Classification
In everyday language, the term worm is also applied to various other living forms such as larvae, insects, centipedes, shipworms (teredo worms), or even some vertebrates (creatures with a backbone) such as blindworms and caecilians. Worms can be divided into several groups, but are still technically decomposers.
- The first of these, Platyhelminthes, includes the flatworms, tapeworms, and flukes. They have a flat, ribbon- or leaf-shaped body with a pair of eyes at the front. Some are parasites.
- The second group contains the threadworms, roundworms, and hookworms. This phylum is called Nematoda. Threadworms may be microscopic, such as the vinegar eelworm, or more than 1 metre (3 feet) long. They are found in damp earth, moss, decaying substances, fresh water, or salt water. Some roundworms are also parasites. The Guinea worm, for example, gets under the skin of the feet and legs of people living in tropical countries.
- The third group consists of the segmented worms, with bodies divided into segments, or rings. This phylum is called Annelida. Among these are the earthworms and the bristle worms of the sea.
In earlier taxonomic classification, all the above were included in the now obsolete group Vermes.
There are hundreds of thousands of species that live in a wide variety of habitats other than soil. Over time this broad definition narrowed to the modern definition, although this still includes several different animal groups. Phyla that include worms include:
- Annelida (segmented worms)
- Arthropoda (inchworms, sometimes called "canker worms")
- Chaetognatha (arrow worms)
- Gnathostomulid (jaw worms)
- Hemichordata (acorn/tongue worms)
- Nematoda (roundworms)
- Nematomorpha (horsehair worms)
- Nemertea (ribbonworms)
- Onychophora (velvet worms)
- Phoronida (horseshoe worms)
- Platyhelminthes (flatworms)
- Priapulida (phallus worms)
- Sipuncula (peanut worms)
To most people the most familiar worms are the earthworms, members of phylum Annelida. Earthworms in general have been around for 120 million years. They enrich and aerate the soil; Charles Darwin found that worms turn over the top six inches (15 cm) of topsoil every 20 years in a field near where he lived. In less favourable conditions, such as waterlogged soils or hard soil deficient in suitable organic matter, or in seasonally dry soils, they cannot achieve anything like that. They lack a brain in the sense of the vertebrate brain, but have nerve centers (called ganglia); they also lack eyes but can sense light with photoreceptors. Worms are hermaphrodites (both sexes in one animal) but can cross fertilize.
Other invertebrate groups may be called worms, especially colloquially. In particular, many unrelated insect larvae are called "worms", such as the railroad worm, woodworm, glowworm, bloodworm, inchworm, mealworm, silkworm, Wooly bear worm.
Worms may also be called helminths, particularly in medical 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 it is infested with parasitic worms, typically roundworms or tapeworms.
"Ringworm" is not a worm at all, but a skin fungus.
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