired gonads and nephridia and a single nerve cord, which is regarded as lying ventral. The anterior body forms an introvert, in which the mouth is central. The introvert is alternately pushed out and withdrawn to burrow. Prey is caught by curved teeth on the everted mouth and pharynx lining and swallowed during withdrawal of the introvert. The gut is simple and ends in the anus. In case a tail is present, it arises ventrally to the anus. The tail bears many vesicular branches, which contains extensions of the coelom and are thought to be respiratory. Sexes are separate. Reproduction through loricate larvae, which stay in the adult environment.