Temporal range: Late Pennsylvanian–Recent (Priapulid-like burrows from Cambrian)
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, in comparatively shallow waters up to 90 metres (300 ft). 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 sixteen extant species of priapulid worms are known.
Priapulids are cylindrical worm-like animals, ranging from 0.2-0.3  to 39 centimetres ( 0.08-0.12 to 15.35 in) in length, 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 body is ringed and often has circles of spines, which are continued into the slightly protrusible pharynx. Some species may also have a tail or a pair of caudal appendages. The body has a chitinous cuticle that is moulted as the animal grows.
There is a wide body-cavity, it has no connection with the renal or reproductive organs, so it is not a coelom; it is probably a blood-space or hemocoel. There are no vascular or respiratory systems, but the body cavity does contain phagocytic amoebocytes and cells containing the respiratory pigment haemerythrin.
The alimentary canal is straight, consisting of an eversible pharynx, an intestine, and a short rectum. The pharynx is muscular and lined by teeth. The anus is terminal, although in Priapulus one or two hollow ventral diverticula of the body-wall stretch out behind it.
The nervous system comprises a nerve ring around the pharynx and a prominent cord running the length of the body with ganglia and longitudinal and transversal neurites consistent with an orthogonal organisation. The nervous system retains a basiepidermal configuration with a connection with the ectoderm, forming part of the body wall. There are no specialized sense organs, but there are sensory nerve endings in the body, especially on the proboscis.
The priapulids are gonochoristic, having two separate sexes (i.e. male and female) Their male and female organs are closely associated with the excretory protonephridia. They comprise a pair of branching tufts, each of which opens to the exterior on one side of the anus. The tips of these tufts enclose a flame-cell like those found in flatworms and other animals, and these probably function as excretory organs. As the animals mature, diverticula arise on the tubes of these organs, which develop either spermatozoa or ova. These sex cells pass out through the ducts.
Reproduction and development 
Priapulid development has only been reappraised recently because early studies reported abnormal developmental caused by high temperature of embryo culture. For the species Priapulus caudatus, the 80 µm egg is undergoing a total and radial cleage following a symmetrical and subequal pattern. Development is remarkably slow with the first cleavage taking place 15hr after fertilization, gastrulation after several days and hatchling of the first 'lorica' larva after 15 to 20 days. In current systematics they are described as protostomes, even if they have a deuterostomic development.
Fossil record 
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. Trace fossils that are morphologically almost identical to the burrows (Treptichnus pedum) officially mark the start of the Cambrian period, suggesting that priapulids, or at least close anatomical relatives, evolved around this time. Crown-group priapulid body fossils are first known from the Cambrian
- "Class" Palaeoscolecida
- Class †Archaeopriapulida
- Class Priapulimorpha
- Class Halicryptomorpha
- Class Seticoronaria
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- Histological studies on Halicryptus spinulosus (Priapulida) with regard to environmental hydrogen sulfide resistance
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- Multicellular Animals: Order in Nature - System Made by Man
- Halicryptus higginsi n.sp. (Priapulida): A Giant New Species from Barrow, Alaska
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- Pechenik, J. A. (2009). Biology of the Invertebrates (6th ed.). New York: McGraw-Hill. p. 454. ISBN 978-0073028262.
- Wennberg, S. A.; Janssen, R.; Budd, G. E. (2008). "Early embryonic development of the priapulid worm Priapulus caudatus". Evolution & Development 10 (3): 326. doi:10.1111/j.1525-142X.2008.00241.x.
- Janssen, R.; Wennberg, S. A.; Budd, G. E. (2009). "The hatching larva of the priapulid worm Halicryptus spinulosus". Frontiers in Zoology 6: 8. doi:10.1186/1742-9994-6-8. PMC 2693540. PMID 19470151.
- Deuterostomic Development in the Protostome Priapulus caudatus
- Vannier, J.; Calandra, I.; Gaillard, C.; Żylińska, A. (2010). "Priapulid worms: Pioneer horizontal burrowers at the Precambrian-Cambrian boundary". Geology 38 (8): 711–714. doi:10.1130/G30829.1.
- This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). Encyclopædia Britannica (11th ed.). Cambridge University Press.
|Wikispecies has information related to: Priapulida|
- "Evolution of the penis worm". Press Releases (University of Bristol). 2006-08-09.
- Play It Again, Life: Natural History, February 1986[dead link]