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The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the
phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number. The plural of a noun typically denotes a q ...
Nematoda (also called Nemathelminthes), with plant-parasitic nematodes also known as eelworms. They are a diverse animal phylum inhabiting a broad range of environments. Taxonomically, they are classified along with
insect Insects (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be in ...

insect
s and other
moulting In biology, moulting (British English), or molting (American English), also known as sloughing, shedding, or in many invertebrates, ecdysis, is the manner in which an animal routinely casts off a part of its body (often, but not always, an outer ...
animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells ...

animal
s in the
clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic – that is, composed of a common ancestor and all its lineage (evolution), lineal descendants - on a phylogenetic tree. R ...

clade
Ecdysozoa Ecdysozoa () is a group of protostome Protostomia () is the clade A clade (), also known as a monophyletic group or natural group, is a group of organism In biology, an organism () is any organic, life, living system that functio ...

Ecdysozoa
, and unlike
flatworm The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a count ...
s, have tubular
digestive system The human digestive system consists of the human gastrointestinal tract, gastrointestinal tract plus the accessory organs of digestion (the tongue, salivary glands, pancreas, liver, and gallbladder). Digestion involves the breakdown of food in ...
s with openings at both ends. Like
tardigrade Tardigrades (), known colloquially as water bears or moss piglets, are a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatica ...

tardigrade
s, they have a reduced number of
Hox gene Hox genes, a subset of homeobox genes, are a group of related genes that specify regions of the body plan of an embryo An embryo is the early stage of development of a multicellular organism. In general, in organism In biology, an ...
s, but as their sister phylum
Nematomorpha Nematomorpha (sometimes called Gordiacea, and commonly known as horsehair worms, hairsnakes, or Gordian worms) are a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languag ...
has kept the ancestral protostome Hox genotype, it shows that the reduction has occurred within the nematode phylum. Nematode
species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individu ...

species
can be difficult to distinguish from one another. Consequently, estimates of the number of nematode species described to date vary by author and may change rapidly over time. A 2013 survey of animal biodiversity published in the
mega journalA mega journal (also mega-journal and megajournal) is a peer-reviewed academic journal, academic open access journal designed to be much larger than a traditional journal by exercising low selectivity among accepted articles. It was pioneered by ''PL ...
''
Zootaxa ''Zootaxa'' is a peer-reviewed Peer review is the evaluation of work by one or more people with similar competencies as the producers of the work ( peers). It functions as a form of self-regulation by qualified members of a profession within t ...
'' puts this figure at over 25,000. Estimates of the total number of
extant Extant is the opposite of the word extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the endling, last individual o ...
species are subject to even greater variation. A widely referenced article published in 1993 estimated there may be over 1 million species of nematode. A subsequent publication challenged this claim, estimating the figure to be at least 40,000 species. Although the highest estimates (up to 100 million species) have since been deprecated, estimates supported by rarefaction curves, together with the use of
DNA barcoding DNA barcoding is a method of species identification using a short section of DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consi ...
and the increasing acknowledgment of widespread
cryptic species In biology, a species complex is a group of closely related organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties ...
among nematodes, have placed the figure closer to 1 million species. Nematodes have successfully adapted to nearly every
ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syst ...

ecosystem
: from marine (salt) to fresh water, soils, from the polar regions to the tropics, as well as the highest to the lowest of elevations (including mountains). 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 the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individu ...

species
counts, and are found in locations as diverse as mountains, deserts, and
oceanic trench , while the lithosphere is subducted back into the asthenosphere at trenches Oceanic trenches are prominent long, narrow topography, topographic depressions of the ocean floor. They are typically wide and below the level of the surrounding ocean ...
es. They are found in every part of the earth's
lithosphere A lithosphere ( grc, λίθος [] for "rocky", and [] for "sphere") is the rigid, outermost shell of a terrestrial planet, terrestrial-type planet or natural satellite. On Earth, it is composed of the crust (geology), crust and the portion o ...
, even at great depths, below the surface of the Earth in gold mines in South Africa. They represent 90% of all animals on the
ocean floor The ocean (also the sea or the world ocean) is the body of salt water which covers approximately 71% of the surface of the Earth.
ocean floor
. In total, 4.4 × 1020 nematodes inhabit the Earth's topsoil, or approximately 60 billion for each human, with the highest densities observed in tundra and boreal forests. Their numerical dominance, often exceeding a million individuals per square meter and accounting for about 80% of all individual animals on earth, their diversity of lifecycles, and their presence at various trophic levels point to an important role in many ecosystems. They have been shown to play crucial roles in polar ecosystems. The roughly 2,271 
genera Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxonomy), circumscribing) and classifying gr ...
are placed in 256 
families In human society, family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of families is to maintain the w ...
. The many parasitic forms include
pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a Germ theory ...
s in most plants and animals. A third of the genera occur as
parasite Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives on or inside another organism, the Host (biology), host, causing it some harm, and is adaptation (biology), adapted structurally to this w ...
s of
vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an indiv ...
s; about 35 nematode species occur in humans.
Nathan Cobb Nathan Augustus Cobb (30 June 1859, in Spencer, Massachusetts – 4 June 1932, in Baltimore, Maryland Baltimore ( , locally: ) is the List of municipalities in Maryland, most populous city in the U.S. state of Maryland, as well as List of Uni ...
, a
nematologist Nematology is the scientific discipline concerned with the study of nematodes, or roundworms. Although nematological investigation dates back to the days of Aristotle or even earlier, nematology as an independent discipline has its recognizable b ...
, described the ubiquity of nematodes on Earth as thus:
In short, if all the matter in the universe except the nematodes were swept away, our world would still be dimly recognizable, and if, as disembodied spirits, we could then investigate it, we should find its mountains, hills, vales, rivers, lakes, and oceans represented by a film of nematodes. The location of towns would be decipherable since, for every massing of human beings, there would be a corresponding massing of certain nematodes. Trees would still stand in ghostly rows representing our streets and highways. The location of the various plants and animals would still be decipherable, and, had we sufficient knowledge, in many cases even their species could be determined by an examination of their erstwhile nematode parasites.


Etymology

The word ''nematode'' comes from the
Modern Latin New Latin (also called Neo-Latin or Modern Latin) is the List of revived languages, revival of Latin used in original, scholarly, and scientific works since about 1500. Modern scholarly and technical nomenclature, such as in zoological and botan ...
compound of ''nemat-'' "thread" (from Greek ''nema'', genitive ''nematos'' "thread," from stem of ''nein'' "to spin"; see ''needle'') + ''-odes'' "like, of the nature of" (see ''-oid'').


Taxonomy and systematics


History

In 1758, Linnaeus described some nematode genera (e.g., ''
Ascaris ''Ascaris'' is a genus of parasite, parasitic nematode worms known as the "small intestinal roundworms", which is a type of parasitic worm. One species, ''Ascaris lumbricoides'', affects humans and causes the disease ascariasis. Another species, ...

Ascaris
''), then included in the
Vermes Vermes ("worm Worms are many different distantly related bilateral animals that typically have a long cylindrical tube-like body, no limb Limb can refer to: *Limb (anatomy), an appendage of a human or animal *Limb Music, a record label *Li ...

Vermes
. The name of the group Nematoda, informally called "nematodes", came from Nematoidea, originally defined by
Karl Rudolphi Karl Asmund Rudolphi (14 July 1771 – 29 November 1832) was a Swedish-born German naturalist Natural history is a domain of inquiry involving organisms, including animals, fungus, fungi, and plants, in their natural environment, leaning mor ...

Karl Rudolphi
(1808), from
Ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply as Greek (, ), refers collectively to the diale ...
νῆμα (''nêma, nêmatos'', 'thread') and -eiδἠς (''-eidēs'', 'species'). It was treated as
family In human society A society is a Social group, group of individuals involved in persistent Social relation, social interaction, or a large social group sharing the same spatial or social territory, typically subject to the same Politic ...
Nematodes by Burmeister (1837). At its origin, the "Nematoidea" erroneously included Nematodes and
Nematomorpha Nematomorpha (sometimes called Gordiacea, and commonly known as horsehair worms, hairsnakes, or Gordian worms) are a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languag ...
, attributed by von Siebold (1843). Along with
Acanthocephala Acanthocephala (Greek language, Greek , ', thorn + , ', head) is a phylum (biology), phylum of parasitic worms known as acanthocephalans, thorny-headed worms, or spiny-headed worms, characterized by the presence of an wiktionary:evert, eversibl ...

Acanthocephala
,
Trematoda Trematoda is a class within the phylum Platyhelminthes. It includes two groups of parasitic flatworms, known as flukes. They are internal parasites of molluscs Mollusca is the second-largest phylum of invertebrate animals after the Arth ...
, and Cestoidea, it formed the obsolete group Entozoa, created by Rudolphi (1808). They were also classed along with Acanthocephala in the obsolete
phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number. The plural of a noun typically denotes a q ...
Nemathelminthes by Gegenbaur (1859). In 1861, K. M. Diesing treated the group as order Nematoda. In 1877, the
taxon In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechani ...
Nematoidea, including the family Gordiidae (horsehair worms), was promoted to the rank of phylum by
Ray Lankester Sir Edwin Ray Lankester (15 May 1847 – 13 August 1929) was a British zoologist Zoology ()The pronunciation of zoology as is typically regarded as nonstandard, though it is not uncommon. is the branch of biology that studies the animal king ...

Ray Lankester
. The first clear distinction between the nemas and gordiids was realized by Vejdovsky when he named a group to contain the horsehair worms the order Nematomorpha. In 1919,
Nathan Cobb Nathan Augustus Cobb (30 June 1859, in Spencer, Massachusetts – 4 June 1932, in Baltimore, Maryland Baltimore ( , locally: ) is the List of municipalities in Maryland, most populous city in the U.S. state of Maryland, as well as List of Uni ...
proposed that nematodes should be recognized alone as a phylum. He argued they should be called "nema" in English rather than "nematodes" and defined the taxon Nemates (later emended as Nemata, Latin plural of ''nema''), listing Nematoidea ''sensu restricto'' as a synonym. However, in 1910, Grobben proposed the phylum Aschelminthes and the nematodes were included in as class Nematoda along with class Rotifera, class Gastrotricha, class Kinorhyncha, class Priapulida, and class Nematomorpha (The phylum was later revived and modified by in 1951 as Pseudoceolomata, but remained similar). In 1932, Potts elevated the class Nematoda to the level of phylum, leaving the name the same. Despite Potts' classification being equivalent to Cobbs', both names have been used (and are still used today) and Nematode became a popular term in zoological science. Since Cobb was the first to include nematodes in a particular phylum separated from Nematomorpha, some researchers consider the valid taxon name to be Nemates or Nemata, rather than Nematoda, because of the zoological rule that gives priority to the first used term in case of synonyms.


Phylogeny

The
phylogenetic In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanism ...

phylogenetic
relationships of the nematodes and their close relatives among the
protostomia Protostomia is the clade A clade (; from grc, , ''klados'', "branch"), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic—that is, composed of a common ancestor and all its lineag ...
n
Metazoa Animals (also called Metazoa) are multicellular Multicellular organisms are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the L ...

Metazoa
are unresolved. Traditionally, they were held to be a lineage of their own, but in the 1990s, they were proposed to form the group
Ecdysozoa Ecdysozoa () is a group of protostome Protostomia () is the clade A clade (), also known as a monophyletic group or natural group, is a group of organism In biology, an organism () is any organic, life, living system that functio ...

Ecdysozoa
together with
moulting In biology, moulting (British English), or molting (American English), also known as sloughing, shedding, or in many invertebrates, ecdysis, is the manner in which an animal routinely casts off a part of its body (often, but not always, an outer ...
animals, such as
arthropod An arthropod (, (gen. ποδός)) is an invertebrate animal having an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Euarthropoda,Reference showing that Euarthropoda is a phylum: ...
s. 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 a
sister taxon In phylogenetics In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiolo ...
to the parasitic
Nematomorpha Nematomorpha (sometimes called Gordiacea, and commonly known as horsehair worms, hairsnakes, or Gordian worms) are a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languag ...
; together, they make up the
Nematoida Nematoida is a grouping of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellu ...
. Along with the
Scalidophora Scalidophora is a group of marine body cavity, pseudocoelomate protostomes that was proposed on morphological grounds to unite three phylum (biology), phyla: the Kinorhyncha, the Priapulida and the Loricifera. The three phyla have four characters ...

Scalidophora
(formerly Cephalorhyncha), the Nematoida form the clade
Cycloneuralia Cycloneuralia is a clade of ecdysozoan animals including the Scalidophora (Kinorhyncha Kinorhyncha ( grc, κινέω, kīnéō, I move, ' "snout") is a phylum In biology, a phylum (; plural The plural (sometimes list of glossing ab ...
, but much disagreement occurs 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 (; plural The plural (sometimes abbreviated An abbreviation (from Latin ''brevis'', meaning ''short'') is a shortened form of a word or phrase, by any method. It may consist of a group of letters, or words taken fr ...
.


Nematode systematics

Due to the lack of knowledge regarding many nematodes, their systematics is contentious. An early and influential classification was proposed by Chitwood and Chitwood—later revised by Chitwood—who divided the phylum into two classes— Aphasmidia and Phasmidia. These were later renamed (gland bearers) and Secernentea (secretors), respectively. The Secernentea share several characteristics, including the presence of phasmids, a pair of sensory organs located in the lateral posterior region, and this was used as the basis for this division. This scheme was adhered to in many later classifications, though the Adenophorea were not in a uniform group. Initial studies of incomplete
DNA sequence DNA sequencing is the process of determining the nucleic acid sequence A nucleic acid sequence is a succession of bases signified by a series of a set of five different letters that indicate the order of nucleotides Nucleotides are organic ...
s suggested the existence of five
clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic – that is, composed of a common ancestor and all its lineage (evolution), lineal descendants - on a phylogenetic tree. R ...

clade
s: *
Dorylaimida Dorylaimida (dorylaims) is a diverse order of nematode The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-parasitic nematodes being known as e ...
*
Enoplia The Enoplia are a subclass of nematode The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-parasitic nematodes being known as eelworms. They ar ...
* Spirurina * Tylenchina * Rhabditina The Secernentea seem to be a natural group of close relatives, while the "Adenophorea" appear to be a
paraphyletic In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and all descendants of that ancestor excluding a few—typically only one or two—Monophyly, monophyletic subgroups. The group is said to be paraphyleti ...

paraphyletic
assemblage of roundworms that retain a good number of ancestral traits. The old
Enoplia The Enoplia are a subclass of nematode The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-parasitic nematodes being known as eelworms. They ar ...
do not seem to be monophyletic, either, but do contain two distinct lineages. The old group " Chromadoria" seems to be another paraphyletic assemblage, with the
Monhysterida The Monhysterida are an order in the phylum Nematoda. Usually the stoma In botany Botany, also called , plant biology or phytology, is the science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic ...
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. The understanding of roundworm systematics and
phylogeny A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram A diagram is a symbolic representation Representation may refer to: Law a ...

phylogeny
as of 2002 is summarised below: Phylum Nematoda *
Basal Basal or basilar is a term meaning ''base'', ''bottom'', or ''minimum''. Science * Basal (anatomy), an anatomical term of location for features associated with the base of an organism or structure * Basal (medicine), a minimal level that is neces ...
order
Monhysterida The Monhysterida are an order in the phylum Nematoda. Usually the stoma In botany Botany, also called , plant biology or phytology, is the science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic ...
* Class
Dorylaimida Dorylaimida (dorylaims) is a diverse order of nematode The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-parasitic nematodes being known as e ...
* Class
Enoplea Enoplea (enopleans) is a Class (biology), class, which with the classes SecernenteaTree of Life Web Project (ToL) (2002)Nematoda Version of January 1, 2002. Retrieved November 2, 2008. and Chromadorea make up the phylum Nematode, Nematoda in curr ...
* Class
Secernentea Secernentea was a class (biology), class of nematodes in the Classical Phylogeny System (Chitwood, 1958) and is no longer in use. This morphological-based classification system has been replaced by the Modern Phylogeny system, where taxonomy assig ...
** Subclass Diplogasteria (disputed) ** Subclass Rhabditia (paraphyletic?) ** Subclass Spiruria ** Subclass Tylenchia (disputed) * "
Chromadorea The Chromadorea are a class of the roundworm phylum, nematode, Nematoda. They contain a single Class (biology), subclass (Chromadoria) and several orders. With such a redundant arrangement, the Chromadoria are liable to be split up if the orders a ...
" assemblage Later work has suggested the presence of 12 clades. The Secernentea—a group that includes virtually all major animal and plant 'nematode' parasites—apparently arose from within the Adenophorea. In 2019, a study identified one conserved signature indel (CSI) found exclusively in members of the phylum Nematoda through comparative genetic analyses. The CSI consists of a single amino acid insertion within a conserved region of a Na(+)/H(+) exchange regulatory factor protein NRFL-1 and is a molecular marker that distinguishes the phylum from other species. A major effort by a collaborative wiki called 959 Nematode Genomes is underway to improve the systematics of this phylum. An analysis of the mitochondrial DNA suggests that the following groupings are valid *subclass Dorylaimia *orders
Rhabditida Rhabditida is an order of free-living, zooparasitic, and phytoparasitic microbivorous nematodes living in soil. The Cephalobidae, Panagrolaimidae, Steinernematidae, and Strongyloididae seem to be closer to the Tylenchia, regardless of whether t ...
, Trichinellida and
Mermithida Mermithida is an order (biology), order of nematode worms. The order includes two families, and most members are endoparasites on arthropods. One of the morphological characteristic of the order is the presence of a stichosome.Chitwood, B. G. & Ch ...
*suborder Rhabditina *infraorders Spiruromorpha and Oxyuridomorpha


Anatomy

Nematodes are very small, slender worms: typically about 5 to 100 µm thick, and 0.1 to 2.5 mm long. The smallest nematodes are microscopic, while free-living species can reach as much as , and some parasitic species are larger still, reaching over in length. The body is often ornamented with ridges, rings, bristles, or other distinctive structures. The head of a nematode is relatively distinct. 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 edges. An adhesive 'caudal gland' is often found at the tip of the tail. The
epidermis The epidermis is the outermost of the three layers that comprise the skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate Vertebrates () comprise all species of animal Animals (also calle ...
is either a
syncytium A syncytium or symplasm (; plural syncytia; from Greek: σύν ''syn'' "together" and κύτος ''kytos'' "box, i.e. cell") is a multinucleate cell which can result from multiple cell fusions of uninuclear cells (i.e., cells with a single nucleu ...
or a single layer of cells, and is covered by a thick
collagen Collagen () is the main structural protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowder ...

collagen
ous
cuticle A cuticle (), or cuticula, is any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticle" are non- homologous, differing in their origin, structu ...
. The cuticle is often of a complex structure and may have two or three distinct layers. Underneath the epidermis lies a layer of longitudinal
muscle Skeletal muscles (commonly referred to as muscles) are organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly cat ...

muscle
cells. The relatively rigid cuticle works with the muscles to create a hydroskeleton, as nematodes lack circumferential muscles. Projections run from the inner surface of muscle cells towards the nerve cords; this is a unique arrangement in the animal kingdom, in which nerve cells normally extend fibers into the muscles rather than ''vice versa''.


Digestive system

The oral cavity is lined with cuticle, which is often strengthened with structures, such as ridges, especially in carnivorous species, which 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. The oral cavity opens into a muscular, sucking
pharynx The pharynx (plural: pharynges) is the part of the throat behind the human mouth, mouth and nasal cavity, and above the esophagus and trachea – the tubes going down to the stomach and the lungs. It is found in vertebrates and invertebrates, thou ...

pharynx
, also lined with cuticle. Digestive glands are found in this region of the gut, producing
enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates in ...
s that start to break down the food. In stylet-bearing species, these may even be injected into the prey. No
stomach The stomach is a muscular, in the of humans and many other animals, including several s. The stomach has a dilated structure and functions as a vital organ. In the digestive system the stomach is involved in the second phase of digestion, ...

stomach
is present, with the pharynx connecting directly to a muscleless
intestine The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system The human digestive system consists of the gastrointestinal tract The gastrointestinal tract, (GI tract, GIT, d ...

intestine
that forms the main length of the gut. This produces further enzymes, and also absorbs nutrients through its single-cell-thick lining. The last portion of the intestine is lined by cuticle, forming a
rectum The rectum is the final straight portion of the large intestine The large intestine, also known as the large bowel, is the last part of the gastrointestinal tract and of the digestive system in vertebrates. Water is absorbed here and the re ...

rectum
, which expels waste through the
anus The anus (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be in ...

anus
just below and in front of the tip of the tail. The movement of food through the digestive system is the result of the body movements of the worm. The intestine has valves or
sphincter A sphincter is a circular muscle Skeletal muscles (commonly referred to as muscles) are organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A give ...
s at either end to help control the movement of food through the body.


Excretory system

Nitrogenous waste Metabolic wastes or excrements are substance Substance may refer to: * Substance (Jainism), a term in Jain ontology to denote the base or owner of attributes * Chemical substance, a material with a definite chemical composition * Matter, anything ...
is excreted in the form of
ammonia Ammonia is a chemical compound, compound of nitrogen and hydrogen with the chemical formula, formula NH3. A Binary compounds of hydrogen, stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct ch ...

ammonia
through the body wall, and is not associated with any specific organs. However, the structures for excreting salt to maintain
osmoregulation Osmoregulation is the active regulation of the osmotic pressure of an organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical ...
are typically more complex. In many marine nematodes, one or two unicellular ' renette glands' excrete salt through a pore on the underside of the animal, close to the pharynx. In most other nematodes, these specialized 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.


Nervous system

Four peripheral
nerve A nerve is an enclosed, cable-like bundle of fibers (called axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences Despite the various English dialects Dialect The term diale ...

nerve
s run along the length of the body on the dorsal, ventral, and lateral surfaces. Each nerve lies within a cord of connective tissue lying beneath the cuticle and between the muscle cells. The ventral nerve is the largest, and has a double structure forward of the excretory
pore Pore may refer to: Biology Animal biology and microbiology * Sweat pore Perspiration, also known as sweating, is the production of fluids secreted by the sweat glands in the skin Skin is the layer of usually soft, flexible outer tissue ...
. The dorsal nerve is responsible for motor control, while the lateral nerves are sensory, and the ventral combines both functions. The nervous system is also the only place in the nematode body that contains
cilia The cilium (; the plural is cilia) is an organelle In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, ...

cilia
, which are all nonmotile and with a sensory function. At the anterior end of the animal, the nerves branch from a dense, circular nerve (
nerve ringA circumesophageal or circumpharyngeal nerve ring is an arrangement of nerve ganglia A ganglion is a group of neuron cell bodies in the peripheral nervous system. In the somatic nervous system this includes Dorsal root ganglion, dorsal root gang ...
) round surrounding the pharynx, and serving as the
brain A brain is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tis ...

brain
. Smaller nerves run forward from the ring to supply the sensory organs of the head. The bodies of nematodes are covered in numerous sensory
bristle A bristle is a stiff hair Hair is a protein filament In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, ...
s and papillae that together provide a sense of touch. Behind the sensory bristles on the head lie two small pits, or ' amphids'. These are well supplied with nerve cells and are probably
chemoreception A chemoreceptor, also known as chemosensor, is a specialized sensory receptor Sensory neurons, also known as afferent neurons, are neurons in the nervous system In Biology, biology, the nervous system is a Complex system, highly complex ...
organs. A few aquatic nematodes possess what appear to be
pigment A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally dyes are often organic compound , CH4; is among the simplest organic compou ...
ed eye-spots, but whether or not these are actually sensory in nature is unclear.


Reproduction

Most nematode species are
dioecious Dioecy (; Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 1 ...
, with separate male and female individuals, though some, such as ''
Caenorhabditis elegans ''Caenorhabditis elegans'' () is a free-living transparent nematode The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-parasitic nematodes a ...

Caenorhabditis elegans
'', are
androdioeciousAndrodioecy is a sexual reproduction, reproductive system characterized by the coexistence of males and hermaphrodites. Androdioecy is rare in comparison to the other major reproductive systems: dioecy, gynodioecy and hermaphroditism. In animals, an ...
, consisting of
hermaphrodite In reproductive biology Reproductive biology includes both sexual and asexual reproduction. Reproductive biology includes a wide number of fields: * Reproductive systems * Endocrinology Endocrinology (from '' endocrine'' + '' -ology'') is a ...

hermaphrodite
s and rare males. Both sexes possess one or two tubular
gonad A gonad, sex gland, or reproductive gland is a mixed gland that produces the gametes (sex cells) and sex hormones of an organism. In the female of the species the reproductive cells are the egg cells, and in the male the reproductive cells are t ...
s. In males, the sperm are produced at the end of the gonad and migrate along its length as they mature. The testis opens into a relatively wide
seminal vesicle The seminal vesicles (also called vesicular glands, or seminal glands), are a pair of two convoluted tubular glands that lie behind the urinary bladder The urinary bladder, or simply bladder, is a hollow muscular organ in humans and other ver ...
and then during intercourse into a glandular and muscular ejaculatory duct associated with the
vas deferens The vas deferens, or ductus deferens, is part of the male reproductive system The male reproductive system consists of a number of sex organs that play a role in the process of human reproduction. These organs are located on the outside of the ...

vas deferens
and
cloaca In animal anatomy, a cloaca (plural cloacae or ) is the posterior that serves as the only opening for the , reproductive, and s (if present) of many animals. All s, s, birds, and a few mammals (s, s, s, and s) have this orifice, from which ...
. In females, the ovaries each open into an
oviduct The oviduct is the passageway in animals from an ovary The ovary is an organ found in the female reproductive system The female reproductive system is made up of the internal and external sex organs that function in reproduction of new of ...
(in hermaphrodites, the eggs enter a
spermatheca The spermatheca (pronounced plural: spermathecae ), also called receptaculum seminis (plural: receptacula seminis), is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar f ...
first) and then a glandular
uterus The uterus (from Latin "uterus", plural ''uteri'') or womb () is the main female hormone-responsive, sex organ, secondary sex organ of the reproductive system in humans and most other mammals. Things occurring in the uterus are described with t ...

uterus
. The uteri both open into a common vulva/vagina, usually located in the middle of the morphologically ventral surface. Reproduction is usually sexual, though hermaphrodites are capable of self-fertilization. Males are usually smaller than females or hermaphrodites (often much smaller) and often have a characteristically bent or fan-shaped tail. During
copulation Sexual intercourse (or coitus or copulation) is a sexual activity typically involving the insertion and Pelvic thrust, thrusting of the penis into the vagina for Sexual stimulation, sexual pleasure, sexual reproduction, reproduction, or both.S ...
, one or more
chitin Chitin (carbon, C8hydrogen, H13oxygen, O5nitrogen, N)n ( ) is a long-chain polymer of N-Acetylglucosamine, ''N''-acetylglucosamine, an amide derivative of glucose. The second most abundant polysaccharide in nature (behind only cellulose), it ...

chitin
ized
spicule Spicules are any of various small needle-like anatomical structures occurring in organisms Spicule may also refer to: *Spicule (sponge), small skeletal elements of sea sponges *Spicule (nematode), reproductive structures found in male nematodes (r ...
s move out of the cloaca and are inserted into the genital pore of the female.
Amoeboid An amoeba (; less commonly spelt ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of cell or unicellular organism A unicellular organism, also known as a single-celled organism, is an organism ...
sperm Sperm is the male reproductive Cell (biology), cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as ...

sperm
crawl along the spicule into the female worm. Nematode sperm is thought to be the only
eukaryotic cell Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus enclosed within a nuclear envelope. Eukaryotes belong to the Domain (biology), domain Eukaryota or Eukarya; their name comes from the Greek language, Greek wi ...
without the globular protein
G-actin Actin is a family In human society A society is a Social group, group of individuals involved in persistent Social relation, social interaction, or a large social group sharing the same spatial or social territory, typically subje ...
. Eggs may be
embryonatedEmbryonated, unembryonated and de-embryonated are terms generally used in reference to eggs or, in botany Botany, also called , plant biology or phytology, is the science Science (from the Latin word ''scientia'', meaning "knowledge") is ...
or unembryonated when passed by the female, meaning their fertilized eggs may not yet be developed. A few species are known to be
ovoviviparous Ovoviviparity, ovovivipary, ovivipary, or aplacental viviparity is an outmoded term used as a "bridging" form of reproduction between egg-laying oviparous and live-bearing viviparous reproduction. Ovoviviparous animals have the embryos develop ...
. The eggs are protected by an outer shell, secreted by the uterus. In free-living roundworms, the eggs hatch into
larva A larva (plural larvae ) is a distinct juvenile form many animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that f ...
e, which appear essentially identical to the adults, except for an underdeveloped reproductive system; in parasitic roundworms, the lifecycle is often much more complicated. Nematodes as a whole possess a wide range of modes of reproduction. Some nematodes, such as ''
Heterorhabditis ''Heterorhabditis'' is a genus of nematodes belonging to the order Rhabditida. All species of this genus are obligate parasites of insects, and some are used as biological pest control, biological control agents for the control of pest insects. '' ...
'' spp., undergo a process called endotokia matricida: intrauterine birth causing maternal death. Some nematodes are hermaphrodite, hermaphroditic, and keep their self-fertilized eggs inside the
uterus The uterus (from Latin "uterus", plural ''uteri'') or womb () is the main female hormone-responsive, sex organ, secondary sex organ of the reproductive system in humans and most other mammals. Things occurring in the uterus are described with t ...

uterus
until they hatch. The juvenile nematodes then ingest the parent nematode. This process is significantly promoted in environments with a low food supply. The nematode model species ''C. elegans'', ''Caenorhabditis briggsae, C. briggsae'', and ''Pristionchus pacificus'', among other species, exhibit androdioecy, which is otherwise very rare among animals. The single genus ''Meloidogyne'' (root-knot nematodes) exhibits a range of reproductive modes, including sexual reproduction, facultative sexuality (in which most, but not all, generations reproduce asexually), and both meiosis, meiotic and mitosis, mitotic parthenogenesis. The genus ''Mesorhabditis'' exhibits an unusual form of parthenogenesis, in which sperm-producing males copulate with females, but the sperm do not fuse with the ovum. Contact with the sperm is essential for the ovum to begin dividing, but because no fusion of the cells occurs, the male contributes no genetic material to the offspring, which are essentially cloning, clones of the female.


Free-living species

Different free-living species feed on materials as varied as bacteria, algae, fungi, small animals, fecal matter, dead organisms, and living tissues. Free-living marine nematodes are important and abundant members of the meiobenthos. They play an important role in the decomposition process, aid in recycling of nutrients in marine environments, and are sensitive to changes in the environment caused by pollution. One roundworm of note, Caenorhabditis elegans, ''C. elegans'', lives in the soil and has found much use as a model organism. ''C. elegans'' has had its entire genome sequenced, the developmental fate of every cell determined, and every neuron mapped.


Parasitic species

Nematodes that commonly parasitise humans include ascarids (''Ascaris''), filarias, hookworms, pinworm (parasite), pinworms (''Enterobius''), and whipworms (''Trichuris trichiura''). The species ''Trichinella spiralis'', commonly known as the 'trichina worm', occurs in rats, pigs, bears, and humans, and is responsible for the disease trichinosis. ''Baylisascaris'' usually infests wild animals, but can be deadly to humans, as well. ''Dirofilaria immitis'' is known for causing heartworm disease by inhabiting the hearts, arteries, and lungs of dogs and some cats. ''Haemonchus contortus'' is one of the most abundant infectious agents in sheep around the world, causing great economic damage to sheep. In contrast, entomopathogenic nematodes parasitize insects and are mostly considered beneficial by humans, but some attack beneficial insects. One form of nematode is entirely dependent upon fig wasps, which are the sole source of ficus, fig fertilization. They prey upon the wasps, riding them from the ripe fig of the wasp's birth to the fig flower of its death, where they kill the wasp, and their offspring await the birth of the next generation of wasps as the fig ripens. A newly discovered parasitic tetradonematid nematode, ''Myrmeconema neotropicum'', apparently induces fruit mimicry in the tropical ant ''Cephalotes atratus''. Infected ants develop bright red gaster (insect anatomy), gasters (abdomens), tend to be more sluggish, and walk with their gasters in a conspicuous elevated position. These changes likely cause frugivorous birds to confuse the infected ants for berries, and eat them. Parasite eggs passed in the bird's feces are subsequently collected by foraging ''C. atratus'' and are fed to their
larva A larva (plural larvae ) is a distinct juvenile form many animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that f ...
e, thus completing the lifecycle of ''M. neotropicum''. Similarly, multiple varieties of nematodes have been found in the abdominal cavities of the primitively social sweat bee, ''Lasioglossum zephyrus''. Inside the female body, the nematode hinders ovarian development and renders the bee less active, thus less effective in pollen collection. Plant-parasitic nematodes include several groups causing severe crop losses, taking 10% of crops worldwide every year. The most common genera are ''Aphelenchoides'' (foliar nematodes), ''Ditylenchus'', ''Globodera'' (potato cyst nematodes), ''Heterodera'' (soybean cyst nematodes), ''Longidorus'', ''Meloidogyne'' (root-knot nematodes), ''Nacobbus'', ''Pratylenchus'' (lesion nematodes), ''Trichodorus'', and ''Xiphinema'' (dagger nematodes). Several phytoparasitic nematode species cause histological damages to roots, including the formation of visible galls (e.g. by root-knot nematodes), which are useful characters for their diagnostic in the field. Some nematode species transmit plant viruses through their feeding activity on roots. One of them is ''Xiphinema index'', vector of grapevine fanleaf virus, an important disease of grapes, another one is ''Xiphinema diversicaudatum'', vector of arabis mosaic virus''.'' Other nematodes attack bark and forest trees. The most important representative of this group is ''Bursaphelenchus xylophilus'', the pine wood nematode, present in Asia and America and recently discovered in Europe.


Agriculture and horticulture

Depending on its species, a nematode may be beneficial or detrimental to plant health. From agricultural and horticulture perspectives, the two categories of nematodes are the predatory ones, which kill garden pests such as cutworms and Helicoverpa zea, corn earworm moths, and the pest nematodes, such as the root-knot nematode, which attack plants, and those that act as vector (epidemiology), vectors spreading plant viruses between crop plants. Plant-parasitic nematodes are often known as eelworms and attack leaves and buds. Predatory nematodes can be bred by soaking a specific recipe of leaves and other detritus in water, in a dark, cool place, and can even be purchased as an Organic movement, organic form of pest control. Rotations of plants with nematode-resistant species or varieties is one means of managing parasitic nematode infestations. For example, Marigold (common), marigolds, grown over one or more seasons (the effect is cumulative), can be used to control nematodes. Another is treatment with natural antagonists such as the fungus ''Gliocladium roseum''. Chitosan, a natural Biological pest control, biocontrol, elicits plant defense responses to destroy parasitic cyst nematodes on roots of soybean, corn, sugar beet, potato, and tomato crops without harming beneficial nematodes in the soil. Soil steam sterilization, Soil steaming is an efficient method to kill nematodes before planting a crop, but indiscriminately eliminates both harmful and beneficial soil fauna. The golden nematode ''Globodera rostochiensis'' is a particularly harmful variety of nematode pest that has resulted in quarantines and crop failures worldwide. CSIRO has found a 13- to 14-fold reduction of nematode population densities in plots having Indian mustard ''Brassica juncea'' green manure or seed meal in the soil.


Epidemiology

A number of intestinal nematodes cause diseases affecting human beings, including ascariasis, trichuriasis, and hookworm disease. Filarial nematodes cause filariasis, filariases.


Soil ecosystems

About 90% of nematodes reside in the top 15 cm (6") of soil. Nematodes do not decompose organic matter, but, instead, are parasitic and free-living organisms that feed on living material. Nematodes can effectively regulate bacterial population and community composition—they may eat up to 5,000 bacteria per minute. Also, nematodes can play an important role in the nitrogen cycle by way of nitrogen mineralization. One group of carnivorous fungus, carnivorous fungi, the Nematophagous fungus, nematophagous fungi, are predators of soil nematodes. They set enticements for the nematodes in the form of lassos or adhesive structures.


Survivability

Nematode worms (''C. elegans''), part of an ongoing research project conducted on the 2003 Space Shuttle Columbia, Space Shuttle ''Columbia'' mission STS-107, survived the Space Shuttle Columbia disaster#Recovery of debris, re-entry breakup. It is believed to be the first known life form to survive a virtually unprotected atmospheric descent to Earth's surface. In a research project published in 2012, it was found that the Antarctic Nematodes (P. davidi) was able to withstand intracellular freezing depending on how well it was fed. When compared between fed and starved nematodes, the survival rate increased in the fed group and decreased in the starved group.


See also

* * * * * : A helminth infection of humans caused by the dog or cat roundworm, ''Toxocara canis'' or ''Toxocara cati'' *


References


Further reading

* * * [in Russian]. * * * * *


External links


Harper Adams University College Nematology Research

Nematodes/roundworms of man
* http://www.ucmp.berkeley.edu/phyla/ecdysozoa/nematoda.html
European Society of Nematologists

Nematode.net: Repository of parasitic nematode sequences.
* http://webarchive.loc.gov/all/20020914155908/http://www.nematodes.org/
NeMys World free-living Marine Nematodes database



International Federation of Nematology Societies

Society of Nematologists

Australasian Association of Nematologists

Research on nematodes and longevity

Nematode on BBC

Nematode worms in an aquarium


on the University of Florida, UF / *Institute of Food and Agricultural Sciences, IFAS Featured Creatures Web site {{Authority control Nematodes, Articles containing video clips Nematoida Composting Garden pests Organic gardening Pest control Ediacaran first appearances Extremophiles Cosmopolitan animals