|Orange thread-gilled worm
|Ringed sylid worm
Polychaetes are segmented worms, generally less than 10 centimetres in length, although ranging at the extremes from 1 millimetre to 3 metres . They are often brightly coloured, and may be iridescent or even luminescent. Each segment bears a pair of paddle-like and highly vascularized parapodia, which are used for movement and, in many species, act as the worm's primary respiratory surfaces. Bundles of bristles, called setae, project from the parapodia.
However, polychaetes vary widely from this generalised pattern, and can display a range of different body forms. The most generalised polychaetes are those that crawl along the bottom, but others have adapted to many different ecological niches, including burrowing, pelagic life, tube-dwelling or boring, commensalism, and parasitism, requiring various modifications to their body structure.
The head, or prostomium, is relatively well developed, compared with other annelids. It projects forward over the mouth, which therefore lies on the animal's underside. The head normally includes two to four pair of eyes, although there are some blind species. These are typically fairly simple structures, capable of distinguishing only light and dark, although some species have large eyes with lenses that may be capable of more sophisticated vision.
The head also includes a pair of antennae, tentacle-like palps, and a pair of pits lined with cilia, known as "nuchal organs". These latter appear to be chemoreceptors, and help the worm to seek out food.
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