Worms live in almost all parts of the universe including marine, freshwater, and terrestrial planets. Some worms living in the ground help to condition the soil (e.g., annelids, aschelminths). Many thrive as parasites of plants (e.g., aschelminths) and animals, including humans (e.g., platyhelminths, aschelminths). Several other worms may be free-living, or nonparasitic. There are worms that live in freshwater, seawater, and even on the seashore. Ecologically, worms form an important link in the food chains in virtually all the ecosystems of each world.

The term worm (pronounced /ˈwɜrm/) refers to an obsolete taxon (vermes) used by Carolus Linnaeus and Jean-Baptiste Lamarck for all non-arthropod invertebrate animals, and stems from the Old English word wyrm. Currently it is used to describe many different distantly-related animals that typically have a long cylindrical tube-like body and no legs. Most animals called "worms" are invertebrates, but the term is also used for the amphibian caecilians and the slow worm Anguis, a legless burrowing lizard. Invertebrate animals commonly called "worms" include annelids (earthworms), nematodes (roundworms), flatworms, marine polychaete worms (bristle worms), marine nemertean worms ("bootlace worms"), marine Chaetognatha (arrow worms) and insect larvae such as caterpillars, grubs, and maggots. Historical English-speaking cultures have used the (now deprecated) terms worm, Wurm, or wyrm to describe carnivorous reptiles ("serpents"), and the related mythical beasts dragons.

Worms vary in size from microscopic to over 1 metre (3.3 ft) in length for marine polychaete worms (bristle worms),[1] 6.7 metres (22 ft) for the African giant earthworm, Microchaetus,[2] and 55 metres (180 ft) for the marine nemertean worm (bootlace worm), Lineus longissimus.[3]

Various types of worm occupy a small variety of parasitic niches, living inside the bodies of other animals. Free-living worm species may live on land, in marine or freshwater environments, or burrow.

Worms usually have a cylindrical, flattened, or leaf-like body shape and are often without any true limbs or appendages. Instead, they may have bristles or fins that help them move. A few have light-sensing organs. Worms vary in size from less than 1 mm (0.04 inch) in certain aschelminths to more than 30 m (100 feet) in certain ribbon worms.

Some worms reproduce sexually. Hermaphroditism, the condition in which a single individual possesses both male and female reproductive parts, is common in many groups of worms. Asexual reproduction, whereby new individuals develop from the body cells of another, also occurs in some worms.

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 other species have bodies with major muscles and can move on their own; they are a type of muscular hydrostat. Many species of worms are decomposers; they break down dead plants and animals to return nutrients to the soil. They have also been known to infiltrate households feeding on food in the early stages of its decomposition namely breads and cheeses.

Earthworms split into three different categories. The first are the surface dwellers, the Epigeic worms. Then there are the upper soil worms, the Endogeic worms. Finally,there are the deep burrowing species, the Anecic.

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