Editing Polychaete (section)

==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>