Amphinomida 
 

Capitellida
Chaetopterida
Cirratulida
Cossurida
Ctenodrillidae
Eunicida
Flabelligerida
Magelonida
Myzostomida
Nerillida
Opheliida
Orbiniida
Orweniida
Phyllodocida
Pisionidae
Polygordiida
Protodrilida
Psammodrilidae
Sabellida
Spionida
Spintheridae
Sternaspida
Terebellida

The Polychaeta or Polychaetes are a class of annelid worms, generally marine, with a pair of fleshy protrusions on each body segment called parapodia that bear many bristles, called setae, 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, but common representatives are the lugworm (Arenicola marina) and the sandworm or clam worm Nereis.

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

Taxonomically, the polychaetes are thought to be paraphyletic, meaning an assemblage of not very closely related forms, but all of which come from the same source.

Polychaetes are extremely variable in both form and lifestyle and include freeliving (with many swimming among plankton), burrowing, and tube-dwelling species as well as commensals. The free living 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 filter-feeding, similar to the fanworms .

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.

Another remarkable polychaete is Hesiocaeca methanicola, which lives on methane clathrate deposits.

The bobbit worm is also a member of this family.

Reference

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