Polychaeta () is a paraphyletic class of generally marine annelid worms, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia which bear many chitinous bristles called chaetae, hence their name. More than 10,000 species have been described in this diverse and widespread class; in addition to inhabiting all of the world's oceans, polychaetes occur at all ocean depths, from planktonic species living near the surface, to a small undescribed species observed through ROV at the deepest region in the Earth's oceans, Challenger Deep. In addition, many species live on the abyssal plains, coral reefs, parasitically, and a few within fresh water. Commonly encountered representatives include the lugworms, bloodworms, and species of Alitta such as the clam worm and sandworm or ragworm; these species inhabit shallow water marine environments and coastlines of subtropical and temperate regions around the world and may be used as fishing bait. More exotic species include the stinging fireworms, the predatory and large-bodied bobbit worm, the culturally important palolo worm, the bone-eating worms, and giant tube worms, which are extremophiles that tolerate near-boiling water near hydrothermal vents.
Description
Polychaetes are segmented worms, generally less than 10 cm (4 in) in length, although ranging at the extremes from 1 mm (0.04 in) to 3 m (10 ft), in Eunice aphroditois. They can sometimes be 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, the chaetae, project from the parapodia. However, polychaete body plans vary widely from this generalized 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 swimming, dwelling in self-created tubes or ones bored out of a substrate, commensalism, and parasitism; such varied lifestyles requires a divergence from the basic body plan of the common ancestor.
The head, or prostomium, is relatively well developed, compared with other annelids. It projects forward over the mouth, which is located on the succeeding section; the peristomium. The mouthparts vary in form depending on their diets, since the group includes predators, herbivores, filter feeders, scavengers, and parasites. In general, however, they possess a pair of jaws and a pharynx that can be rapidly everted, allowing the worms to grab food and pull it into their mouths. In some species, the pharynx is modified into a lengthy proboscis. Their jaws are formed from sclerotised collagen. The digestive tract is a simple tube, usually with a stomach partway along. The head may include two to four pairs of eyes, although some species are eyeless. The eyes 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, an example being the complex eyes of Alciopidae, which rival those of cephalopods and vertebrates. The head also includes a pair of antennae, tentacle-like palps, and a pair of pits lined with cilia known as nuchal organs, which are chemoreceptors that help the worm to seek out food.
The outer surface of the body wall consists of a simple columnar epithelium covered by a thin cuticle, constructed from cross-linked collagen fibers and may be 2 to 13 millimetres (0.079 to 0.512 in) thick. Sclerotized collagen makes up their setae. The papillae of polychaetes are tiny, fleshy projections on the worm's body or parapodia, often associated with sensory and locomotive functions. Some papillae have sensory receptors to help the worm detect environmental changes, such as touch, water currents, or chemical signals, and support movement by working with parapodia to aid grip and friction during crawling or burrowing. Other papilla types play a protective role, specifically in the secretion of mucus used to deter predators. Underneath the cuticle, in order, are a thin layer of connective tissue, a layer of circular muscle, a layer of longitudinal muscle, and a peritoneum surrounding the coelom (body cavity). Additional oblique muscles move the parapodia. In most species the body cavity is divided into separate compartments by sheets of peritoneum between each segment, but in some species it is more continuous.
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