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Temporal range: Late Pennsylvanian–Recent[1] (Stem-group from Cambrian)
Priapulus caudatus.jpg
Priapulus caudatus
Scientific classification e
Kingdom: Animalia
Subkingdom: Eumetazoa
Clade: ParaHoxozoa
Clade: Bilateria
Clade: Nephrozoa
(unranked): Protostomia
Superphylum: Ecdysozoa
Clade: Scalidophora
Phylum: Priapulida
Théel, 1906[2]

Priapulida (priapulid worms, from Gr. πριάπος, priāpos 'Priapus' + Lat. -ul-, diminutive), 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 (300 ft) deep.[3] Some species show a remarkable tolerance for hydrogen sulfide and anoxia.[4] 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.[5]

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. Fossil findings show that the mouth design of the stem-arthropod Pambdelurion is identical with that of priapulids, indicating that their mouth is an original trait inherited from the last common ancestor of both priapulids and arthropods, even if modern arthropods no longer possess it.[6] Among Ecdysozoa, their nearest relatives are Kinorhyncha and Loricifera, with which they constitute the Scalidophora clade named after the spines covering the introvert (scalids).[7] 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.[1] About 20 extant species of priapulid worms are known, half of them being of meiobenthic size.[8]


Priapulids are cylindrical worm-like animals, ranging from 0.2–0.3[9] to 39 centimetres[10] ( 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 body is ringed and often has circles of spines, which are continued into the slightly protrusible pharynx.[3] 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.[11]

There is a wide body-cavity, which has no connection with the renal or reproductive organs, so it is not a coelom; it is probably a blood-space or hemocoel.[3] There are no vascular or respiratory systems, but the body cavity does contain phagocytic amoebocytes and cells containing the respiratory pigment haemerythrin.[11]

The alimentary canal is straight, consisting of an eversible pharynx, an intestine, and a short rectum. The pharynx is muscular and lined by teeth.[11] The anus is terminal, although in Priapulus one or two hollow ventral diverticula of the body-wall stretch out behind it.[3]

The nervous system consists of 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.[12] 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.[11]

The priapulids are gonochoristic, having two separate sexes (i.e. male and female).[13] 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.[3] The perigenital area of the genus Tubiluchus exhibit sexual dimorphism.[14]

Reproduction and development[edit]

Priapulid development has been reappraised recently because early studies reported abnormal development caused by high temperature of embryo culture. For the species Priapulus caudatus, the 80 µm egg undergoes a total and radial cleavage following a symmetrical and subequal pattern.[15] Development is remarkably slow, with the first cleavage taking place 15 hours after fertilization, gastrulation after several days and hatching of the first 'lorica' larvae after 15 to 20 days.[16] The species Meiopriapulus fijiensis have direct development.[17] In current systematics, they are described as protostomes, despite having a deuterostomic development.[18] Because the group is so ancient, it is assumed the deuterostome condition which appears to be ancestral for bilaterians have been maintained.[19]

Fossil record[edit]

Ottoia tricuspida in the Burgess Shale. Middle Cambrian
Microfossil of a priapulid tooth (Ottoia, Cambrian). From Smith et al. 2015

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.[20] In addition, isolated microfossils (corresponding to the various teeth and spines that line the pharynx and introvert) are widespread in Cambrian deposits, allowing the distribution of priapulids – and even individual species – to be tracked widely through Cambrian oceans.[21] Trace fossils that are morphologically almost identical to modern priapulid burrows (Treptichnus pedum) officially mark the start of the Cambrian period, suggesting that priapulids, or at least close anatomical relatives, evolved around this time.[20] Crown-group priapulid body fossils are first known from the Carboniferous.[1]


External phylogeny[edit]


Priapulida Priapulus caudatus 20150625.jpg

Kinorhyncha Pycnophyes zelinkaei.jpg


Nematoda CelegansGoldsteinLabUNC.jpg

Nematomorpha Paragordius tricuspidatus.jpeg


Onychophora Velvet worm.jpg


Tardigrada Echiniscus L.png

Arthropoda Long nosed weevil edit.jpg

>529 mya

Internal phylogeny[edit]

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 living species:[22]

Illustration of Ottoia, a prehistoric priapulid.

Stem-group Scalidophora

Stem-group Palaeoscolecida

  • Family †Selkirkiidae Conway Morris 1977
    • Genus †Selkirkia Walcott 1911 non Hemsley 1884
  • Order †Ottoiomorpha Adrianov & Malakhov 1995
  • Phylum Priapulida Théel 1906
    • Class Priapulimorpha Salvini-Plawen 1974
      • Order Meiopriapulomorpha Adrianov & Malakhov 1995 (assigned its own order by [23])
        • Family Tubiluchidae van der Land 1970 [Meiopriapulidae Adrianov & Malakhov 1995]
          • Genus Tubiluchus (10 species)
        • Family Meiopriapulidae
          • Genus Meiopriapulus (1 species)
      • Order Priapulimorpha Salvini-Plawen 1974 [Halicryptomorpha Adrianov & Malakhov 1995; Seticoronaria Salvini-Plawen 1974; Eupriapulida Lemburg, 1999]
        • Family Priapulidae Gosse 1855 [Xiaoheiqingidae (sic) Hu 2002]
          • Genus Acanthopriapulus (1 species)
          • Genus Priapulopsis (3 species)
          • Genus Priapulus (3 species)
        • Family Halicryptidae Salvini-Plawen 1974
          • GenusHalicryptus (2 species)
        • Family Maccabeidae Por & Bromley 1974 [Chaetostephanidae Salvini-Plawen 1974]
          • Genus Maccabeus (2 species)


  1. ^ a b c Budd, G. E.; Jensen, S. (May 2000). "A critical reappraisal of the fossil record of the bilaterian phyla". Biological Reviews of the Cambridge Philosophical Society. 75 (2): 253–95. doi:10.1111/j.1469-185X.1999.tb00046.x. PMID 10881389. S2CID 39772232.
  2. ^ Théel, Hjalmar (1905–1906). "Northern and Arctic Invertebrates in the Collection of the Swedish State Museum (Riksmuseum). II. Priapulids, Echiurids etc". Kungl. Svenska Vetenskapsakademiens Handlingar. 40 (4): 8–13.
  3. ^ a b c d e  One or more of the preceding sentences incorporates text from a publication now in the public domainShipley, Arthur Everett (1911). "Priapuloidea". In Chisholm, Hugh (ed.). Encyclopædia Britannica. Vol. 22 (11th ed.). Cambridge University Press. p. 313.
  4. ^ Oeschger, R.; Janssen, H. H. (September 1991). "Histological studies on Halicryptus spinulosus (Priapulida) with regard to environmental hydrogen sulfide resistance". Hydrobiologia. 222: 1–12. doi:10.1007/BF00017494. S2CID 31342308.
  5. ^ Margulis, Lynn; Chapman, Michael J. (19 March 2009). Kingdoms and Domains: An Illustrated Guide to the Phyla of Life on Earth. ISBN 9780080920146.
  6. ^ "Ancestor of arthropods had the mouth of a penis worm".
  7. ^ Dunn, C. W.; Hejnol, A.; Matus, D. Q.; Pang, K.; Browne, W. E.; Smith, S. A.; Seaver, E.; Rouse, G. W.; Obst, M. (10 April 2008). "Broad Phylogenomic Sampling Improves Resolution of the Animal Tree of Life". Nature. 452 (7188): 745–749. Bibcode:2008Natur.452..745D. doi:10.1038/nature06614. PMID 18322464. S2CID 4397099.
  8. ^ Giere, Olav (November 2008). Meiobenthology: The Microscopic Motile Fauna of Aquatic Sediments. ISBN 9783540686613.
  9. ^ Ax, Peter (2003-04-08). Multicellular Animals: Order in Nature – System Made by Man. ISBN 978-3-540-00146-1.
  10. ^ Shirley, Thomas C.; Storch, Volker (1999). "Halicryptus higginsi n.sp. (Priapulida): A Giant New Species from Barrow, Alaska". Invertebrate Biology. 118 (4): 404–413. doi:10.2307/3227009. JSTOR 3227009.
  11. ^ a b c d Barnes, R. D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. pp. 873–877. ISBN 978-0-03-056747-6.
  12. ^ Rothe, B. H.; Schmidt-Rhaesa, A. (Winter 2010). "Structure of the nervous system in Tubiluchus troglodytes (Priapulida)". Invertebrate Biology. 129: 39–58. doi:10.1111/j.1744-7410.2010.00185.x.
  13. ^ Pechenik, J. A. (2009). Biology of the Invertebrates (6th ed.). New York: McGraw-Hill. p. 454. ISBN 978-0-07-302826-2.
  14. ^ Todaro, M. Antonio; Shirley, Thomas C. (2003). "A new meiobenthic priapulid (Priapulida, Tubiluchidae) from a Mediterranean submarine cave". Italian Journal of Zoology. 70: 79–87. doi:10.1080/11250000309356499. S2CID 84539380.
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  16. ^ Janssen, R.; Wennberg, S. A.; Budd, G. E. (26 May 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.
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  19. ^ "Penis worms show the evolution of the digestive system".
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  21. ^ Smith, M. R.; Harvey, T. H. P.; Butterfield, N. J. (July 2015). "The macro- and microfossil record of the Cambrian priapulid Ottoia" (PDF). Palaeontology. 58 (4): 705–721. doi:10.1111/pala.12168.
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External links[edit]