IntroductionAnnelida (ənĕlˈĭdə) [key] [Lat., anellus = a ring], phylum of soft-bodied, bilaterally symmetrical (see symmetry, biological), segmented animals, known as the segmented, or annelid, worms. Over 12,000 known species are grouped in three classes: the earthworms and freshwater worms (oligochaetes), the leeches (hirudineans), and the marine worms (polychaetes). Annelids are found throughout the world, from deep ocean bottoms to high mountain glaciers. They live in protected habitats such as mud, sand, and rock crevices, and in and among other invertebrate animals, such as sponges. Many live in tubes they secrete around themselves.
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