Polychaeta

From Palaeos

Lophotrochozoa
`==Hallkeriida
   `--+--Wiwaxia corrugata †
      `--+--Canadia spinosa †
         `==Polychaeta (paraphyletic) (Annelida) 
            |--Myzostomida
            |--Pogonophora
            |--Vestimentifera
            `--+--+--Banffida †
               |  `--+-- Echiura [Echiuroidea] 
               `--Clitellata
                  |--Acanthobdellida
                  |--Branchiobdellida
                  |--Hirudinea
                  `--Oligochaeta       

Tomopteris, a planktonic worm

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 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.

Taxonomically, the polychaetes are now known 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. In other words they represent an evolutionary grade of worlm-like animals.

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 or in the bottom, and some live as commensals. A few are parasitic. The mobile forms tend to have well-developed sense organs and jaws, while the stationary forms lack them but may have specialized gills or tentacles used for respiration and deposit or filter feeding, e.g., 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.

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.

Hallkeriida †
`--Polychaeta (paraphyletic) (Annelida)
   |--Scolecida
   `--Palpata
      |--Aciculata
      |  |--Eunicida
      |  `--Phyllodocida
      `--Canalipalpata 
         |--Sabellida (paraphyletic)
         |  '--Pogonophora
         |--Spionida
         `--+--Terebellida
            `--+--+--Banffida †
               |  `--+-- Echiura [Echiuroidea] 
               `--Clitellata  

[edit] Credits