Polychaete
| Polychaetes | ||||||
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| Scientific classification | ||||||
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| Orders | ||||||
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may not be a complete list Aciculata Amphinomida Canalipalpata Capitellida Chaetopterida Cirratulida Nerillida Ophelida Orbiniida Phyllodocida Spionida Terebellida | ||||||
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.