Any of about 5,400 species of marine worms of the annelid class Polychaeta, having a segmented body with many setae (bristles) on each segment. Species, often brightly coloured, range from less than 1 in. (2.5 cm) to about 10 ft (3 m) long. Most body segments bear two bristly parapodia (lobelike outgrowths). The head has short sensory projections and tentacles. Adults may be free-swimming or sedentary; larvae are free-swimming. Found worldwide, polychaetes are important for turning over sediment on the ocean bottom. One species, the bloodworm, is a popular saltwater fish bait. Seealso tube worm.
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Any worm that lives in a marine environment is considered a marine worm. Marine worms are found in several different phyla, including the Platyhelminthes, Nematoda, Annelida, Chaetognatha, Hemichordata, etc. Some of them are segmented and some of them are not. These worms are also called polychaetes,which means "many bristles". Have segments with bristles; they occur in bundles.
These worms have specialized tentacles used for exchanging oxygen and giving out carbon dioxide
One of these worms can include the tube worm, which lives in waters near under water volcanoes, and can withstand temperatures up to 90 degrees Celsius. The tube worm kind of looks like giant blood vessels, and veins.