Polychaeta
Amphinomida
Capitellida
Chaetopterida
Cirratulida
Cossurida
Ctenodrillidae
Eunicida
Flabelligerida
Magelonida
Myzostomida
Nerillida
Opheliida
Orbiniida
Orweniida
Phyllodocida
Pisionidae
Polygordiida
Protodrilida
Psammodrilidae
Sabellida
Spionida
Spintheridae
Sternaspida
Terebellida
</table>
The
Polychaeta or
Polychaetes are a class of
annelid worms, generally marine, with a pair of fleshy protrusions on each body segment called parapodia ("almost feet") which bear many bristles called setae which are made of
Chitin.
Polychaeta means "many-bristled" (as opposed to the
Oligochaeta which are "few-bristled"), and the polychaetes are sometimes called
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 paddle-like and highly vascularized parapodia are used for movement and act as the
animal's primary gas-exchange 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, that is an assemblage of not very closely related forms, but which all came 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 as in 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.
Reference
- Campbell, Reece, and Mitchell. Biology. 1999.
