Template:Short description Template:Automatic taxobox
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, except for a few tropical meiobenthic species which live in medium- to coarse-grained sands, and in comparatively shallow waters up to Template:Convert deep and no warmer than 12–13°C.<ref name=EB1911/><ref>Priapulid neoichnology, ecosystem engineering, and the Ediacaran–Cambrian transition</ref> Some species show a remarkable tolerance for hydrogen sulfide, anoxia and low salinity.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> Halicryptus spinulosus appears to prefer brackish shallow waters.<ref>The phylogeny, classification and zoogeography of the class Priapulida. II. Revision of the family Priapulidae and zoogeography of priapulids</ref> 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 (5,390 per square foot).<ref>Template:Cite book</ref>
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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Among Ecdysozoa, their nearest relatives are Kinorhyncha and Loricifera, with which they constitute the Scalidophora clade named after the spines covering the introvert (scalids).<ref>Template:Cite journal</ref> They feed on slow-moving invertebrates, such as polychaete worms.
Some analyses suggest that Priapulida may represent a basal lineage within Ecdysozoa, leading to their classification as "living fossils".<ref>Template:Cite journal</ref> 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.<ref name=Budd2000>Template:Cite journal</ref> 22 extant species of priapulid worms are known, half of them being of meiobenthic size.<ref>Template:Cite book</ref>
AnatomyEdit
Priapulids are cylindrical worm-like animals, ranging from 0.2 to 0.3<ref>Template:Cite book</ref> to 39 centimetres<ref>Template:Cite journal</ref> (0.08–0.12 to 15.35 in) long, with a median anterior mouth quite devoid of any armature or tentacles. They show both radial and bilateral symmetry. The gonads, protonephridia and ventral nerve cord are bilateral, while the introvert, pharynx and brain show radial symmetry, and appears to be a secondary trait.<ref>Aspects of priapulid development</ref><ref>Symmetry of priapulids (Priapulida). 1. Symmetry of adults</ref> Also the larvae show inernal and external characteristics of radial symmetry.<ref>Symmetry of priapulids (Priapulida). 2. Symmetry of larvae</ref> The adult body is divided into a main trunk or abdomen and a somewhat swollen proboscis region ornamented with longitudinal ridges. In addition it is ringed and often has circles of spines, which are continued into the slightly protrusible pharynx.<ref name=EB1911/> Family Priapulidae have species with a tail or a pair of caudal appendages. A slender tail or tail filament is also found in family Tubiluchidae. Appendages are absent in the remaining families.<ref>Template:Cite journal</ref><ref>Template:Cite book</ref> The body has a chitinous cuticle that is moulted as the animal grows.<ref name=IZ>Template:Cite book</ref> Members of the family Chaetostephanidae also secretes a gelatinous tube, open in both ends, which they live in.<ref>Template:Cite book</ref>
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.<ref name=EB1911/> There are no vascular or respiratory systems, but the body cavity does contain phagocytic amoebocytes and cells containing the respiratory pigment haemerythrin.<ref name=IZ />
The alimentary canal is straight, consisting of an eversible pharynx, an intestine, and a short rectum. The pharynx is muscular and lined by teeth.<ref name=IZ /> Three of the five extant families have gone through a significant miniaturization and become detritivores (Tubiluchidae and Meiopriapulidae) and filter feeders (Chaetostephanidae). 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.<ref name="Wernström">Template:Cite journal</ref><ref>Template:Cite journal</ref> The anus is terminal, although in Priapulus one or two hollow ventral diverticula of the body-wall stretch out behind it.<ref name=EB1911/>
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.<ref name=Rothe2010>Template:Cite journal</ref> 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.<ref name=IZ />
The priapulids are gonochoristic, having two separate sexes (i.e. male and female).<ref name=pech>Template:Cite book</ref> 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.<ref name=EB1911>{{#if: |
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}}{{#ifeq: ||}}</ref> The perigenital area of the genus Tubiluchus exhibit sexual dimorphism.<ref>Template:Cite journal</ref>
Reproduction and developmentEdit
For the species Priapulus caudatus, the 80 μm egg undergoes a total and radial cleavage following a symmetrical and subequal pattern.<ref name=Wennberg2008>Template:Cite journal</ref> 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.<ref name="Janssen-2009">Template:Cite journal</ref> The species Meiopriapulus fijiensis have direct development.<ref>Template:Cite journal</ref> In current systematics, they are described as protostomes, despite having a deuterostomic development.<ref>Template:Cite journal</ref> Because the group is so ancient, it is assumed the deuterostome condition which appears to be ancestral for bilaterians have been maintained.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Fossil recordEdit
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.<ref name=Vannier2010>Template:Cite journal</ref> In addition, isolated microfossils (corresponding to the various teeth and spines that line the pharynx and introvert) are widespread in Cambrian deposits,<ref>Template:Cite journal</ref> allowing the distribution of priapulids – and even individual species – to be tracked widely through Cambrian oceans.<ref>Template:Cite journal</ref><ref name="Wernström"/> 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.<ref name=Vannier2010 /> Crown-group priapulid body fossils are first known from the Carboniferous.<ref name=Budd2000 />
PhylogenyEdit
External phylogenyEdit
Template:Citation needed Template:Clade
Internal phylogenyEdit
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ClassificationEdit
There are 22 known extant species:<ref>Template:Cite book</ref><ref>2019 Annual Checklist : Browse taxonomic classification phylum: Cephalorhyncha, class: Priapulida</ref>
Phylum Priapulida Théel 1906
- Order Halicryptomorpha Salvini-Plawen 1974 [Adrianov & Malakhov 1995; Salvini-Plawen 1974; Eupriapulida Lemburg, 1999]
- Family Halicryptidae Salvini-Plawen 1974
- Genus Halicryptus
- Species H. higginsi (Shirley & Storch, 1999)
- Species H. spinulosus (von Siebold, 1849)
- Genus Halicryptus
- Family Halicryptidae Salvini-Plawen 1974
- Order Priapulomorpha Adrianov & Malakhov 1995 (assigned its own order by <ref>Adrianov A. V, Malakhov V. V. 2001. Symmetry of priapulids (Priapulida). 1. Symmetry of adults. 247:99–110.</ref>)
- Family Priapulidae Gosse 1855 [Xiaoheiqingidae (sic) Hu 2002]
- Genus Acanthopriapulus
- Species A. horridus (Théel, 1911)
- Genus Priapulopsis
- Species P. australis (de Guerne, 1886)
- Species P. bicaudatus (Danielssen, 1869)
- Species P. cnidephorus (Salvini-Plawen, 1973)
- Genus Priapulus
- Species P. abyssorum (Menzies, 1959)
- Species P. caudatus (Lamarck, 1816)
- Species P. tuberculatospinosus (Baird, 1868)
- Genus Acanthopriapulus
- Family Tubiluchidae van der Land 1970 [Meiopriapulidae Adrianov & Malakhov 1995]
- Genus Tubiluchus
- Species T. arcticus (Adrianov, Malakhov, Tchesunov & Tzetlin, 1989)
- Species T. australensis (van der Land, 1985)
- Species T. corallicola (van der Land, 1968)
- Species T. lemburgi (Schmidt-Rhaesa, Rothe & Martínez, 2013)
- Species T. pardosi (Schmidt-Rhaesa, Panpeng & Yamasaki, 2017)
- Species T. philippinensis (van der Land, 1985)
- Species T. remanei (van der Land, 1982)
- Species T. soyoae (Schmidt-Rhaesa, Panpeng & Yamasaki, 2017)
- Species T. troglodytes (Todaro & Shirley, 2003)
- Species T. vanuatensis (Adrianov & Malakhov, 1991)
- Genus Tubiluchus
- Genus Meiopriapulus
- Species M. fijiensis
- Family Priapulidae Gosse 1855 [Xiaoheiqingidae (sic) Hu 2002]
(Morse, 1981)
- Order Seticoronaria
- Family Chaetostephanidae Por & Bromley 1974 [Chaetostephanidae Salvini-Plawen 1974]
- Genus Maccabeus
- Species M. cirratus (Malakhov, 1979)
- Species M. tentaculatus (Por, 1973)
- Genus Maccabeus
- Family Chaetostephanidae Por & Bromley 1974 [Chaetostephanidae Salvini-Plawen 1974]
Extinct groupsEdit
Stem-group †Scalidophora
- Order †Ancalagonida Adrianov & Malakhov 1995 [Fieldiida Adrianov & Malakhov 1995]
- Family †Ancalagonidae Conway Morris 1977
- Genus †Ancalagon Conway Morris 1977
- Family †Fieldiidae Conway Morris 1977
- Genus †Fieldia Walcott 1912
- Family †Ancalagonidae Conway Morris 1977
Stem-group †Palaeoscolecida
- Family †Selkirkiidae Conway Morris 1977
- Genus †Selkirkia Walcott 1911 non Hemsley 1884
- Order †Ottoiomorpha Adrianov & Malakhov 1995
- Genus †Scolecofurca Conway Morris 1977
- Family †Ottoiidae Walcott 1911
- Genus †Ottoia Walcott 1911
- Family †Corynetidae Huang, Vannier & Chen 2004
- Genus †Corynetis Luo & Hu 1999 [Anningvermis Huang, Vannier & Chen 2004]
- Family †Miskoiidae Walcott 1911
ReferencesEdit
External linksEdit
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