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The 'Polychaeta' or 'polychaetes' are a class of annelid worms, generally marine. Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made of chitin. ''Polychaeta'' means "many-bristled" (as opposed to the Oligochaeta which are "few-bristled"), and indeed the polychaetes are sometimes referred to as 'bristle worms'. More than 10,000 species are described in this class. Common representatives include the lugworm (''Arenicola marina'') and the sandworm or clam worm ''Nereis''.

Contents

Anatomy and physiology

Ecology

Evolutionary history

Taxonomy

See also

References

External links

Anatomy and physiology

The polychaetes' paddle-like and highly vascularized parapodia are used for movement and act as the annelid's primary respiratory surfaces (parapodia can be thought of as kinds of external gills that are also used for locomotion). Polychaeta also have well-developed heads compared to other annelids.

Ecology

Polychaetes are extremely variable in both form and lifestyle and include a few taxa that swim among the plankton. Most burrow or build tubes on the bottom, and some live as commensals. A few are parasitic. The mobile forms or Errantia tend to have well-developed sense organs and jaws, while the Sedentaria (or stationary forms) lack them but may have specialized gills or tentacles used for respiration and deposit or filter feeding, e.g., fanworms.
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.

One notable polychaete, the Pompeii worm (''Alvinella pompejana'') is endemic to the hydrothermal vents of the Pacific Ocean. Pompeii worms are thought to be the most heat-tolerant complex animals known.
A recently discovered genus Osedax includes the Bone-eating snot flower.
Another remarkable polychaete is ''Hesiocaeca methanicola'', which lives on methane clathrate deposits.
''Lamellibrachia luymesi'' is a cold seep tube worm that reaches lengths of over 3 meters and may be the most long lived animal at over 250 years old.

Evolutionary history

Cloudina is an Ediacaran fossil that may be an early Serpulid worm. The oldest definite polychaetes are from the Cambrian Period, such as Canadia from the Burgess Shale.
Polychaetes are more commonly known from their fossilized jaws, known as scolecodonts, and from the mineralized tubes that some of them secrete.

Taxonomy

Taxonomically, the polychaetes are thought to be paraphyletic, meaning that as a group it contains its most recent common ancestor, but does not contain all the descendants of that ancestor. Groups that may be descended from the polychaetes include the earthworms, the leeches, sipunculans, and echiurans. 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.

★ 'Subclass Palpata'

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★ Order Aciculata

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★ Suborder uncertain

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★ Family Aberrantidae

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★ Family Nerillidae

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★ Family Spintheridae

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★ Suborder Eunicida

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★ Family Amphinomidae

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★ Family Diurodrilidae

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★ Family Dorvilleidae

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★ Family Eunicidae

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★ Family Euphrosinidae

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★ Family Hartmaniellidae

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★ Family Histriobdellidae

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★ Family Lumbrineridae

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★ Family Oenonidae

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★ Family Onuphidae

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★ Suborder Phyllodocida

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★ Family Acoetidae

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★ Family Alciopidae

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★ Family Aphroditidae

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★ Family Chrysopetalidae

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★ Family Eulepethidae

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★ Family Glyceridae

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★ Family Goniadidae

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★ Family Hesionidae

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★ Family Ichthyotomidae

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★ Family Iospilidae

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★ Family Lacydoniidae

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★ Family Lopadorhynchidae

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★ Family Myzostomatidae

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★ Family Nautillienellidae

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★ Family Nephtyidae

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★ Family Nereididae

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★ Family Paralacydoniidae

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★ Family Pholoidae

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★ Family Phyllodocidae

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★ Family Pilargidae

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★ Family Pisionidae

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★ Family Polynoidae

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★ Family Pontodoridae

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★ Family Sigalionidae

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★ Family Sphaeodoridae

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★ Family Syllidae

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★ Family Typhloscolecidae

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★ Family Tomopteridae

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★ Order Canalipalpata

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★ Suborder uncertain

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★ Family Polygordiidae

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★ Family Protodrilidae

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★ Family Protodriloididae

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★ Family Saccocirridae

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★ Suborder Sabellida

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★ Family Oweniidae

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★ Family Siboglinidae (formerly the phyla Pogonophora & Vestimentifera)

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★ Family Serpulidae

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★ Family Sabellidae

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★ Family Sabellariidae

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★ Family Spirorbidae

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★ Suborder Spionida

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★ Family Apistobranchidae

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★ Family Chaetopteridae

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★ Family Longosomatidae

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★ Family Magelonidae

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★ Family Poecilochaetidae

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★ Family Spionidae

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★ Family Trochochaetidae

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★ Family Uncispionidae

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★ Suborder Terebellida

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★ Family Acrocirridae

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★ Family Alvinellidae

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★ Family Ampharetidae

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★ Family Cirratulidae

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★ Family Ctenodrilidae

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★ Family Fauveliopsidae

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★ Family Flabelligeridae

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★ Family Flotidae

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★ Family Pectinariidae

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★ Family Poeobiidae

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★ Family Sternaspidae

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★ Family Terebellidae

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★ Family Trichobranchidae

★ 'Subclass Scolecida'

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★ Family Aeolosomatidae

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★ Family Arenicolidae

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★ Family Capitellidae

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★ Family Cossunidae

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★ Family Maldanidae

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★ Family Ophelidae

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★ Family Orbiniidae

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★ Family Paraonidae

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★ Family Parergodrilidae

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★ Family Potamodrilidae

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★ Family Psammodrilidae

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★ Family Questidae

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★ Family Scalibregmatidae

See also

★ Epitoky, a form of reproduction of ''Polychaetae''.

References

★ Campbell, Reece, and Mitchell. Biology. 1999.

★ Recent views on the status, delineation, and classification of the Annelida, Rouse, Greg W.; Fauchald, Kristian, , , American Zoologist, 1998

External links

★ Special issue dedicated to polychaete published in ''Marine Ecology''. 'Read the article abstracts online'