The nematodes (pronounced /ˈnɛməˌtoʊdz/) or roundworms (phylum In biology, a phylum [note 1] is a taxonomic rank below Kingdom and above Class. "Phylum" is equivalent to the botanical term division Nematoda) are the most diverse phylum In biology, a phylum [note 1] is a taxonomic rank below Kingdom and above Class. "Phylum" is equivalent to the botanical term division. Phylum is one of the major biological divisions called Taxa. Although "phylum" is often used as if it were a clearly defined term, no satisfactory definition of it exists. In fact, "phylum& of pseudocoelomates By the broadest definition, a body cavity is any fluid filled space in a multicellular organism. However, the term usually refers to the space, located between an animal’s outer covering and the outer lining of the gut cavity, where internal organs develop. "The body cavity" of the human body cavities normally refers to the ventral, and one of the most diverse of all animals Animals are a major group of mostly multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and independently. All animals are also. Nematode species In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are often used, such as similarity of DNA, morphology or are very difficult to distinguish; over 28,000 have been described,^[1] of which over 16,000 are parasitic Parasitism is a type of symbiotic relationship between organisms of different species where one organism, the parasite, benefits at the expense of the host. It has been estimated that the total number of nematode species might be approximately 1,000,000.^[2] Unlike cnidarians Cnidaria is a phylum containing over 9,000 species of animals found exclusively in aquatic and mostly marine environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance, sandwiched between two layers of epithelium that are or flatworms The flatworms, known in scientific literature as Platyhelminthes or Plathelminthes (from the Greek πλατύ, platy, meaning "flat" and ἕλμινς , helminth-, meaning worm) are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals. Unlike other bilaterians, they have no body cavity, and no, roundworms have a digestive system In mammals, food enters the mouth, being chewed by teeth, with chemical processing beginning with chemicals in the saliva from the salivary glands. Then it travels down the esophagus into the stomach, where hydrochloric acid kills most contaminating microorganisms and begins mechanical break down of some food , and chemical alteration of some. The that is like a tube with openings at both ends.

Habitats

Nematodes have successfully adapted to nearly every ecosystem An ecosystem consists of all the organisms living in a particular area, as well as all the nonliving, physical components of the environment with which the organisms interact, such as air, soil, water, and sunlight. It is all the organisms in a given area, along with the nonliving factors with which they interact; a biological community and its from marine to fresh water, from the polar regions to the tropics, as well as the highest to the lowest of elevations. They are ubiquitous in freshwater, marine, and terrestrial environments, where they often outnumber other animals in both individual and species In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are often used, such as similarity of DNA, morphology or counts, and are found in locations as diverse as Guam and oceanic trenches Trenches define one of the most important natural boundaries on the Earth’s solid surface: the one between two lithospheric plates. There are three types of lithospheric plate boundaries: divergent , convergent (where one lithospheric plate sinks beneath another and returns to the mantle), and transform (where two lithospheric plates slide past. They represent, for example, 90% of all life on the seafloor of the Earth.^[3] Their many parasitic forms include pathogens A pathogen, (from Greek πάθος pathos "suffering, passion", and γἰγνομαι gignomai (gen-) "I give birth to") an infectious agent, or more commonly germ, is a biological agent that causes disease to its host. There are several substrates and pathways whereby pathogens can invade a host; the principal pathways have in most plants and animals (including humans Humans, known taxonomically as Homo sapiens , are the only living species in the Homo genus of bipedal primates in Hominidae, the great ape family. However, in some cases "human" is used to refer to any member of the genus Homo). Some nematodes can undergo cryptobiosis Cryptobiosis is an ametabolic state of life entered by an organism in response to adverse environmental conditions such as desiccation, freezing, and oxygen deficiency. In the cryptobiotic state, all metabolic procedures stop, preventing reproduction, development, and repair. An organism in a cryptobiotic state can essentially live indefinitely.

Taxonomy and systematics

The group was originally defined by Karl Rudolphi in 1808^[4] under the name Nematoidea, from Ancient Greek Ancient Greek is the historical stage in the development of the Greek language spanning the Archaic , Classical (c. 5th–4th centuries BC), and Hellenistic (c. 3rd century BC – 6th century AD) periods of ancient Greece and the ancient world. It is predated in the 2nd millennium BC by Mycenaean Greek. Its Hellenistic phase is known as Koine (& νῆμα (nêma, nêmatos, 'thread') and -eiδἠς (-eidēs, 'like'). The vernacular A vernacular, mother tongue or mother language, and less frequently one sense of idiom and dialect, is the native language of a population located in a country or in a region defined on some other basis, such as a locality. For example, Navajo is a local language in the southwest of the United States, and English is the state language of a number word "nematode" is a corruption of this taxon A taxon is a group of (one or more) organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement. Defining what belongs or does not belong to such a taxonomic group is done by a taxonomist. It is not uncommon for one taxonomist to disagree with another on what exactly belongs to, reclassified as family What does and does not belong to each family is determined by a taxonomist. Similarly for the question if a particular family should be recognized at all. Often there is no exact agreement, with different taxonomists each taking a different position. There are no hard rules that a taxonomist needs to follow in describing or recognizing a family Nematodes by Burmeister in 1837^[4] and order Nematoda by K. M. Diesing in 1861.^[4]

At the origin, the "Nematoidea" included both roundworms and horsehair worms. Along with Acanthocephala, Trematoda and Cestoidea, it formed the group Entozoa.^[5] The first differentiation of roundworms from horsehair worms Nematomorpha are a phylum of parasitic animals that are morphologically and ecologically similar to nematode worms, hence the name. They range in size in most species from 50 to 100 centimetres (20 to 39 in) long and can reach in extreme cases up to 2 meters, and 1 to 3 millimetres (0.039 to 0.12 in) in diameter. Horsehair worms can be discovered, though erroneous, is due to von Siebold (1843) with orders Nematoidea and Gordiacei (Gordiacea). They were classed along with Acanthocephala in the new phylum Nemathelminthes (today obsolete) by Gegenbaur (1859). Then the taxon Nematoidea has been promoted to the rank of phylum by Ray Lankester (1877) including the family Gordiidae (horsehair worms). In 1919, Nathan Cobb proposed that roundworms should be recognized alone as a phylum. He argued that they should be called nema(s) in English rather than "nematodes"^[6] and defined the taxon Nemates (Latin plural of nema). For ITIS, the taxon Nematoda is invalid.^[7] Since Cobb was the first to exclude all but nematodes from the group, the valid taxon should be Nemates Cobb 1919 or Nemata Cobb 1919.

Phylogeny

The relationships of the nematodes and their close relatives among the protostomian Protostomia are a clade of animals. Together with the deuterostomes and a few smaller phyla, they make up the Bilateria, mostly comprising animals with bilateral symmetry and three germ layers. The major distinctions between deuterostomes and protostomes are found in embryonic development Metazoa Animals are a major group of mostly multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and independently. All animals are also are unresolved. Traditionally, they were held to be a lineage of their own, but in the 1990s it was proposed that they form a clade A clade[note 1] is a group consisting of an organism and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological classification. In together with moulting In biology, moulting signifies the manner in which an animal routinely casts off a part of its body (often but not always an outer layer or covering), either at specific times of year, or at specific points in its life-cycle animals such as arthropods An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the Phylum Arthropoda (from Greek ἄρθρον arthron, "joint", and ποδός podos "foot", which together mean "jointed feet"), and include the insects, arachnids, crustaceans, and. This group has been named Ecdysozoa The Ecdysozoa are a grouping of protostome animals, including the Arthropoda (insects, chelicerata, crustaceans, and myriapods), Nematoda, and several smaller phyla. They were first defined by Aguinaldo et al. in 1997, based mainly on trees constructed using 18S ribosomal RNA genes. A large study in 2008 by Dunn et al. strongly supported the. However, the monophyly In common cladistic usage, monophyletic describes a group of organisms that form a clade, consisting of a last common ancestor and all of its descendants. The term is not synonymous with the less common term holophyly, which does not include a last common ancestor. It is contrasted with the terms paraphyly, which is a taxonomic group consisting of of the Ecdysozoa was never unequivocally accepted: while most researchers consider at least the placement of arthropods as more distant relatives of annelids The annelids , formally called Annelida (from French annelés "ringed ones", ultimately from Latin anellus "little ring"), are a large phylum of segmented worms, with over 17,000 modern species including ragworms, earthworms and leeches. They are found in marine environments from tidal zones to hydrothermal vents, in freshwater, — with which they were formerly united — to be warranted, the presumed close relationships of the nematodes and relatives with the arthropods has been a major point of contention.^[8]

Even though the amount of data since accumulated in regard to this problem is staggering, the situation seems if anything less clear these days. DNA sequence data, initially strongly supporting the Ecdysozoa hypothesis, has become rather equivocal on ecdysozoan monophyly, and is simply unable to refute either a close or a more distant relationship between the arthropod and nematode lineages. That the roundworms have a large number of peculiar apomorphies Cladistics is a method of classifying species of organisms into groups called clades, which consist of 1) all the descendants of an ancestral organism and 2) the ancestor itself. For example, birds, dinosaurs, crocodiles, and all descendants of their most recent common ancestor form a clade. In the terms of biological systematics, a clade is a and in many cases a parasitic lifestyle confounds morphological analyses. Genetic analyses of roundworms^{[citation needed]} suggest that — as is also indicated by their unique morphological features — the group has been under intense selective Natural selection is a natural law by which genetically heritable traits become more or less common in a population over successive generations. This selection in interaction with the production of variation, the possible genetic fixation process and possibly, in several cases, whith little epigenetic process determine the evolution of the species pressure during its early radiation, resulting apparently in accelerated rates of both morphological and molecular evolution Evolution is the change in the inherited traits of a population of organisms through successive generations. This change results from interactions between processes which introduce variation into a population, and other processes which remove it. As a result, variants with particular traits become more, or less, common. A trait is a particular. Furthermore, no distinctive apomorphies of Ecdysozoa are known; even moulting has recently been confirmed to occur outside the presumed clade.^[8]

Conversely, the identity of the closest living relatives of the Nematoda has always been considered to be well resolved. Morphological characters and molecular phylogenies agree with placement of the roundworms as sister taxon to the parasitic horsehair worms (Nematomorpha); together they make up the Nematoida Nematoida is a grouping of animals, including the roundworms and horsehair worms. Together with the Scalidophora Scalidophora is a group of marine pseudocoelomate invertebrates, consisting of the three phyla Kinorhyncha, Priapulida, and Loricifera. The members of the group share a number of characteristics, including introvert larvae and moulting of the cuticle . Their closest relatives are thought to be the Panarthropoda, Nematoda and Nematomorpha; they are (formerly Cephalorhyncha), the Nematoida form the Introverta. It is entirely unclear whether the Introverta are, in turn, the closest living relatives of the enigmatic Gastrotricha The gastrotrichs are a phylum of microscopic (0.06-3.0 mm) animals abundant in fresh water and marine environments. Most fresh water species are part of the periphyton and benthos. Marine species are found mostly interstitially in between sediment particles. The Gastrotrich has the shortest life span of all animals, living for just 3 days; if so, they are considered a clade Cycloneuralia Cycloneuralia is a clade of ecdysozoan animals including the Scalidophora and the Nematoida (Nematodes, Nematomorphs). Its sister group is the panarthropoda, but there is much disagreement both between and among the available morphological and molecular data. The Cycloneuralia or the Introverta — depending on the validity of the former — are often ranked as a superphylum In biology, a phylum [note 1] is a taxonomic rank below Kingdom and above Class. "Phylum" is equivalent to the botanical term division. Phylum is one of the major biological divisions called Taxa. Although "phylum" is often used as if it were a clearly defined term, no satisfactory definition of it exists. In fact, "phylum&.^[8]

Nematode systematics

Due to the lack of knowledge regarding many nematodes, their systematics is contentious. Traditionally, they are divided into two classes The composition of each class is determined by a taxonomist. Often there is no exact agreement, with different taxonomists taking different positions. There are no hard rules that a taxonomist needs to follow in describing a class, but for well-known animals there is likely to be consensus. For example, dogs are usually assigned to the phylum, the Adenophorea Onchocerca volvulus · Loa loa (Loa loa filariasis) · Mansonella (Mansonelliasis) · Dirofilaria repens (Dirofilariasis) and the Secernentea Secernentea are the main class of nematodes, characterised by numerous caudal papillae and an excretory system possessing lateral canals. Like all nematodes, they have no circulatory or respiratory system, and initial DNA sequence studies^{[verification needed]} suggested the existence of five clades A clade[note 1] is a group consisting of an organism and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological classification. In:^[9]

• Dorylaimia
• Enoplia
• Spirurina
• Tylenchina
• Rhabditina

As it seems, the Secernentea Secernentea are the main class of nematodes, characterised by numerous caudal papillae and an excretory system possessing lateral canals. Like all nematodes, they have no circulatory or respiratory system are indeed a natural group of closest relatives. But the "Adenophorea" appear to be a paraphyletic A group of taxa is said to be paraphyletic if the group contains its last common ancestor but does not contain all the descendants of that ancestor. This term is used in both phylogenetics[note 1] and linguistics assemblage of roundworms simply retaining a good number of ancestral traits In cladistics, a symplesiomorphy or symplesiomorphic character is a trait which is shared between two or more taxa, but which is also shared with other taxa which have an earlier last common ancestor with the taxa under consideration. They are therefore not an indication that the taxa be considered more closely related to each other than to the. The old Enoplia do not seem to be monophyletic either but to contain two distinct lineages. The old group "Chromadoria" seem to be another paraphyletic assemblage, with the Monhysterida representing a very ancient minor group of nematodes. Among the Secernentea, the Diplogasteria may need to be united with the Rhabditia. while the Tylenchia might be paraphyletic with the Rhabditia.^[10]

The understanding of roundworm systematics and phylogeny as of 2002 is summarised below:

Phylum Nematoda

• Basal In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram order Monhysterida
• Class Dorylaimea
• Class Enoplea
• Class Secernentea Secernentea are the main class of nematodes, characterised by numerous caudal papillae and an excretory system possessing lateral canals. Like all nematodes, they have no circulatory or respiratory system
  • Subclass Diplogasteria (disputed)
  • Subclass Rhabditia (paraphyletic?)
  • Subclass Spiruria
  • Subclass Tylenchia (disputed)
• "Chromadorea" assemblage

Anatomy

Nematodes are slender, worm-like animals, typically less than 2.5 millimetres (0.10 in) long. The smallest nematodes are microscopic, while free-living species can reach as much as 5 centimetres (2.0 in) and some parasitic species are larger still. The body is often ornamented with ridges, rings, warts, bristles or other distinctive structures.^[11]

The head of a nematode is relatively distinctive. Whereas the rest of the body is bilaterally symmetrical, the head is radially symmetrical, with sensory bristles and, in many cases, solid head-shields radiating outwards around the mouth. The mouth has either three or six lips, which often bear a series of teeth on their inner edge. An adhesive caudal gland is often found at the tip of the tail.^[11]

The epidermis is either a syncytium In biology, a syncytium is a large cell-like structure filled with cytoplasm containing many nuclei. Most cells in eukaryotic organisms have a single nucleus; syncytia are specialized forms used by various organisms or a single layer of cells, and is covered by a thick collagenous Collagen is a group of naturally occurring proteins. In nature, it is found exclusively in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content. Collagen, in the form of elongated cuticle. The cuticle is often of complex structure, and may have two or three distinct layers. Underneath the epidermis lies a layer of muscle cells. Projections run from the inner surface of these cells towards the nerve cords; this is a unique arrangement in the animal kingdom, in which nerve cells normally extend fibres into the muscles rather than vice versa.^[11]

The muscle layer surrounds the body cavity, which is filled with a fluid that lacks any form of blood cells. The gut runs down the centre of the cavity.^[11]

Digestive system

The oral cavity is lined with cuticle, which is often strengthened with ridges or other structures, and, especially in carnivorous species, may bear a number of teeth. The mouth often includes a sharp stylet which the animal can thrust into its prey. In some species, the stylet is hollow, and can be used to suck liquids from plants or animals.^[11]

The oral cavity opens into a muscular sucking pharynx The pharynx is the part of the neck and throat situated immediately posterior to (behind) the mouth and nasal cavity, and cranial, or superior, to the esophagus, larynx, and trachea, also lined with cuticle. Digestive glands are found in this region of the gut, producing enzymes that start to break down the food. In stylet-bearing species, these may even be injected into the prey.^[11]

There is no stomach, with the pharynx connecting directly to the intestine that forms the main length of the gut. This produces further enzymes, and also absorbs nutrients through its lining. The last portion of the intestine is lined by cuticle, forming a rectum which expels waste through the anus just below and in front of the tip of the tail. The intestine also has valves or sphincters at either end to help control the movement of food through the body.^[11]

Excretory system

Nitrogenous waste is excreted in the form of ammonia through the body wall, and is not associated with any specific organs. However, the structures for excreting salt to maintain osmoregulation are typically more complex.^[11]

In many marine nematodes, there are one or two unicellular renette glands that excrete salt through a pore on the underside of the animal, close to the pharynx. In most other nematodes, these specialised cells have been replaced by an organ consisting of two parallel ducts connected by a single transverse duct. This transverse duct opens into a common canal that runs to the excretory pore.^[11]

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