Editing Priapulida (section)

== Anatomy ==
Priapulids are cylindrical worm-like animals, ranging from 0.2 to 0.3<ref>{{cite book|url=https://books.google.com/books?id=MKUab3WegL0C&q=Tubiluchus+mm&pg=PA29 |title=Multicellular Animals: Order in Nature – System Made by Man|isbn=978-3-540-00146-1|last1=Ax|first1=Peter|date=2003-04-08|publisher=Springer }}</ref> to 39 centimetres<ref>{{cite journal|jstor=3227009|title= Halicryptus higginsi n.sp. (Priapulida): A Giant New Species from Barrow, Alaska |journal= Invertebrate Biology | volume=118 |issue= 4 | pages=404–413|last1= Shirley |first1= Thomas C. |last2= Storch |first2= Volker |year= 1999 |doi= 10.2307/3227009 |bibcode= 1999InvBi.118..404S }}</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, [[Nephridium#Protonephridia|protonephridia]] and ventral nerve cord are bilateral, while the introvert, pharynx and brain show radial symmetry, and appears to be a secondary trait.<ref>[https://www.diva-portal.org/smash/get/diva2:172290/FULLTEXT01.pdf Aspects of priapulid development]</ref><ref>[https://onlinelibrary.wiley.com/doi/abs/10.1002/1097-4687%28200102%29247%3A2%3C99%3A%3AAID-JMOR1005%3E3.0.CO%3B2-0 Symmetry of priapulids (Priapulida). 1. Symmetry of adults]</ref> Also the larvae show inernal and external characteristics of radial symmetry.<ref>[https://pubmed.ncbi.nlm.nih.gov/11223922/ 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>{{Cite journal |last1=Schmidt-Rhaesa |first1=Andreas |last2=Rothe |first2=Birgen H. |last3=Martínez |first3=Alejandro García |date=November 2013 |title=Tubiluchus lemburgi, a new species of meiobenthic Priapulida |url=https://linkinghub.elsevier.com/retrieve/pii/S004452311300051X |journal=Zoologischer Anzeiger - A Journal of Comparative Zoology |language=en |volume=253 |issue=2 |pages=158–163 |doi=10.1016/j.jcz.2013.08.004|bibcode=2013ZooAn.253..158S }}</ref><ref>{{Cite book |last1=Margulis |first1=Lynn |url=https://books.google.com/books?id=9IWaqAOGyt4C&dq=Priapulus+Meiopriapulus+Halicryptus+Maccabeus+tails&pg=PA286 |title=Kingdoms and Domains: An Illustrated Guide to the Phyla of Life on Earth |last2=Chapman |first2=Michael J. |date=2009-03-19 |publisher=Academic Press |isbn=978-0-08-092014-6 |language=en}}</ref> The body has a [[chitin]]ous cuticle that is moulted as the animal grows.<ref name=IZ>{{cite book | author = Barnes, R. D. | year = 1982 | title = Invertebrate Zoology | publisher = Holt-Saunders International | location = Philadelphia, PA | pages = 873–877 | isbn = 978-0-03-056747-6 }}</ref> Members of the family Chaetostephanidae also secretes a gelatinous tube, open in both ends, which they live in.<ref>{{Cite book |last=Schmidt-Rhaesa |first=Andreas |url=https://books.google.com/books?id=NBP4bvxp1loC&q=Maccabeus%2520gelatinous%2520tube&pg=PA173 |title=Handbook of Zoology |date=2012-12-21 |publisher=Walter de Gruyter |isbn=978-3-11-027253-6 |volume=11 |language=en}}</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 [[Circulatory system|vascular]] or [[Respiratory system|respiratory]] systems, but the body cavity does contain [[phagocytosis|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">{{Cite journal |last1=Wernström |first1=Joel Vikberg |last2=Slater |first2=Ben J. |last3=Sørensen |first3=Martin V. |last4=Crampton |first4=Denise |last5=Altenburger |first5=Andreas |date=2023-08-12 |title=Geometric morphometrics of macro- and meiofaunal priapulid pharyngeal teeth provides a proxy for studying Cambrian "tooth taxa" |journal=Zoomorphology |volume=142 |issue=4 |pages=411–421 |language=en |doi=10.1007/s00435-023-00617-4 |issn=1432-234X|doi-access=free |hdl=10037/30213 |hdl-access=free }}</ref><ref>{{Cite journal |last1=Higgins |first1=Robert P. |last2=Storch |first2=Volker |date=1991 |title=Evidence for Direct Development in Meiopriapulus fijiensis (Priapulida) |url=https://www.jstor.org/stable/3226738 |journal=Transactions of the American Microscopical Society |volume=110 |issue=1 |pages=37–46 |doi=10.2307/3226738 |jstor=3226738 }}</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 [[ganglion|ganglia]] and longitudinal and transversal neurites consistent with an orthogonal organisation.<ref name=Rothe2010>{{Cite journal | last1 = Rothe | first1 = B. H. | last2 = Schmidt-Rhaesa | first2 = A. | doi = 10.1111/j.1744-7410.2010.00185.x | title = Structure of the nervous system in ''Tubiluchus troglodytes'' (Priapulida) | journal = Invertebrate Biology | volume = 129 | pages = 39–58 | date = Winter 2010 | issue = 1 | bibcode = 2010InvBi.129...39R }}</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]] [[organ (anatomy)|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>{{cite book | last = Pechenik | first = J. A. | title = Biology of the Invertebrates | edition = 6th | year = 2009 | publisher = McGraw-Hill | location = New York | isbn = 978-0-07-302826-2 | page = 454 }}</ref> Their male and female organs are closely associated with the excretory [[protonephridium|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 [[flatworm]]s 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>{{EB1911|last=Shipley|first=Arthur Everett|author-link=Arthur Shipley|wstitle=Priapuloidea|volume=22|page=313|inline=1}}</ref> The perigenital area of the genus Tubiluchus exhibit sexual dimorphism.<ref>{{cite journal| title = A new meiobenthic priapulid (Priapulida, Tubiluchidae) from a Mediterranean submarine cave| year = 2003| doi = 10.1080/11250000309356499| last1 = Todaro| first1 = M. Antonio| last2 = Shirley| first2 = Thomas C.| journal = Italian Journal of Zoology| volume = 70| pages = 79–87| s2cid = 84539380| doi-access = free| hdl = 11380/303453| hdl-access = free}}</ref>

=== Reproduction and development ===
For the species ''Priapulus caudatus'', the 80&nbsp;μm egg undergoes a total and radial cleavage following a symmetrical and subequal pattern.<ref name=Wennberg2008>{{Cite journal | last1 = Wennberg | first1 = S. A. | last2 = Janssen | first2 = R. | last3 = Budd | first3 = G. E. | doi = 10.1111/j.1525-142X.2008.00241.x | title = Early embryonic development of the priapulid worm ''Priapulus caudatus'' | journal = Evolution & Development | volume = 10 | issue = 3 | pages = 326–338 | date = May–June 2008 | pmid = 18460094| s2cid = 11175247 | url = http://uu.diva-portal.org/smash/get/diva2:172287/FULLTEXT01 }}</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">{{Cite journal | last1 = Janssen | first1 = R. | last2 = Wennberg | first2 = S. A. | last3 = Budd | first3 = G. E. | title = The hatching larva of the priapulid worm ''Halicryptus spinulosus'' | doi = 10.1186/1742-9994-6-8 | journal = Frontiers in Zoology | volume = 6 | page = 8 | date = 26 May 2009 | pmid = 19470151| pmc = 2693540 | doi-access = free }}</ref> The species ''Meiopriapulus fijiensis'' have direct development.<ref>{{cite journal |title=Evidence for Direct Development in Meiopriapulus fijiensis (Priapulida) |journal=Transactions of the American Microscopical Society |volume=110 |issue=1 |pages=37–46 |date=January 1991 | jstor = 3226738 |last1=Higgins |first1=Robert P. |last2=Storch |first2=Volker |doi=10.2307/3226738 }}</ref> In current systematics, they are described as protostomes, despite having a deuterostomic development.<ref>{{Cite journal | doi = 10.1016/j.cub.2012.09.037|title = Deuterostomic development in the protostome ''Priapulus caudatus'' | journal = Current Biology| volume = 22| issue = 22| pages = 2161–2166 | year = 2012| last1 = Martín-Durán | first1 = J. M. | last2 = Janssen | first2 = R. | last3 = Wennberg | first3 = S. | last4 = Budd | first4 = G. E. | last5 = Hejnol | first5 = A. | pmid=23103190| doi-access = free |bibcode = 2012CBio...22.2161M }}</ref> Because the group is so ancient, it is assumed the deuterostome condition which appears to be ancestral for [[Nephrozoa|bilaterians]] have been maintained.<ref>{{cite web| url = https://www.sciencedaily.com/releases/2012/10/121026084350.htm| title = Penis worms show the evolution of the digestive system}}</ref>