Editing Polychaete

{{short description|Class of annelid worms}}
{{Redirect|Polychaeta|the genus of flies|Polychaeta (fly)}}
{{Paraphyletic group
| fossil_range = {{fossilrange|Cambrian Stage 3|0|[[Cambrian]] <small>(or earlier?)</small> – present}}
| name = Polychaetes
| image = Libr0409.jpg
| image_caption = "A variety of marine worms": plate from ''Das Meer'' by [[Matthias Jakob Schleiden|M. J. Schleiden]] (1804–1881)
| image_upright = 1.2
| auto = yes
| taxon = Polychaeta
| authority = Grube, 1850
| includes = *[[Oweniidae]]
*[[Magelonidae]]
*[[Chaetopteridae]]<ref name="Struck2011">{{Cite journal | last1 = Struck | first1 = T. H. | last2 = Paul | first2 = C. | last3 = Hill | first3 = N. | last4 = Hartmann | first4 = S. | last5 = Hösel | first5 = C. | last6 = Kube | first6 = M. | last7 = Lieb | first7 = B. | last8 = Meyer | first8 = A. | last9 = Tiedemann | first9 = R. | last10 = Purschke | doi = 10.1038/nature09864 | first10 = G. N. | last11 = Bleidorn | first11 = C. | title = Phylogenomic analyses unravel annelid evolution | journal = Nature | volume = 471 | issue = 7336 | pages = 95–98 | year = 2011 | pmid =  21368831| bibcode = 2011Natur.471...95S | s2cid = 4428998 }}</ref>
*[[Amphinomida]]
*[[Errantia]]
*[[Sedentaria]]
| excludes = *[[Sipuncula]]
*[[Myzostomida]]
*[[Clitellata]]
}}

'''Polychaeta''' ({{IPAc-en|ˌ|p|ɒ|l|ɪ|ˈ|k|iː|t|ə}}) is a [[paraphyletic]]<ref name="Struck2011" /> class of generally marine [[Annelid|annelid worms]], [[common name|commonly called]] '''bristle worms''' or '''polychaetes''' ({{IPAc-en|ˈ|p|ɒ|l|ɪ|ˌ|k|iː|t|s}}). Each body segment has a pair of fleshy protrusions called [[parapodia]] that bear many bristles, called [[chaeta]]e, which are made of [[chitin]]. More than 10,000 species are described in this class.  Common representatives include the [[lugworm]] (''Arenicola marina'') and the [[Alitta virens|sandworm]] or [[Alitta succinea|clam worm]] ''Alitta''.

Polychaetes as a class are robust and widespread, with species that live in the coldest ocean temperatures of the [[abyssal plain]], to forms which tolerate the extremely high temperatures near [[hydrothermal vent]]s. Polychaetes occur throughout the Earth's oceans at all depths, from forms that live as [[plankton]] near the surface, to a 2- to 3-cm specimen (still unclassified) observed by the robot ocean probe [[Nereus (underwater vehicle)|''Nereus'']] at the bottom of the [[Challenger Deep]], the deepest known spot in the Earth's oceans.<ref>[http://ns.gov.gu/geography.html Geography of Guam ] {{Webarchive|url=https://web.archive.org/web/19961027163532/http://ns.gov.gu/geography.html |date=1996-10-27 }} ns.gov.gu Accessed Oct. 8, 2009</ref> Only 168 species (less than 2% of all polychaetes) are known from fresh waters.<ref>{{cite journal |first1=Cristopher |last1=Glasby |first2=Tarmo |last2=Timm |year=2008 |title=Global diversity of polychaetes (Polychaeta: Annelida) in freshwater |journal=[[Hydrobiologia]] |volume=595 |issue=1: Freshwater Animal Diversity Assessment |pages=107–115 |doi=10.1007/s10750-007-9008-2 |citeseerx=10.1.1.655.4467 |s2cid=13143924 |editor1=E. V. Balian |editor2=C. Lévêque |editor3=H. Segers |editor4=K. Martens }}</ref>

==Description==
Polychaetes are segmented worms, generally less than {{convert|10|cm|in|0|abbr=on}} in length, although ranging at the extremes from {{convert|1|mm|in|2|abbr=on}} to {{convert|3|m|ft|-1|abbr=on}}, in ''[[Eunice aphroditois]]''. They can sometimes be brightly coloured, and may be [[iridescent]] or even [[luminescent]]. Each segment bears a pair of paddle-like and highly vascularized [[parapodia]], which are used for movement and, in many species, act as the worm's primary [[gas exchange|respiratory]] surfaces. Bundles of bristles, called [[chaeta]]e, project from the parapodia.<ref name=IZ>{{cite book |author= Barnes, Robert D. |year=1982 |title= Invertebrate Zoology |publisher= Holt-Saunders International |location= Philadelphia, PA|pages= 469–525|isbn= 978-0-03-056747-6}}</ref>

However, polychaetes vary widely from this generalized pattern, and can display a range of different body forms. The most generalised polychaetes are those that crawl along the bottom, but others have adapted to many different [[ecological niche]]s, including burrowing, swimming, [[pelagic]] life, tube-dwelling or boring, [[commensal]]ism, and [[parasite|parasitism]], requiring various modifications to their body structures.

The head, or [[prostomium]], is relatively well developed, compared with other annelids. It projects forward over the mouth, which therefore lies on the animal's underside. The head normally includes two to four pair of eyes, although some species are blind. These are typically fairly simple structures, capable of distinguishing only light and dark, although some species have large eyes with lenses that may be capable of more sophisticated vision,<ref name=IZ/> including the Alciopids' complex eyes which rival cephalopod and vertebrate eyes.<ref>[https://www.cell.com/current-biology/fulltext/S0960-9822(24)00237-9 High-resolution vision in pelagic polychaetes]</ref><ref>{{Cite web|url=https://www.smithsonianmag.com/science-nature/14-fun-facts-about-marine-bristle-worms-180955773/|title=14 Fun Facts About Marine Bristle Worms}}</ref>

Many species show [[bioluminescence]]; eight families have luminous species.<ref>{{Cite journal |last1=Kanie |first1=Shusei |last2=Miura |first2=Daisuke |last3=Jimi |first3=Naoto |last4=Hayashi |first4=Taro |last5=Nakamura |first5=Koji |last6=Sakata |first6=Masahiko |last7=Ogoh |first7=Katsunori |last8=Ohmiya |first8=Yoshihiro |last9=Mitani |first9=Yasuo |date=2021-09-27 |title=Violet bioluminescent Polycirrus sp. (Annelida: Terebelliformia) discovered in the shallow coastal waters of the Noto Peninsula in Japan |journal=Scientific Reports |language=en |volume=11 |issue=1 |pages=19097 |doi=10.1038/s41598-021-98105-6 |pmid=34580316 |pmc=8476577 |bibcode=2021NatSR..1119097K |issn=2045-2322}}</ref><ref>{{Cite journal |last1=Zörner |first1=S. A. |last2=Fischer |first2=A. |date=22 Dec 2006 |title=The spatial pattern of bioluminescent flashes in the polychaete Eusyllis blomstrandi (Annelida) |journal=Helgoland Marine Research |language=en |volume=61 |issue=1 |pages=55–66 |doi=10.1007/s10152-006-0053-4 |s2cid=2473677 |issn=1438-3888|doi-access=free }}</ref>

The head also includes a pair of [[antenna (biology)|antennae]], tentacle-like [[palp]]s, and a pair of pits lined with [[cilia]], known as "nuchal organs". These latter appear to be [[chemoreceptor]]s, and help the worm to seek out food.<ref name=IZ/>

===Internal anatomy and physiology===
[[File:Polychaeta anatomy en.svg|thumb|400px|{{center|General anatomy of a polychaete}}]]
[[File:Phyllodoce rosea.jpg|thumb| {{center|''Phyllodoce rosea''}}]]

The outer surface of the body wall consists of a simple [[columnar epithelium]] covered by a thin [[cuticle]]. Underneath this, in order, are a thin layer of connective tissue, a layer of circular muscle, a layer of longitudinal muscle, and a [[peritoneum]] surrounding the [[coelom|body cavity]]. Additional oblique muscles move the parapodia. In most species the body cavity is divided into separate compartments by sheets of peritoneum between each segment, but in some species it is more continuous.

The mouth of polychaetes is located on the [[peristomium]], the segment behind the [[prostomium]], and varies in form depending on their diets, since the group includes predators, herbivores, filter feeders, scavengers, and parasites. In general, however, they possess a pair of jaws and a [[pharynx]] that can be rapidly everted, allowing the worms to grab food and pull it into their mouths. In some species, the pharynx is modified into a lengthy [[proboscis]]. The digestive tract is a simple tube, usually with a stomach part way along.

The smallest species, and those adapted to burrowing, lack [[gill]]s, breathing only through their body surfaces. Most other species have external gills, often associated with the parapodia.

A simple but well-developed circulatory system is usually present. The two main blood vessels furnish smaller vessels to supply the parapodia and the gut. Blood flows forward in the dorsal vessel, above the gut, and returns down the body in the ventral vessel, beneath the gut. The blood vessels themselves are contractile, helping to push the blood along, so most species have no need of a heart. In a few cases, however, muscular pumps analogous to a heart are found in various parts of the system. Conversely, some species have little or no circulatory system at all, transporting oxygen in the [[Coelom#Coelomic fluid|coelomic fluid]] that fills their body cavities.<ref name=IZ/>

The blood may be colourless, or have any of three different respiratory pigments. The most common of these is [[haemoglobin]], but some groups have [[haemerythrin]] or the green-coloured [[chlorocruorin]], instead.

The nervous system consists of a single or double ventral nerve cord running the length of the body, with [[ganglion|ganglia]] and a series of small nerves in each segment. The brain is relatively large, compared with that of other annelids, and lies in the upper part of the head. An [[endocrine gland]] is attached to the ventral posterior surface of the brain, and appears to be involved in reproductive activity. In addition to the sensory organs on the head, photosensitive eye spots, [[statocyst]]s, and numerous additional sensory nerve endings, most likely involved with the sense of touch, also occur on the body.<ref name=IZ/>

Polychaetes have a varying number of [[protonephridia]] or [[metanephridia]] for excreting waste, which in some cases can be relatively complex in structure. The body also contains greenish "[[chloragogen]]" tissue, similar to that found in [[oligochaete]]s, which appears to function in metabolism, in a similar fashion to that of the vertebrate [[liver]].<ref name=IZ/>

The cuticle is constructed from cross-linked fibres of [[collagen]] and may be 200&nbsp;nm to 13&nbsp;mm thick. Their jaws are formed from [[sclerite|sclerotised]] collagen, and their [[setae]] from sclerotised [[chitin]].<ref name=Briggs1993>{{cite journal |last1=Briggs |first1=Derek E. G. |last2=Kear |first2=Amanda J. |title=Decay and preservation of polychaetes: taphonomic thresholds in soft-bodied organisms |journal=Paleobiology |date=8 February 2016 |volume=19 |issue=1 |pages=107–135 |doi=10.1017/S0094837300012343 |jstor=2400774 |bibcode=1993Pbio...19..107B |s2cid=84073818 }}</ref>

==Ecology==
[[File:Alvinella pompejana01.jpg|thumb| The [[Pompeii worm]]  lives at great depths by hydrothermal vents at temperatures up to 80 °C]]
[[File:Hesiocaeca methanicola noaa.jpg|thumb| ''[[Hesiocaeca methanicola]]'' lives at great depths on [[methane ice]]]]
[[File:Lamellibrachia luymesi1.png|thumb| The cold seep tube worm  ''[[Lamellibrachia]]'' can live over 250 years]]
[[File:Eunice aphroditois.jpg|thumb| The predatory [[Bobbit worm]]]]

Polychaetes are predominantly marine, but many species also live in freshwater, and a few in terrestrial environments.<ref>[https://www.mdpi.com/1424-2818/13/2/98 Annelids in Extreme Aquatic Environments: Diversity, Adaptations and Evolution]</ref> They are extremely variable in both form and lifestyle, and include a few taxa that swim among the [[plankton]] or above the [[abyssal plain]]. Most burrow or build tubes in the sediment, and some live as [[commensal]]s.  A few species, roughly 80 (less than 0.5% of species), are parasitic.<ref name=":0">{{Cite journal |last1=Martin |first1=Daniel |last2=Nygren |first2=Arne |last3=Cruz-Rivera |first3=Edwin |date=2017-06-01 |title=Proceraea exoryxae sp. nov. (Annelida, Syllidae, Autolytinae), the first known polychaete miner tunneling into the tunic of an ascidian |journal=PeerJ |language=en |volume=5 |pages=e3374 |doi=10.7717/peerj.3374 |pmid=28584710 |pmc=5457667 |issn=2167-8359 |doi-access=free }}</ref><ref name=":1">{{Cite book |date=1998 |chapter=SYMBIOTIC POLYCHAETES: REVIEW OF KNOWN SPECIES |chapter-url=https://www.taylorfrancis.com/chapters/edit/10.1201/b12646-22/symbiotic-polychaetes-review-known-species-martin-britayev |title=Oceanography and Marine Biology: An Annual Review |language=en |pages=225–254 |doi=10.1201/b12646-22|doi-broken-date=1 November 2024 | hdl=10261/39328| hdl-access=free|last1=Martin |first1=Daniel |last2=Britayev |first2=Temir A. |publisher=CRC Press |isbn=9780429210600 }}</ref> These include both [[ectoparasites]] and [[endoparasites]]. Ectoparasitic polychaetes feed on skin, blood, and other secretions, and some are adapted to bore through hard, usually calcerous surfaces, such as the shells of mollusks.<ref name=":1" /> These "boring" polychaetes may be parasitic, but may be opportunistic or even obligate symbionts (commensals).<ref>{{Cite journal |last1=Abe |first1=Hirokazu |last2=Hoshino |first2=Osamu |last3=Yamada |first3=Kazuyuki |last4=Ogino |first4=Tetsuya |last5=Kawaida |first5=Shun |last6=Sato-Okoshi |first6=Waka |date=2022-06-28 |title=A novel symbiotic relationship between ascidians and a new tunic-boring polychaete (Annelida: Spionidae: Polydora) |url=https://mapress.com/zt/article/view/zootaxa.5159.1.1 |journal=Zootaxa |language=en |volume=5159 |issue=1 |pages=1–22 |doi=10.11646/zootaxa.5159.1.1 |pmid=36095560 |issn=1175-5334|doi-access=free |url-access=subscription }}</ref><ref name=":1" /><ref name=":0" />

The mobile forms ([[Errantia]]) tend to have well-developed sense organs and jaws, while the stationary forms ([[Sedentaria]]) lack them, but may have specialized gills or tentacles used for respiration and deposit or filter feeding, e.g., [[fanworm]]s.
Underwater polychaetes have eversible mouthparts used to capture prey.<ref>{{cite web |url=http://www.mesa.edu.au/friends/seashores/e_complanata.html |title=Bristleworm |work=MESA}}{{self-published source|date=July 2019}}</ref>{{self-published inline|date=July 2019}} A few groups have evolved to live in terrestrial environments, like [[Namanereidinae]] with many terrestrial species, but are restricted to humid areas. Some have even evolved cutaneous invaginations for aerial gas exchange.

== Notable polychaetes ==
*One notable polychaete, the [[Pompeii worm]] (''Alvinella pompejana''), is endemic to the [[hydrothermal vent]]s of the Pacific Ocean. Pompeii worms are among the most heat-tolerant complex animals known.
*A recently discovered genus, ''[[Osedax]]'', includes a species nicknamed the "[[bone-eating snot flower]]".<ref name="BBCNews_Science&Environment_18Oct2005_Osedaxspp">{{cite news|url=http://news.bbc.co.uk/2/hi/4354286.stm|title='Zombie worms' found off Sweden |date=18 October 2005|work=[[BBC News]]|access-date=12 February 2010}}</ref>
*Another remarkable polychaete is ''[[Hesiocaeca methanicola]]'', which lives on [[methane clathrate]] deposits.
*''[[Lamellibrachia luymesi]]'' is a [[cold seep]] [[tube worm (body plan)|tube worm]] that reaches lengths of over 3 m and may be the most long-lived annelid, being over 250 years old. 
*A still unclassified multilegged predatory polychaete worm was identified only by observation from the underwater vehicle [[Nereus (underwater vehicle)|''Nereus'']] at the bottom of the [[Challenger Deep]], the greatest depth in the oceans, near {{cvt|10,902|m|ft}} in depth. It was about an inch long visually, but the probe failed to capture it, so it could not be studied in detail.<ref>[http://ns.gov.gu/geography.html Accessed Oct. 8, 2009] {{Webarchive|url=https://web.archive.org/web/19961027163532/http://ns.gov.gu/geography.html |date=1996-10-27 }} Geography of the ocean floor near Guam with some notes on exploration of the Challenger Deep.</ref>
*The Bobbit worm (''[[Eunice aphroditois]]'') is a predatory species that can achieve a length of {{convert|3|m|ft|0|abbr=on}}), with an average diameter of {{convert|25|mm|in|0|abbr=on}}.
*''Dimorphilus gyrociliatus'' has the smallest known genome of any annelid. The species shows extreme [[sexual dimorphism]]. Females measure ~1 mm long and have simplified bodies containing six segments, a reduced coelom, and no appendages, parapodia, or chaetae. The males are only 50 μm long and consist of just a few hundred cells. They lack a digestive system and have just 68 neurons, and only live for roughly a week.<ref>[https://ecoevocommunity.nature.com/posts/a-tiny-worm-sheds-light-into-genome-compaction A tiny worm sheds light into genome compaction]</ref><ref>{{cite journal | doi=10.1038/s41559-020-01327-6 | title=Conservative route to genome compaction in a miniature annelid | year=2020 | last1=Martín-Durán | first1=José M. | last2=Vellutini | first2=Bruno C. | last3=Marlétaz | first3=Ferdinand | last4=Cetrangolo | first4=Viviana | last5=Cvetesic | first5=Nevena | last6=Thiel | first6=Daniel | last7=Henriet | first7=Simon | last8=Grau-Bové | first8=Xavier | last9=Carrillo-Baltodano | first9=Allan M. | last10=Gu | first10=Wenjia | last11=Kerbl | first11=Alexandra | last12=Marquez | first12=Yamile | last13=Bekkouche | first13=Nicolas | last14=Chourrout | first14=Daniel | last15=Gómez-Skarmeta | first15=Jose Luis | last16=Irimia | first16=Manuel | last17=Lenhard | first17=Boris | last18=Worsaae | first18=Katrine | last19=Hejnol | first19=Andreas | journal=Nature Ecology & Evolution | volume=5 | issue=2 | pages=231–242 | pmid=33199869 | pmc=7854359 | bibcode=2020NatEE...5..231M }}</ref>

==Reproduction==
Most polychaetes have separate sexes, rather than being hermaphroditic. The most primitive species have a pair of [[gonad]]s in every segment, but most species exhibit some degree of specialisation. The gonads shed immature [[gamete]]s directly into the body cavity, where they complete their development. Once mature, the gametes are shed into the surrounding water through ducts or openings that vary between species, or in some cases by the complete rupture of the body wall (and subsequent death of the adult). A few species [[animal sexual behavior|copulate]], but most fertilize their eggs externally.

The fertilized eggs typically hatch into [[trochophore]] larvae, which float among the [[plankton]], and eventually [[metamorphosis|metamorphose]] into the adult form by adding segments. A few species have no larval form, with the egg hatching into a form resembling the adult, and in many that do have larvae, the trochophore never feeds, surviving off the yolk that remains from the egg.<ref name=IZ/>

However, some polychaetes exhibit remarkable reproductive strategies. Some species reproduce by [[epitoky]]. For much of the year, these worms look like any other burrow-dwelling polychaete, but as the breeding season approaches, the worm undergoes a remarkable transformation as new, specialized segments begin to grow from its rear end until the worm can be clearly divided into two halves. The front half, the atoke, is asexual. The new rear half, responsible for breeding, is known as the epitoke. Each of the epitoke segments is packed with eggs and sperm and features a single eyespot on its surface. The beginning of the last lunar quarter is the cue for these animals to breed, and the epitokes break free from the atokes and float to the surface. The eye spots sense when the epitoke reaches the surface and the segments from millions of worms burst, releasing their eggs and sperm into the water.<ref>{{cite book |author-link=Ross Piper |last=Piper |first=Ross |year=2007 |title=Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals |url=https://archive.org/details/extraordinaryani0000pipe |url-access=registration |publisher=[[Greenwood Press (publisher)|Greenwood Press]]|isbn=9780313339226 }}</ref>

A similar strategy is employed by the deep sea worm ''[[Syllis ramosa]]'', which lives inside a [[sponge]]. The rear ends of the worm develop into "stolons" containing the eggs or sperm; these stolons then become detached from the parent worm and rise to the sea surface, where fertilisation takes place.<ref name=Smithsonian>{{cite web |url=http://www.smithsonianmag.com/science-nature/14-fun-facts-about-marine-bristle-worms-180955773/ |title=Some polychaetes have sex lives out of a science fiction movie |last1=Frost |first1=Emily |last2=Waters |first2=Hannah |date=1 July 2015 |work=14 fun facts about marine bristle worms |publisher=Smithsonian.com |access-date=9 August 2017}}</ref>

==Fossil record==
[[Crown group#Stem groups|Stem-group]] polychaete fossils are known from the [[Sirius Passet]] [[Lagerstätte]], a rich, sedimentary deposit in Greenland tentatively dated to the late [[Atdabanian]] (early [[Cambrian]]). The oldest found is ''[[Phragmochaeta canicularis]]''.<ref>{{Cite journal | last1 = Conway Morris | first1 = S. | last2 = Peel | first2 = J. S. | title = The Earliest Annelids: Lower Cambrian Polychaetes from the Sirius Passet Lagerstätte, Peary Land, North Greenland | journal = Acta Palaeontologica Polonica | volume = 53 | pages = 137–148 | year = 2008 | doi = 10.4202/app.2008.0110 | doi-access = free }}</ref> Many of the more famous [[Burgess Shale]] organisms, such as ''[[Canadia (annelid)|Canadia]]'', may also have polychaete affinities. ''[[Wiwaxia]]'', long interpreted as an annelid,<ref name="JSTORExpressionerror">{{Cite journal| last1 = Butterfield | first1 = N. J.| title = A reassessment of the enigmatic Burgess Shale fossil ''Wiwaxia corrugata'' (Matthew) and its relationship to the polychaete ''Canadia spinosa'' Walcott| jstor = 2400789| journal = Paleobiology| volume = 16| issue = 3| pages = 287–303| year = 1990 |doi=10.1017/S0094837300010009 | bibcode = 1990Pbio...16..287B| s2cid = 88100863}}</ref> is now considered to represent a mollusc.<ref name="Smith2012">{{cite journal | last = Smith | first = M. R. | year = 2012 | title = Mouthparts of the Burgess Shale fossils ''Odontogriphus ''and ''Wiwaxia'': Implications for the ancestral molluscan radula | journal = Proceedings of the Royal Society B | volume = 279| issue = 1745| pages = 4287–4295 | doi = 10.1098/rspb.2012.1577 | pmid = 22915671 | pmc = 3441091}}</ref><ref name="Smithin press">{{cite journal | last1 = Smith | first1 = M. R. | year = 2014 | title = Ontogeny, morphology and taxonomy of the soft-bodied Cambrian 'mollusc' ''Wiwaxia'' | journal = Palaeontology | volume = 57 | issue = 1 | pages =  215–229 | doi = 10.1111/pala.12063| bibcode = 2014Palgy..57..215S | s2cid = 84616434 | doi-access = free }}</ref> An even older fossil, ''[[Cloudina]]'', dates to the terminal [[Ediacaran]] period; this has been interpreted as an early polychaete, although consensus is absent.<ref name=Miller>{{Cite book
 | author = Miller, A. J.
 | year = 2004
 | title = A revised morphology of ''Cloudina'' with ecological and phylogenetic implications
 | citeseerx = 10.1.1.526.5035
 }}</ref><ref name=VinnZaton>{{cite journal |last1=Vinn |first1=Olev |last2=Zatoń |first2=Michał |title=Inconsistencies in proposed annelid affinities of early biomineralized organism ''Cloudina'' (Ediacaran): structural and ontogenetic evidences |journal= Carnets de Géologie |date=March 2012 |issue=Lettres |doi=10.4267/2042/46095 |doi-access=free }}</ref>

Being [[soft-bodied organisms]], the fossil record of polychaetes is dominated by their fossilized jaws, known as [[scolecodont]]s, and the [[mineral]]ized tubes that some of them secrete.<ref name="VinnMutvei2009tubeworms">{{cite journal |last1=Vinn |first1=O |last2=Mutvei |first2=H |title=Calcareous tubeworms of the Phanerozoic |journal=Estonian Journal of Earth Sciences |date=2009 |volume=58 |issue=4 |pages=286 |doi=10.3176/earth.2009.4.07 |doi-access=free }}</ref> Most important [[biomineralising polychaetes]] are [[Serpulidae|serpulids]], [[Sabellidae|sabellids]], and [[Cirratulidae|cirratulids]]. Polychaete cuticle does have some [[preservation potential]]; it tends to survive for at least 30 days after a polychaete's death.<ref name="Briggs1993"/> Although biomineralisation is usually necessary to preserve soft tissue after this time, the presence of polychaete muscle in the nonmineralised Burgess shale shows this need not always be the case.<ref name=Briggs1993/> Their preservation potential is similar to that of [[jellyfish]].<ref name="Briggs1993"/>

==Taxonomy and systematics==
{{see also|List of annelid families}}
[[File:Phyllodoce lineata.jpg|thumb| Head of ''Phyllodoce lineata'']]
[[File:Pacific Feather Duster Sabellastarte sp.jpg|thumb|right| The plumes of the [[feather duster worm]] are used to filter water]]
[[File:Tomopteriskils.jpg|thumb|right| [[Planktonic]] bristleworm ''[[Tomopteris]]'']]
[[File:Spirobrancheus giganteus.jpg|thumb| [[Christmas tree worm]]s]]
[[File:Nereis pelagica.jpg|thumb| [[Rag worm]]s can be dangerous to touch, giving painful burns]]
[[File:Nereis virens.jpg|thumb| [[Alitta virens|Sandworms]] eat seaweed and microorganisms and can be longer than  four feet]]
[[File:Riftia tube worm colony Galapagos 2011.jpg|thumb| [[Giant tube worm]]s can tolerate extremely high hydrogen sulfide levels]]

Taxonomically, polychaetes are thought to be [[paraphyletic]],<ref>{{cite journal |last=Westheide |first=W. |year=1997 |title=The direction of evolution within the Polychaeta |journal=Journal of Natural History |volume=31 |issue=1 |pages=1–15 |doi=10.1080/00222939700770011 |bibcode=1997JNatH..31....1W }}</ref> meaning the group excludes some descendants of its most recent common ancestor.  Groups that may be descended from the polychaetes include the [[clitellate]]s ([[earthworm]]s and [[leech]]es), [[sipuncula]]ns, and [[echiura]]ns.  The Pogonophora and [[Vestimentifera]] were once considered separate phyla, but are now classified in the polychaete family [[Siboglinidae]].

Much of the classification below matches Rouse & Fauchald, 1998, although that paper does not apply ranks above family.

Older classifications recognize many more (sub)orders than the layout presented here. As comparatively few polychaete [[taxa]] have been subject to [[cladistic]] analysis, some groups which are usually considered invalid today may eventually be reinstated.

These divisions were shown to be mostly paraphyletic in recent years.

*'''[[Basal (evolution)|Basal]] or ''[[incertae sedis]]'''''
**Family [[Diurodrilidae]]
**Family [[Histriobdellidae]]
**Family [[Nerillidae]]
**Family [[Parergodrilidae]]
**Family [[Potamodrilidae]]
**Family [[Psammodrilidae]]
**Family [[Spintheridae]]
**Family [[Protodriloididae]]
**Family [[Saccocirridae]]
**Order [[Haplodrili]]
**Order [[Myzostomida]]
***Family [[Endomyzostomatidae]]
***Family [[Asteromyzostomatidae]]
***Family [[Myzostomatidae]]
*'''Subclass [[Palpata]]'''
**Family [[Protodrilidae]]
**Family [[Polygordiidae]]
*'''Subclass [[Aciculata]]'''
**Family [[Levidoridae]]
**Order [[Amphinomida]]
***Family [[Amphinomidae]]
***Family [[Euphrosinidae]]
**Order [[Eunicida]]
***Family [[Dorvilleidae]]
***Family [[Eunicidae]]
***Family [[Hartmaniellidae]]
***Family [[Ichthyotomidae]]
***Family [[Lumbrineridae]]
***Family [[Oenonidae]]
***Family [[Onuphidae]]
**Order [[Phyllodocida]]
***Suborder [[Aphroditiformia]]
****Family [[Acoetidae]]
****Family [[Aphroditidae]]
****Family [[Eulepethidae]]
****Family [[Iphionidae]]
****Family [[Pholoidae]]
****Family [[Polynoidae]]
****Family [[Sigalionidae]]
***Suborder [[Glyceriformia]]
****Family [[Glyceridae]]
****Family [[Goniadidae]]
****Family [[Lacydoniidae]]
****Family [[Paralacydoniidae]]
***Suborder [[Nereidiformia]]
****Family [[Antonbruunidae]]
****Family [[Chrysopetalidae]]
****Family [[Hesionidae]]
****Family [[Nereididae]]
****Family [[Pilargidae]]
****Family [[Syllidae]]
***Suborder [[Phyllodocida incertae sedis]]
****Family [[Iospilidae]]
****Family [[Nautiliniellidae]]
****Family [[Nephtyidae]]
****Family [[Typhloscolecidae]]
****Family [[Tomopteridae]]
***Suborder [[Phyllodociformia]]
****Family [[Alciopidae]]
****Family [[Lopadorrhynchidae]]
****Family [[Phyllodocidae]]
****Family [[Pontodoridae]]
*'''Subclass [[Sedentaria]]'''
**Family [[Chaetopteridae]]
**Infraclass [[Canalipalpata]]
***Order [[Sabellida]]
****Family [[Caobangidae]]
****Family [[Fabriciidae]]
****Family [[Oweniidae]]
****Family [[Sabellariidae]]
****Family [[Sabellidae]]
****Family [[Serpulidae]]
****Family [[Siboglinidae]] (formerly the phyla Pogonophora & Vestimentifera)
***Order [[Spionida]]
****Suborder [[Spioniformia]]
*****Family [[Apistobranchidae]]
*****Family [[Longosomatidae]]
*****Family [[Magelonidae]]
*****Family [[Poecilochaetidae]]
*****Family [[Spionidae]]
*****Family [[Trochochaetidae]]
*****Family [[Uncispionidae]]
***Order [[Terebellida]]
****Suborder [[Cirratuliformia]]
*****Family [[Acrocirridae]] (sometimes placed in Spionida)
*****Family [[Cirratulidae]] (sometimes placed in Spionida)
*****Family [[Ctenodrilidae]] (sometimes own suborder Ctenodrilida)
*****Family [[Fauveliopsidae]] (sometimes own suborder Fauveliopsida)
*****Family [[Flabelligeridae]] (sometimes suborder Flabelligerida)
*****Family [[Flotidae]] (sometimes included in Flabelligeridae)
*****Family [[Poeobiidae]] (sometimes own suborder Poeobiida or included in Flabelligerida)
*****Family [[Sternaspidae]] (sometimes own suborder Sternaspida)
****Suborder [[Terebellomorpha]]
*****Family [[Alvinellidae]]
*****Family [[Ampharetidae]]
*****Family [[Pectinariidae]]
*****Family [[Terebellidae]]
*****Family [[Trichobranchidae]]
**Infraclass [[Scolecida]]
***Family [[Arenicolidae]]
***Family [[Capitellidae]]
***Family [[Cossuridae]]
***Family [[Maldanidae]]
***Family [[Opheliidae]]
***Family [[Orbiniidae]]
***Family [[Paraonidae]]
***Family [[Scalibregmatidae]]
***Order [[Capitellida]] ([[nomen dubium]])
***Order [[Cossurida]] (nomen dubium)
***Order [[Opheliida]] (nomen dubium)
***Order [[Orbiniida]] (nomen dubium)
***Order [[Questida]] (nomen dubium)
***Order [[Scolecidaformia]] (nomen dubium)
*'''Subclass [[Echiura]]'''
** Order [[Bonelliida]]
*** Family [[Bonelliidae]] 
*** Family [[Ikedidae]] 
** Order [[Echiurida]]
*** Family [[Echiuridae]] 
*** Family [[Thalassematidae]] 
*** Family [[Urechidae]]

==See also==
* [[Aelosoma]]
* [[Edith Berkeley]]
*''[[Australonuphis]]''

==References==

===Bibliography===
* Campbell, Reece, and Mitchell. Biology. 1999.
* {{cite journal
 |author1=Rouse, Greg W. |author2=Fauchald, Kristian | year = 1998
 | title = Recent views on the status, delineation, and classification of the Annelida
 | journal = American Zoologist
 | volume = 38 | pages = 953–964
 | doi = 10.1093/icb/38.6.953
 | issue = 6
| doi-access = free}}

===Notes===
{{Reflist}}

==External links==
{{Wikispecies|Polychaeta}}
{{Commons category|Polychaeta}}
* [http://www.marinespecies.org/polychaeta/aphia.php?p=sourcedetails&id=198042 World Polychaeta Database]
* [https://onlinelibrary.wiley.com/toc/14390485/2005/26/3%E2%80%904 Special issue of ''Marine Ecology''] dedicated to polychaetes
* [https://web.archive.org/web/20080720124908/http://www.tafi.org.au/zooplankton/imagekey/annelida/index.html Marine Polychaete Larva], a guide to the marine zooplankton of south eastern Australia
* [http://www.nhm.ac.uk/research-curation/research/projects/taxinfo//key/family_key.htm Key to Families of Polychaetes], [[Natural History Museum, London|Natural History Museum]]

{{Taxonbar|from=Q18952}}
{{Authority control}}

[[Category:Polychaetes|*]]
[[Category:Extant Cambrian first appearances]]
[[Category:Paraphyletic groups]]