Worm, an animal that has a soft, long body, typically with no legs. A worm is an invertebrate, an animal without a backbone. Worms have a variety of shapes, resembling such objects as threads, ribbons, and torpedoes. Some worms (such as certain spiny-headed worms) are of microscopic size; others (such as certain tapeworms) may grow to a length of more than 75 feet (23 m). Some worms, such as the earthworm, are free-living. Others are parasites of plants or other animals, and often cause serious disease.

Certain insect larvae, such as silkworms and maggots, are commonly called worms. However, biologists do not consider insects to be worms. Biologists classify worms into a number of large groups called phyla. More than 15 phyla of worms are generally recognized; the most common are listed below. Those phyla in which the worms have the simplest body structure are listed first.

Platyhelminthes. Flatworms. This group includes the planarians, flukes, and tapeworms. and references.)

Nemertina, or Rhynchocoela. Ribbon worms. Most live in the sea. A ribbon worm has a long proboscis that it uses to capture prey. .)

Nematoda. Roundworms. Some are parasites of plants or animals; others live in the soil, in seawater, or in freshwater. and references.)

Nematomorpha. Hairworms. Wiry worms that are found in seawater, freshwater, and moist soil. Larvae live as parasites in certain animals. .)

Acanthocephala. Spiny-headed worms. A spiny-headed worm has a spiny proboscis (a long organ at the front of the body) that it uses to anchor itself to an animal's intestine wall, where it lives as a parasite.

The acanthocephalaThe acanthocephala uses its spiny proboscis to anchor itself to an animal's intestine wall.

Annelida. Segmented worms. This group includes earthworms, leeches, and tube worms. and references.)

Chaetognatha. Arrowworms. Marine worms with transparent, torpedo-shaped bodies.

Hemichordata. This group includes acorn worms—marine worms with gill slits; they live in U-shaped burrows.