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31-541: Worms are many different distantly related bilateral animals that typically have a long cylindrical tube-like body, no limbs , and usually no eyes . Worms vary in size from microscopic to over 1 metre (3.3 ft) in length for marine polychaete worms (bristle worms); 6.7 metres (22 ft) for the African giant earthworm, Microchaetus rappi ; and 58 metres (190 ft) for the marine nemertean worm (bootlace worm), Lineus longissimus . Various types of worm occupy

62-587: A backbone) such as blindworms and caecilians . Worms include several groups. The three main phyla are: 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 , butterworm , inchworm , mealworm , silkworm , and woolly bear worm . Worms may also be called helminths , particularly in medical terminology when referring to parasitic worms , especially

93-583: A distinct environment, larvae may be given shelter from predators and reduce competition for resources with the adult population. Animals in the larval stage will consume food to fuel their transition into the adult form. In some organisms like polychaetes and barnacles , adults are immobile but their larvae are mobile, and use their mobile larval form to distribute themselves. These larvae used for dispersal are either planktotrophic (feeding) or lecithotrophic (non-feeding) . Some larvae are dependent on adults to feed them. In many eusocial Hymenoptera species,

124-446: A head (anterior) end and a tail (posterior) end as well as a back (dorsal) and a belly (ventral); therefore they also have a left side and a right side. Having a front end means that this part of the body encounters stimuli, such as food, favouring cephalisation , the development of a head with sense organs and a mouth. The body stretches back from the head, and many bilaterians have a combination of circular muscles that constrict

155-425: A left and a right side that are mirror images of each other) during embryonic development . This means their body plans are laid around a longitudinal axis ( rostral – caudal axis) with a front (or "head") and a rear (or "tail") end, as well as a left–right–symmetrical belly ( ventral ) and back ( dorsal ) surface. Nearly all bilaterians maintain a bilaterally symmetrical body as adults; the most notable exception

186-488: A pejorative epithet to describe a cowardly, weak or pitiable person. Worms can also be farmed for the production of nutrient-rich vermicompost . Bilateria Triploblasts Lankester, 1873 Bilateria ( / ˌ b aɪ l ə ˈ t ɪər i ə / BY -lə- TEER -ee-ə ) is a large clade or infrakingdom of animals called bilaterians ( / ˌ b aɪ l ə ˈ t ɪər i ə n / BY -lə- TEER -ee-ən ), characterized by bilateral symmetry (i.e. having

217-429: 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

248-561: Is generally very different from the adult form ( e.g. caterpillars and butterflies ) including different unique structures and organs that do not occur in the adult form. Their diet may also be considerably different. In the case of smaller primitive arachnids, the larval stage differs by having three instead of four pairs of legs. Larvae are frequently adapted to different environments than adults. For example, some larvae such as tadpoles live almost exclusively in aquatic environments, but can live outside water as adult frogs . By living in

279-669: Is not a worm at all, but a skin fungus. Lobopodians are an informal grouping of extinct panarthropods from the Cambrian to the Carboniferous that are often called worms or "worm-like animals" despite having had legs in the form of stubby lobopods. Likewise, the extant Onychophora are sometimes called velvet worms despite possessing stubby legs. Wyrm was the Old English term for carnivorous reptiles ("serpents") and mythical dragons . "Worm" has also been used as

310-458: Is sometimes used to refer to parasitic worms. The term is more commonly used in medicine , and usually refers to roundworms and tapeworms. 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,

341-597: Is termed the " Urbilaterian ". The nature of the first bilaterian is a matter of debate. One side suggests that acoelomates gave rise to the other groups (planuloid–aceloid hypothesis by Ludwig von Graff , Elie Metchnikoff , Libbie Hyman , or Luitfried von Salvini-Plawen  [ nl ] ), while the other poses that the first bilaterian was a coelomate organism and the main acoelomate phyla ( flatworms and gastrotrichs ) have lost body cavities secondarily (the Archicoelomata hypothesis and its variations such as

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372-778: Is that the Ambulacraria are sister to Xenacoelomorpha together forming the Xenambulacraria . The Xenambulacraria may be sister to the Chordata or to the Centroneuralia (corresponding to Nephrozoa without Ambulacraria, or to Chordata + Protostomia). The phylogenetic tree shown below depicts the latter proposal. Also, the validity of Deuterostomia (without Protostomia emerging from it) is under discussion. The cladogram indicates approximately when some clades radiated into newer clades, in millions of years ago (Mya). While

403-555: Is the echinoderms , which extend to pentaradial symmetry as adults, but are only bilaterally symmetrical as an embryo . Cephalization is also a characteristic feature among most bilaterians, where the special sense organs and central nerve ganglia become concentrated at the front/rostral end. Bilaterians constitute one of the five main metazoan lineages, the other four being Porifera ( sponges ), Cnidaria ( jellyfish , hydrae , sea anemones and corals ), Ctenophora ( comb jellies ) and Placozoa (tiny "flat animals"). For

434-467: The Nematoda (roundworms) and Cestoda (tapeworms). 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 "

465-411: The Old English word wyrm . Most animals called "worms" are invertebrates , but the term is also used for the amphibian caecilians and the slowworm Anguis , a legless burrowing lizard . Invertebrate animals commonly called "worms" include annelids , nematodes , flatworms , nemerteans , chaetognaths , priapulids , and insect larvae such as grubs and maggots . The term " helminth "

496-416: The blastocoel , as pseudocoeloms ) or secondary cavities (that appear de novo , for example the coelom ). Some of the earliest bilaterians were wormlike, and a bilaterian body can be conceptualized as a cylinder with a gut running between two openings, the mouth and the anus. Around the gut it has an internal body cavity, a coelom or pseudocoelom. Animals with this bilaterally symmetric body plan have

527-450: The echinoderms , hemichordates , chordates , and the extinct Vetulicolia . The protostomes include most of the rest, such as arthropods , annelids , mollusks , flatworms , and so forth. There are several differences, most notably in how the embryo develops. In particular, the first opening of the embryo becomes the mouth in protostomes, and the anus in deuterostomes. Many taxonomists now recognize at least two more superphyla among

558-548: 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 (teredo worms), or even some vertebrates (creatures with

589-591: The Gastrea by Haeckel or Sedgwick , the Bilaterosgastrea by Gösta Jägersten  [ sv ] , or the Trochaea by Nielsen). One hypothesis is that the original bilaterian was a bottom dwelling worm with a single body opening, similar to Xenoturbella . Alternatively, it may have resembled the planula larvae of some cnidaria, which have some bilateral symmetry. However, there is evidence that it

620-1541: The Nephrozoa. Subsequently the acoelomorphs were placed in phylum Xenacoelomorpha , together with the xenoturbellids , and the sister relationship between Xenacoelomorpha and Nephrozoa confirmed in phylogenomic analyses. A modern consensus phylogenetic tree for Bilateria is shown below, although the positions of certain clades are still controversial (dashed lines) and the tree has changed considerably since 2000. Cnidaria [REDACTED] Placozoa [REDACTED] Proarticulata ? † [REDACTED] Xenoturbellida [REDACTED] Nemertodermatida Acoela [REDACTED] Echinodermata [REDACTED] Hemichordata [REDACTED] † Cambroernida [REDACTED] Cephalochordata [REDACTED] Tunicata [REDACTED] Craniata / Vertebrata [REDACTED] † Saccorhytus coronarius † Vetulocystids [REDACTED] † Vetulicolians [REDACTED] Nematoda [REDACTED] Nematomorpha [REDACTED] Loricifera [REDACTED] Onychophora [REDACTED] Tardigrada [REDACTED] Arthropoda [REDACTED] Priapulida [REDACTED] Kinorhyncha [REDACTED] Rotifera and allies [REDACTED] Chaetognatha [REDACTED] Platyhelminthes and allies [REDACTED] Mollusca [REDACTED] Annelida and allies [REDACTED] ¿† Kimberella ? [REDACTED] Kimberella ? † [REDACTED] A different hypothesis

651-550: 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 (which included the crustaceans and arachnids) and Vermes have been renamed or broken up. The process was begun in 1793 by Lamarck, who called the Vermes une espèce de chaos (a sort of chaos) and split

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682-522: The below tree depicts Chordata as a sister group to Protostomia according to analyses by Philippe et al., the authors nonetheless caution that "the support values are very low, meaning there is no solid evidence to refute the traditional protostome and deuterostome dichotomy". Cnidaria [REDACTED] Placozoa [REDACTED] † Proarticulata Xenoturbellida [REDACTED] Nemertodermatida Acoela [REDACTED] Echinodermata [REDACTED] Larva A larva's appearance

713-739: The body, making it longer, and an opposing set of longitudinal muscles, that shorten the body; these enable soft-bodied animals with a hydrostatic skeleton to move by peristalsis . Most bilaterians (nephrozoans) have a gut that extends through the body from mouth to anus, while xenacoelomorphs have a bag gut with one opening. Many bilaterian phyla have primary larvae which swim with cilia and have an apical organ containing sensory cells. However, there are exceptions to each of these characteristics; for example, adult echinoderms are radially symmetric (unlike their larvae), and certain parasitic worms have extremely plesiomorphic body structures. The hypothetical most recent common ancestor of all bilateria

744-697: The earliest known bilaterian, but may also represent an infilled bubble. Fossil embryos are known from around the time of Vernanimalcula ( 580  million years ago ), but none of these have bilaterian affinities. Burrows believed to have been created by bilaterian life forms have been found in the Tacuarí Formation of Uruguay, and were believed to be at least 585 million years old. However, more recent evidence shows these fossils are actually late Paleozoic instead of Ediacaran. The Bilateria has traditionally been divided into two main lineages or superphyla . The deuterostomes traditionally include

775-440: The group into three new phyla, worms, echinoderms, and polyps (which contained corals and jellyfish). By 1809, in his Philosophie Zoologique , Lamarck had created 9 phyla apart from vertebrates (where he still had 4 phyla: mammals, birds, reptiles, and fish) and molluscs, namely cirripedes , annelids, crustaceans, arachnids, insects, worms, radiates , polyps, and infusorians . Chordates are remarkably wormlike by ancestry. In

806-407: The larvae are fed by female workers. In Ropalidia marginata (a paper wasp) the males are also capable of feeding larvae but they are much less efficient, spending more time and getting less food to the larvae. The larvae of some organisms (for example, some newts ) can become pubescent and do not develop further into the adult form. This is a type of neoteny . It is a misunderstanding that

837-525: The larval form always reflects the group's evolutionary history . This could be the case, but often the larval stage has evolved secondarily, as in insects. In these cases , the larval form may differ more than the adult form from the group's common origins. Within Insects , only Endopterygotes show complete metamorphosis, including a distinct larval stage. Several classifications have been suggested by many entomologists , and following classification

868-441: The most part, bilateral embryos are triploblastic , having three germ layers : endoderm , mesoderm and ectoderm . Except for a few phyla (i.e. flatworms and gnathostomulids ), bilaterians have complete digestive tracts with a separate mouth and anus . Some bilaterians lack body cavities ( acoelomates , i.e. Platyhelminthes , Gastrotricha and Gnathostomulida ), while others display primary body cavities (deriving from

899-522: The protostomes, Ecdysozoa (molting animals) and Spiralia . The arrow worms ( Chaetognatha ) have proven difficult to classify; recent studies place them in the Gnathifera . The traditional division of Bilateria into Deuterostomia and Protostomia was challenged when new morphological and molecular evidence found support for a sister relationship between the acoelomate taxa, Acoela and Nemertodermatida (together called Acoelomorpha ), and

930-443: The remaining bilaterians. The latter clade was called Nephrozoa by Jondelius et al. (2002) and Eubilateria by Baguña and Riutort (2004). The acoelomorph taxa had previously been considered flatworms with secondarily lost characteristics, but the new relationship suggested that the simple acoelomate worm form was the original bilaterian body plan and that the coelom, the digestive tract, excretory organs, and nerve cords developed in

961-470: Was segmented, as the mechanism for creating segments is shared between vertebrates (deuterostomes) and arthropods (protostomes). The first evidence of bilateria in the fossil record comes from trace fossils in Ediacaran sediments, and the first bona fide bilaterian fossil is Kimberella , dating to 555  million years ago . Earlier fossils are controversial; the fossil Vernanimalcula may be

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