[8] 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.

[9] Among Ecdysozoa, their nearest relatives are Kinorhyncha and Loricifera, with which they constitute the Scalidophora clade named after the spines covering the introvert (scalids).

Some analyses suggest that Priapulida may represent a basal lineage within Ecdysozoa, leading to their classification as "living fossils".

[12] Priapulids are cylindrical worm-like animals, ranging from 0.2 to 0.3[13] to 39 centimetres[14] (0.08–0.12 to 15.35 in) long, with a median anterior mouth quite devoid of any armature or tentacles.

The body is divided into a main trunk or abdomen and a somewhat swollen proboscis region ornamented with longitudinal ridges.

The two remaining families Priapulidae and Halicryptidae are larger carnivores that feed on other animals, although some species also consume detritus as larvae.

The shape of the teeth reflect these different lifestyles, and seem to be adapted mainly towards grasping prey or raking detritus from the sediment into the mouth.

[23] For the species Priapulus caudatus, the 80 μm egg undergoes a total and radial cleavage following a symmetrical and subequal pattern.

[28] Stem-group priapulids are known from the Middle Cambrian Burgess Shale, where their soft-part anatomy is preserved, often in conjunction with their gut contents – allowing a reconstruction of their diets.

[1] [citation needed] Priapulida Kinorhyncha Nematoda Nematomorpha Onychophora Tardigrada Arthropoda [citation needed] †Archaeopriapulida [paraphyletic] †Ancalagonida Adrianov & Malakhov 1995 †Markuelia Valkov 1983 †Palaeoscolecida Conway Morris & Robinson 1986 Kinorhyncha Reinhard 1887 (Spiny crown worms; Mud dragons) †Paratubiluchus Han et al. 2004 Meiopriapulus Morse 1981 Tubiluchus van der Land 1968 Maccabeus Por 1973 Halicryptus von Siebold 1849 †Priapulites Schram 1973 †Xiaoheiqingella Hu 2002 Priapulopsis Koren & Danielssen 1875 Acanthopriapulus van der Land 1970 Priapulus Lamarck 1816 There are 22 known extant species:[32][33] Phylum Priapulida Théel 1906 Stem-group †Scalidophora Stem-group †Palaeoscolecida