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A worm is an elongated soft-bodied invertebrate animal. The best-known is the earthworm, a member of phylum Annelida, but there are hundreds of thousands of different species that live in a wide variety of habitats other than soil.

Originally the word referred to any creeping or a crawling animal of any kind or size, such as a serpent, caterpillar, snail, or the like (this old usage is preserved in the name "slow worm", actually a lizard). Later this definition was narrowed to the modern definition which still includes several different animal groups.

Other invertebrate groups may be called worms, especially colloquially. Many insect larvae are called worms, such as the railroad worm, woodworm, glowworm, or bloodworms.

Worms may also be called helminths, especially in medical or terminology when referring to parasitic worms, especially the Nematoda (roundworms) and Cestoda (tapeworms). Hence helminthology is the study of parasitic worms.

When an animal, such as a dog, is said to have worms, it means that the dog is infested with parasitic worms, typically roundworm or tapeworm.

Worm species differ in their abilities to move about on their own. Many species have bodies with no major muscles, and cannot move on their own. They must be moved by forces or other animals in their environment. Many species have bodies with major muscles, that let them move on their own. They are a type of muscular hydrostat.

The fear of worms is known as 'scoleciphobia'.

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