Editing Polychaete (section)

===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>