Polychaete

Polychaetes
Scientific classification
Kingdom: Animalia Phylum: Annelida Class: Polychaeta
Orders
may not be a complete list Aciculata Canalipalpata Capitellida Phyllodocida

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"). 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.

Reference

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