A class of the segmented worm phylum Anellida. There are approximately 8,000 species of polychaetes which include errant (free-moving) forms and sedentary ones that live in stabilised burrows, galleries or tubes of various degrees of complexity. Some burrow into coral. Many species are common inhabitants of coral reefs, such as the Christmas tree worms, feather duster worms, fanworms, fireworms, scaleworms, threadworms, and others.