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The vast majority of the more than 8,000 known species of polychaete worms are marine; some, however, are found in fresh or brackish water. They are abundant from the intertidal zone to depths of over 16,405 ft (5,000 m). The polychaetes, so named because of the numerous setae (chaetae) they bear, range in length from less than 1/8 in. to more than 9 ft (2 mm to 3 m), but most are from 2 to 4 in. (5–10 cm) long. Their colors are often brilliant, and some species are iridescent. The class has usually been divided on the basis of mode of existence into two groups, the errantia and the sedentaria.
Errant Polychaetes
Errant polychaetes include actively crawling or swimming forms which may, however, also spend time in burrows or crevices, or under rocks on the seashore. A familiar errant polychaete is the clamworm, Nereis, widely used as bait. Errant polychaetes swim, crawl over the ocean bottom, or tunnel through surface sediments. Many are predators on small invertebrates; some are scavengers. In most the first few body segments bear sensory projections called cirri, while the remaining body segments bear conspicuous leglike appendages called parapodia. The parapodia, along with undulahe worm in crawling and swimming; parapodia are tipped with bundles of setae, usually made of chitin. Most errant polychaetes have well-developed head regions, which bear eyes, sensory tentacles, and a specialized organ, the nuchal organ, thought to detect chemicals. The anterior end of the gut often forms a protrusible structure, the proboscis, sometimes equipped with strong chitinous jaws and used in feeding. The setae of some polychaetes, e.g., the tropical fireworm, are composed of calcium carbonate rather than chitin and are hollow. These brittle setae are easily broken off and contain a toxin that produces a painful reaction in humans. In the scaleworms, a series of overlapping scales form a covering over the animal's upper surface. In the sea mouse these scales are completely covered by long, slender, feltlike setae projecting from the parapodia.
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