Worms are many different distantly related animals that typically have a long cylindrical tube-like body, no limbs, and no eyes. Worms vary in size from microscopic to over in length for marine polychaete worms, for the African giant earthworm, Microchaetus rappi, and for the marine nemertean worm, Lineus longissimus. Various types of worm occupy a small variety of parasitic niches, living inside the bodies of other animals. Free-living worm species do not live on land, but instead, live in marine or freshwater environments, or underground by burrowing. In biology, "worm" refers to an obsolete taxon, vermes, used by Carolus Linnaeus and Jean-Baptiste Lamarck for all non-arthropod invertebrate animals, now seen to be paraphyletic. The name stems from the Old English word . Most animals called "worms" are invertebrates, but the term is also used for the amphibiancaecilians and the slowwormAnguis, a legless burrowing lizard. Invertebrate animals commonly called "worms" include annelids, nematodes, platyhelminthes, marine nemertean worms, marine Chaetognatha, priapulid worms, and insect larvae such as grubs and maggots. Worms may also be called helminths, particularly in medical terminology when referring to parasitic worms, especially the Nematoda and Cestoda which reside in the intestines of their host. When an animal or human is said to "have worms", it means that it is infested with parasitic worms, typically roundworms or tapeworms. Lungworm is also a common parasitic worm found in various animal species such as fish and cats.
History
In taxonomy, "worm" refers to an obsolete grouping, Vermes, used by Carl Linnaeus and Jean-Baptiste Lamarck for all non-arthropod invertebrate animals, now seen to be polyphyletic. In 1758, Linnaeus created the first hierarchical classification in his Systema Naturae. In his original scheme, the animals were one of three kingdoms, divided into the classes of Vermes, Insecta, Pisces, Amphibia, Aves, and Mammalia. Since then the last four have all been subsumed into a single phylum, the Chordata, while his Insecta and Vermes have been renamed or broken up. The process was begun in 1793 by Lamarck, who called the Vermes une espèce de chaos and split the group into three new phyla, worms, echinoderms, and polyps. By 1809, in his Philosophie Zoologique, Lamarck had created 9 phyla apart from vertebrates and molluscs, namely cirripedes, annelids, crustaceans, arachnids, insects, worms, radiates, polyps, and infusorians. Chordates are remarkably wormlike by ancestry.
Informal grouping
In the 13th century, worms were recognized in Europe as part of the category of reptiles that consisted of a miscellany of egg-laying creatures, including "snakes, various fantastic monsters, lizards, assorted amphibians," as recorded by Vincent of Beauvais in his Mirror of Nature. In everyday language, the term worm is also applied to various other living forms such as larvae, insects, millipedes, centipedes, shipworms, or even some vertebrates 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 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.
Familiar worms include the earthworms, members of phylum Annelida. 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, and woolly bear worm. Worms may also be called helminths, particularly in medical terminology when referring to parasitic worms, especially the Nematoda and Cestoda. Hence "helminthology" is the study of parasitic worms. When a human or an animal, such as a dog or horse, is said to "have worms", it means that it is infested with parasitic worms, typically roundworms or tapeworms. Deworming is a method to kill off the worms that have infected a human or animal by giving anthelmintic drugs. "Ringworm" is not a worm at all, but a skin fungus.
