Any of thousands of species of unrelated invertebrate animals that typically have a soft, slender, elongated body with no appendages. The major phyla are Platyhelminthes (flatworms), Annelida (annelids, or segmented worms), Nemertea (ribbon worms), Acanthocephala (spiny-headed worms), and Aschelminthes (nematodes and others). There are several minor phyla. Length ranges from microscopic (e.g., some aschelminths) to more than 100 ft (30 m) (some ribbon worms). Worms are found worldwide on land and in water. They may be parasitic or free-living and are important as soil conditioners, parasites, and a link in the food chain in all ecosystems. See also fluke, pinworm, polychaete, rotifer, tapeworm, tube worm.

This entry comes from Encyclopædia Britannica Concise.
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