View source for Nematode

{{Short description |Phylum of worms}}
{{Use dmy dates |date=November 2023}}
{{Automatic taxobox
|fossil_range={{fossil range |400 |0 |earliest=510 |latest=0 |[[Early Devonian]] – Recent<ref>{{cite journal |last1=Poinar |first1=George |last2=Kerp |first2=Hans |last3=Hass |first3=Hagen |title=''Palaeonema phyticum'' gen. n., sp. n. (Nematoda: Palaeonematidae fam. n.), a Devonian nematode associated with early land plants |journal=Nematology |date=January 2008 |volume=10 |issue=1 |pages=9–14 |doi=10.1163/156854108783360159 |url=https://brill.com/view/journals/nemy/10/1/article-p9_2.xml|url-access=subscription }}</ref>}} Possible Cambrian occurrence<ref>{{cite journal |last1=Maas |first1=Andreas |last2=Waloszek |first2=Dieter |last3=Haug |first3=Joachim |last4=Müller |first4=Klaus |title=A possible larval roundworm from the Cambrian 'Orsten' and its bearing on the phylogeny of Cycloneuralia |journal=Memoirs of the Association of Australasian Palaeontologists |date=January 2007 |volume=34 |pages=499–519 |url=https://www.researchgate.net/publication/238695144}}</ref>
|image=CelegansGoldsteinLabUNC.jpg
|image_caption=''[[Caenorhabditis elegans]]'',<br /> a model species of roundworm
|display_parents=7
|taxon=Nematoda
|authority=[[Karl Moriz Diesing|Diesing]], 1861
|subdivision_ranks=Classes
|subdivision=
* [[Chromadorea]]
* [[Enoplea]]
* [[Secernentea]]
(see [[#Systematics|text]])
|synonyms={{Plainlist |
* Nematodes <small>[[Hermann Burmeister|Burmeister]], 1837</small>
* Nematoidea <small>''sensu stricto'' Cobb, 1919</small>
* Nemates <small>Cobb, 1919</small>
* Nemata <small>Cobb, 1919 ''emend.''</small>
}}
}}

The '''nematodes''' ({{IPAc-en | |ˈ |n |ɛ |m |ə |t |əʊ |d |z}} {{respell |NEM |ə |tohdz}} or {{respell |NEEM |-}}; {{langx |grc |Νηματώδη}}; {{langx |la |Nematoda}}), '''roundworms''' or '''eelworms''' constitute the [[phylum]] '''Nematoda'''. Species in the phylum inhabit a broad range of environments. Most species are free-living, feeding on [[microorganism]]s, but many are [[parasitism|parasitic]]. [[Parasitic worms]] (helminths) are the cause of [[Soil-transmitted helminthiasis|soil-transmitted helminthiases]].

They are classified along with [[arthropod]]s, [[tardigrade]]s and other [[moulting]] [[animalia|animal]]s in the [[clade]] [[Ecdysozoa]]. Unlike the [[flatworm]]s, nematodes have a tubular [[digestion|digestive system]], with openings at both ends. Like tardigrades, they have a reduced number of [[Hox gene]]s, but their sister phylum [[Nematomorpha]] has kept the ancestral [[protostome]] Hox genotype, which shows that the reduction has occurred within the nematode phylum.<ref>{{cite journal |last1=Baker |first1=Emily A. |last2=Woollard |first2=Alison |year=2019 |title=How weird is the worm? Evolution of the developmental gene toolkit in ''Caenorhabditis elegans'' |journal=[[Journal of Developmental Biology]] |volume=7 |issue=4 |page=19 |doi=10.3390/jdb7040019 |doi-access=free |pmid=31569401 |pmc=6956190}}</ref>

Nematode [[species]] can be difficult to distinguish from one another. Consequently, estimates of the number of nematode species are uncertain. A 2013 survey of animal biodiversity suggested there are over 25,000.<ref>{{cite journal |last=Hodda |first=M. |year=2011 |title=Phylum Nematoda {{small |(Cobb, 1932)}} |editor=Zhang, Z.-Q. |department=Animal biodiversity: An outline of higher-level classification and survey of taxonomic richness |journal=[[Zootaxa]] |volume=3148 |pages=63–95 |doi=10.11646/zootaxa.3148.1.11}}</ref><ref>{{cite journal |last=Zhang |first=Z. |year=2013 |title=Animal biodiversity: An update of classification and diversity in 2013 |editor=Zhang, Z.-Q. |department=Animal biodiversity: An update of classification and diversity (Addenda 2013) |journal=[[Zootaxa]] |volume=3703 |issue=1 |pages=5–11 |doi=10.11646/zootaxa.3703.1.3 |doi-access=free }}</ref> Estimates of the total number of [[Neontology#Extant taxa versus extinct taxa|extant]] species are subject to even greater variation. A widely referenced 1993 article  estimated there might be over a million species of nematode.<ref name=Lambshead-1993>{{cite journal |last=Lambshead |first=P. John D. |date=January 1993 |title=Recent developments in marine benthic biodiversity research |journal=[[Oceanis]] |volume=19 |issue=6 |pages=5–24 |url=https://www.researchgate.net/publication/279896164 |access-date=November 5, 2018 |df=dmy-all}}</ref> A subsequent publication challenged this claim, estimating the figure to be at least 40,000&nbsp;species.<ref name="RAnderson">{{cite book |last=Anderson |first=Roy C. |title=Nematode Parasites of Vertebrates: Their Development and Transmission |url=https://books.google.com/books?id=lEERbfsvP1EC&pg=PA1 |date=2000 |publisher=CABI |isbn=978-0-85199-421-5 |oclc=559243334 |doi=10.1079/9780851994215.0000 |pages=1–2 |quote=Estimates of 500,000 to a million species have no basis in fact.}}</ref> Although the highest estimates (up to 100 million species) have since been deprecated, estimates supported by [[Rarefaction (ecology)|rarefaction curves]],<ref name=Lambshead_Boucher>{{cite journal |last1=Lambshead |first1=P.J. |last2=Boucher |first2=G. |title=Marine nematode deep-sea biodiversity—hyperdiverse or hype? |journal=Journal of Biogeography |year=2003 |volume=30 |issue=4 |pages=475–485 |doi=10.1046/j.1365-2699.2003.00843.x |bibcode=2003JBiog..30..475L |s2cid=86504164 |url=https://onlinelibrary.wiley.com/doi/abs/10.1046/j.1365-2699.2003.00843.x |url-access=subscription }}</ref><ref name="Qing_Bert">{{cite journal |last1=Qing |first1=X. |last2=Bert |first2=W. |year=2019 |title=Family Tylenchidae (Nematoda): an overview and perspectives |journal=Organisms Diversity & Evolution |volume=19 |issue=3 |pages=391–408 |doi=10.1007/s13127-019-00404-4 |bibcode=2019ODivE..19..391Q |s2cid=190873905 |url=https://link.springer.com/article/10.1007/s13127-019-00404-4|url-access=subscription }}</ref> together with the use of [[DNA barcoding]]<ref name=Floyd2002>{{cite journal |last1=Floyd |first1=Robin |last2=Abebe |first2=Eyualem |last3=Papert |first3=Artemis |last4=Blaxter |first4=Mark |year=2002 |title=Molecular barcodes for soil nematode identification |journal=Molecular Ecology |volume=11 |issue=4 |pages=839–850 |doi=10.1046/j.1365-294X.2002.01485.x |pmid=11972769 |bibcode=2002MolEc..11..839F |s2cid=12955921 |url=https://onlinelibrary.wiley.com/doi/abs/10.1046/j.1365-294X.2002.01485.x?sid=nlm%3Apubmed|url-access=subscription }}</ref> and the increasing acknowledgment of widespread [[Species complex#cryptic species|cryptic species]] among nematodes,<ref name=Derycke_et_al>{{cite journal |last1=Derycke |first1=S. |last2=Sheibani Tezerji |first2=R. |last3=Rigaux |first3=A. |last4=Moens |first4=T. |year=2012 |title=Investigating the ecology and evolution of cryptic marine nematode species through quantitative real-time PCR of the ribosomal ITS region |journal=Molecular Ecology Resources |volume=12 |issue=4 |pages=607–619 |doi=10.1111/j.1755-0998.2012.03128.x |s2cid=4818657 |pmid=22385909 |bibcode=2012MolER..12..607D |hdl=1854/LU-3127487 |hdl-access=free |url=https://biblio.ugent.be/publication/3127487}}</ref> have placed the figure closer to one million species.<ref name=Blaxter2016>{{cite journal |last=Blaxter |first=Mark |year=2016 |title=Imagining Sisyphus happy: DNA barcoding and the unnamed majority |journal=Philosophical Transactions of the Royal Society of London B |volume=371 |issue=1702 |pages=20150329 |doi=10.1098/rstb.2015.0329 |pmid=27481781 |pmc=4971181}}</ref>

Nematodes have successfully adapted to nearly every [[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. They are ubiquitous in freshwater, marine, and terrestrial environments, where they often outnumber other animals in both individual and [[species]] counts, and are found in locations as diverse as mountains, deserts, and [[oceanic trench]]es. They are found in every part of the Earth's [[lithosphere]],<ref name="Borgonie 2011">
{{cite journal  |last1=Borgonie |first1=G. |last2=García-Moyano |first2=A. |last3=Litthauer |first3=D. |last4=Bert |first4=W. |last5=Bester |first5=A. |last6=van Heerden |first6=E. |last7=Möller |first7=C. |last8=Erasmus |first8=M. |last9=Onstott |first9=T. C. |date=June 2011 |title=Nematoda from the terrestrial deep subsurface of South Africa |journal=Nature |volume=474 |issue=7349 |pages=79–82 |doi=10.1038/nature09974 |pmid=21637257 |bibcode=2011Natur.474...79B |s2cid=4399763 |hdl=1854/LU-1269676 |hdl-access=free |url=https://biblio.ugent.be/publication/1269676}}</ref> even at great depths, {{convert |0.9 |– |3.6 |km |ft |abbr=on |sigfig=2}} below the surface of the Earth in gold mines in South Africa.<ref name="Borgonie 2011"/> They represent 90% of all animals on the [[Seabed|ocean floor]].<ref name=pmid18164201>{{cite journal |last1=Danovaro |first1=Roberto |last2=Gambi |first2=Cristina |last3=Dell'Anno |first3=Antonio |last4=Corinaldesi |first4=Cinzia |last5=Fraschetti |first5=Simonetta |last6=Vanreusel |first6=Ann |last7=Vincx |first7=Magda |last8=Gooday |first8=Andrew J. |title=Exponential Decline of Deep-Sea Ecosystem Functioning Linked to Benthic Biodiversity Loss |journal=Current Biology |volume=18 |issue=1 |date=2008 |doi=10.1016/j.cub.2007.11.056 |doi-access=free |pages=1–8 |pmid=18164201 |bibcode=2008CBio...18....1D |url=http://www.cell.com/article/S0960982207023421/pdf |access-date=2024-12-21}}</ref> In total, 4.4 × 10<sup>20</sup> nematodes inhabit the Earth's topsoil, or approximately 60 billion for each human, with the highest densities observed in tundra and boreal forests.<ref name=":0">{{cite journal |last1=van den Hoogen |first1=Johan |last2=Geisen |first2=Stefan |last3=Routh |first3=Devin |last4=Ferris |first4=Howard |last5=Traunspurger |first5=Walter |last6=Wardle |first6=David A. |last7=de Goede |first7=Ron G.M. |last8=Adams |first8=Byron J. |last9=Ahmad |first9=Wasim |display-authors=5 |date=2019-07-24 |title=Soil nematode abundance and functional group composition at a global scale |journal=Nature |volume=572 |issue=7768 |pages=194–198 |doi=10.1038/s41586-019-1418-6 |s2cid=198492891 |pmid=31341281 |bibcode=2019Natur.572..194V |hdl=20.500.11755/c8c7bc6a-585c-4a13-9e36-4851939c1b10 |hdl-access=free |url=https://gitlab.ethz.ch/devinrouth/crowther_lab_nematodes |access-date=2019-12-10 |url-status=dead |archive-url=https://web.archive.org/web/20200302112231/https://gitlab.ethz.ch/devinrouth/crowther_lab_nematodes |archive-date=2020-03-02 |df=dmy-all }}</ref> 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.<ref name=":0"/><ref name=isbn0-903874-22-9>{{cite book |last=Platt |first=H.M. |chapter=foreword |editor1=Lorenzen, S. |editor2=Lorenzen, S.A. |title=The phylogenetic systematics of freeliving nematodes |publisher=Ray Society |series=Ray Society (Series) |volume=162 |location=London, UK |year=1994 |isbn=978-0-903874-22-9 |oclc=1440106662}}</ref> They play crucial roles in polar ecosystems.<ref>{{cite journal |last1=Lee |first1=Charles K. |last2=Laughlin |first2=Daniel C. |last3=Bottos |first3=Eric M. |last4=Caruso |first4=Tancredi |last5=Joy |first5=Kurt |last6=Barrett |first6=John E. |last7=Brabyn |first7=Lars |last8=Nielsen |first8=Uffe N. |last9=Adams |first9=Byron J. |last10=Wall |first10=Diana H. |last11=Hopkins |first11=David W. |last12=Pointing |first12=Stephen B. |last13=McDonald |first13=Ian R. |last14=Cowan |first14=Don A. |last15=Banks |first15=Jonathan C. |last16=Stichbury |first16=Glen A. |last17=Jones |first17=Irfon |last18=Zawar-Reza |first18=Peyman |last19=Katurji |first19=Marwan |last20=Hogg |first20=Ian D. |last21=Sparrow |first21=Ashley D. |last22=Storey |first22=Bryan C. |last23=Allan Green |first23=T. G. |last24=Cary |first24=S. Craig |display-authors=5 |title=Biotic interactions are an unexpected yet critical control on the complexity of an abiotically driven polar ecosystem |journal=Communications Biology |volume=2 |issue=1 |date=2019-02-15 |issn=2399-3642 |pmid=30793041 |pmc=6377621 |doi=10.1038/s42003-018-0274-5 |doi-access=free |url=https://www.nature.com/articles/s42003-018-0274-5.pdf |access-date=2024-12-21 |page=62}}</ref><ref>{{cite journal |last1=Caruso |first1=Tancredi |last2=Hogg |first2=Ian D. |last3=Nielsen |first3=Uffe N. |last4=Bottos |first4=Eric M. |last5=Lee |first5=Charles K. |last6=Hopkins |first6=David W. |last7=Cary |first7=S. Craig |last8=Barrett |first8=John E. |last9=Green |first9=T. G. Allan |last10=Storey |first10=Bryan C. |last11=Wall |first11=Diana H. |last12=Adams |first12=Byron J. |title=Nematodes in a polar desert reveal the relative role of biotic interactions in the coexistence of soil animals |journal=Communications Biology |volume=2 |issue=1 |date=2019-02-15 |issn=2399-3642 |pmid=30793042 |pmc=6377602 |doi=10.1038/s42003-018-0260-y |doi-access=free |url=https://www.nature.com/articles/s42003-018-0260-y.pdf |access-date=2024-12-21 |page=63}}</ref> The roughly 2,271 [[genera]] are placed in 256 [[Family (biology)|families]].<ref name=n>{{cite book |last=Anderson |first=Roy C. |date=8 February 2000 |title=Nematode Parasites of Vertebrates: Their development and transmission |publisher=CABI |isbn=978-0-85199-786-5 |page=1 |url=https://books.google.com/books?id=lEERbfsvP1EC}}</ref> The many parasitic forms include [[pathogen]]s in most plants and animals. A third of the genera occur as [[parasite]]s of [[vertebrate]]s; about 35 nematode species are [[human parasite]]s.<ref name=n/>

== Etymology==
The word ''nematode'' comes from the [[Neo-Latin|Modern Latin]] compound of ''nema-'' 'thread' (from Greek ''nema'', genitive ''nematos'' 'thread', from the stem ''nein'' 'to spin'; cf. ''needle'') + ''-odes'' 'like, of the nature of' (cf. ''-oid''). The addition firstly of '-oid' and then to '-ode' renders 'threadlike'.<ref name="Name">{{cite web |title=Phylum Name |url=http://nemaplex.ucdavis.edu/General/Phylumname.htm |website=nemaplex.ucdavis.edu |access-date=23 December 2024}}</ref>

== Taxonomy and systematics==
{{see also|List of nematode families}}

<gallery class=center mode=nolines widths=180 heights=180>
File:Eophasma jurasicum.JPG|''[[Eophasma |Eophasma jurasicum]]'', a fossilized nematode
File:Celegans wt nhr80rnai.png|''[[Caenorhabditis elegans]]''
File:Hookworms.JPG|[[Rhabditia]]
File:Gravid adult female Nippostrongylus brasiliensis - image.pntd.v07.i08.g001.png|''[[Nippostrongylus brasiliensis]]''
File:Anisakids.jpg|Unidentified [[Anisakidae]] ([[Ascaridina]]: [[Ascaridoidea]])
File:Threadworm.jpg|[[Oxyuridae]] Pinworm
File:Microfilaria.jpg|[[Spiruridae]] ''[[Dirofilaria immitis]]''
File:Nematode Steinernema feltiae.jpg| ''[[Steinernema]] feltiae''
File:Nematode Steinernema Feltiae with size comparison of 1 EURO coin.webm| ''Steinernema feltiae''
</gallery>

=== History ===
[[File:Tropical Diseases - Fig 149.png|thumb| [[Carl Linnaeus]] described nematodes including the parasitic ''[[Dracunculus medinensis]]'', seen here under a person's skin.<ref name="Cox 2002"/>]]

In 1758, [[Carl Linnaeus]] described nematodes of a few genera including ''[[Ascaris]]'' and ''[[Dracunculus (nematode)|Dracunculus]]'', then included in the [[Vermes]].<ref name="Cox 2002">{{cite journal |last=Cox |first=F. E. G. |title=History of Human Parasitology |journal=Clinical Microbiology Reviews |volume=15 |issue=4 |date=2002 |pmid=12364371 |pmc=126866 |doi=10.1128/CMR.15.4.595-612.2002 |doi-access=free |pages=595–612}}</ref>

The name of the group Nematoda, informally called "nematodes", came from [[Nematoidea]], originally defined by [[Karl Rudolphi]] in 1808,<ref name="Chitwood 1957">{{cite journal |last=Chitwood |first=B.G. |title=The English word "Nema" revised |journal=Systematic Biology |volume=4 |issue=45 |page=1619 |year=1957 |doi=10.2307/sysbio/6.4.184}}</ref> from [[Ancient Greek]] νῆμα (''nêma, nêmatos'', 'thread') and -ειδἠς (''-eidēs'', 'species'). It was treated as [[family (biology)|family]] Nematodes by [[Hermann Burmeister|Burmeister]] in 1837.<ref name="Chitwood 1957"/>

At its origin, the "Nematoidea" erroneously included Nematodes and [[Nematomorpha]], attributed by [[Karl Theodor Ernst von Siebold]] in 1843. Along with [[Acanthocephala]], [[Trematoda]], and [[Cestoidea]], it formed the obsolete group [[Entozoa]],<ref name=isbn0-85199-202-1>{{cite book |last=Siddiqi |first=M.R. |chapter=Introduction, Historical Review and Techniques |pages=23–29 |title=Tylenchida: Parasites of plants and insects |publisher=CABI |year=2000 |isbn=978-0-85199-202-0}}</ref> created by Rudolphi in 1808.<ref>{{cite book |last=Schmidt-Rhaesa |first=A. |year=2014 |chapter=Gastrotricha, Cycloneuralia, and Gnathifera: General history and phylogeny |editor=Schmidt-Rhaesa, A. |title=Handbook of Zoology (founded by W. Kükenthal) |volume=1, Nematomorpha, Priapulida, Kinorhyncha, Loricifera |publisher=de Gruyter |location=Berlin, Boston}}</ref> They were classed along with Acanthocephala in the obsolete [[phylum]] [[Aschelminth|Nemathelminthes]] by Gegenbaur in 1859.<ref name="Chitwood 1957"/>

In 1861, [[Karl Moriz Diesing]] treated the group as order Nematoda.<ref name="Chitwood 1957"/> In 1877, the [[taxon]] Nematoidea, including the family [[Gordiidae]] (horsehair worms), was promoted to the rank of phylum by [[Ray Lankester]].<ref name="Chitwood 1957"/> The first clear distinction between the nemas and gordiids was realized by [[František Vejdovsky]] when he named the group containing the horsehair worms the order Nematomorpha in 1886.<ref name="Bleidorn 2002">{{cite journal |last1=Bleidorn |first1=Christoph |last2=Schmidt-Rhaesa |first2=Andreas |last3=Garey |first3=James R. |title=Systematic relationships of Nematomorpha based on molecular and morphological data |journal=Invertebrate Biology |volume=121 |issue=4 |date=2002 |doi=10.1111/j.1744-7410.2002.tb00136.x |doi-access=free |pages=357–364 |bibcode=2002InvBi.121..357B }}</ref>

In 1910, Grobben proposed the phylum Aschelminthes, and the nematodes were included as class Nematoda alongside the classes [[Rotifer]]a, [[Gastrotrich]]a, [[Kinorhyncha]], [[Priapulida]], and [[Nematomorpha]].<ref name="Marcus 1958">{{cite journal |last=Marcus |first=Ernesto |title=On the Evolution of the Animal Phyla |journal=The Quarterly Review of Biology |volume=33 |issue=1 |date=1958 |doi=10.1086/402207 |pages=24–58}}</ref> <!--The Aschelminthes was revived and modified by [[Libbie Hyman]] in 1951 as Pseudocoelomata.-->

In 1919, [[Nathan Cobb]] 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.<ref>{{cite journal |last=Cobb |first=N.A. |year=1919 |title=The orders and classes of nemas. |journal=Contrib. Sci. Nematol. |volume=8 |pages=213–216 |url=https://www.biodiversitylibrary.org/part/58037}}</ref>

In 1932, Potts elevated the class Nematoda to the level of phylum, leaving the name the same. Although Potts' and Cobb's classifications are equivalent, both names are used, and Nematode became a popular term in zoological science.<ref>{{Cite web |last=Wilson |first=E.O. |title=Phylum Nemata |series=Plant and insect parasitic nematodes |website=nematode.unl.edu |url=https://nematode.unl.edu/nemata.htm |url-status=dead |access-date=2018-04-29 |df=dmy-all |archive-url=https://web.archive.org/web/20180430120018/https://nematode.unl.edu/nemata.htm |archive-date=2018-04-30}}</ref>

=== Phylogeny===
The [[Phylogeny|phylogenetic]] relationships of the nematodes and their close relatives among the [[protostome]]s 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]] together with [[moulting]] animals, such as [[arthropod]]s. The identity of the closest living relatives of the Nematoda has always been considered to be well resolved. Morphological and [[molecular phylogenetics]] agree with placing the roundworms as a [[sister taxon]] to the parasitic [[Nematomorpha]]; together, they make up the [[Nematoida]]. Along with the [[Scalidophora]] (formerly Cephalorhyncha), the Nematoida form the clade [[Cycloneuralia]], 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]].<ref name=ToL_2002_Bilateria>{{cite web |title=Bilateria |year=2002 |website=Tree of Life (tolweb.org) |publisher=[[Tree of Life Web Project]] |url=http://tolweb.org/Bilateria/2459/2002.01.01 |access-date=2008-11-02 |df=dmy-all}}</ref><ref>For an up-to-date view (as of 2022), see [https://www.frontiersin.org/articles/10.3389/fevo.2021.769565/full Phylogenomic Analysis of the Phylum Nematoda: Conflicts and Congruences With Morphology, 18S rRNA, and Mitogenomes].</ref>

=== 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<ref name="Chitwood1933">{{cite journal |last1=Chitwood |first1=B.G. |last2=Chitwood |first2=M.B. |title=The characters of a protonematode |journal=Journal of Parasitology |volume=20 |page=130 |year=1933}}</ref>—later revised by Chitwood<ref name="Chitwood1937">{{cite book |last=Chitwood |first=B.G. |chapter=A revised classification of the ''Nematoda'' |title=Papers on Helminthology published in commemoration of the 30&nbsp;year Jubileum of ... K.J. Skrjabin&nbsp;... |pages=67–79 |year=1937 |location=Moscow |publisher=All-Union Lenin Academy of Agricultural Sciences}}</ref>—who divided the phylum into two classes—[[Aphasmidia]] and [[Secernentea|Phasmidia]]. These were later renamed [[Adenophorea]] (gland bearers) and [[Secernentea]] (secretors), respectively.<ref name="Chitwood1958">{{cite journal |last=Chitwood |first=B.G. |title=The designation of official names for higher taxa of invertebrates |journal=Bull Zool Nomencl |volume=15 |pages=860–895 |year=1958 |doi=10.5962/bhl.part.19410 |doi-access=free}}</ref> The Secernentea share several characteristics, including the presence of [[phasmid (nematode)|phasmid]]s, 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 [[Nucleic acid sequence|DNA sequence]]s<ref>{{cite journal |last1=Coghlan |first1=A. |title=Nematode genome evolution |date=7 Sep 2005 |journal=WormBook |pages=1–15 |doi=10.1895/wormbook.1.15.1 |pmid=18050393 |pmc=4781476 |url=http://www.wormbook.org/chapters/www_genomevol/genomevol.pdf |access-date=13 January 2016 |archive-url=https://web.archive.org/web/20160305075651/http://www.wormbook.org/chapters/www_genomevol/genomevol.pdf |archive-date=5 March 2016 |url-status=dead}}</ref> suggested the existence of five [[clade]]s:<ref name="Blaxter_1998">{{cite journal  |last1=Blaxter |first1=Mark L. |last2=De Ley |first2=Paul |last3=Garey |first3=James R. |last4=Liu |first4=Leo X. |last5=Scheldeman |first5=Patsy |last6=Vierstraete |first6=Andy |last7=Vanfleteren |first7=Jacques R. |last8=Mackey |first8=Laura Y. |last9=Dorris |first9=Mark |last10=Frisse |first10=Linda M. |last11=Vida |first11=J. T. |last12=Thomas |first12=W. Kelley |display-authors=5 |title=A molecular evolutionary framework for the phylum Nematoda |journal=Nature |volume=392 |issue=6671 |pages=71–75 |date=March 1998 |pmid=9510248 |doi=10.1038/32160 |bibcode=1998Natur.392...71B |s2cid=4301939 }}</ref>

* [[Dorylaimida]]
* [[Enoplia]]
* [[Spirurina]]
* [[Tylenchina]]
* [[Rhabditina]]

The Secernentea seem to be a natural group of close relatives, while the Adenophorea appear to be a [[paraphyletic]] assemblage of roundworms that retain a good number of [[plesiomorph|ancestral traits]]. The old [[Enoplia]] do not seem to be monophyletic, either, but do contain two distinct lineages. The old group [[Chromadorea]] seems 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.<ref name="ToL:2002_Nematoda">{{cite web |title=Nematoda |date=2002 |url=http://tolweb.org/Nematoda/2472/2002.01.01 |work=Tree of Life Web Project |access-date=2008-11-02 |df=dmy-all}}</ref>

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

'''Phylum Nematoda'''
* [[Basal (evolution)|Basal]] order [[Monhysterida]]
* Class [[Dorylaimida]]
* Class [[Enoplea]]
* Class [[Secernentea]]
** Subclass [[Diplogasteria]] (disputed)
** Subclass [[Rhabditia]] (paraphyletic?)
** Subclass [[Spiruria]]
** Subclass [[Tylenchia]] (disputed)
* "[[Chromadorea]]" assemblage

Later work has suggested the presence of 12 clades.<ref name=Holterman2006>{{cite journal |last1=Holterman |first1=Martijn |last2=van der Wurff |first2=Andre |last3=van den Elsen |first3=Sven |last4=van Megen |first4=Hanny |last5=Bongers |first5=Tom |last6=Holovachov |first6=Oleksandr |last7=Bakker |first7=Jaap |last8=Helder |first8=Johannes |display-authors=5 |year=2006 |title=Phylum-wide analysis of SSU rDNA reveals deep phylogenetic relationships among nematodes and accelerated evolution toward crown Clades |journal=Mol Biol Evol |volume=23 |issue=9 |pages=1792–1800 |doi=10.1093/molbev/msl044 |doi-access=free |pmid=16790472}}</ref> In 2019, a study identified one [[Conserved signature indels|conserved signature indel]] (CSI) found exclusively in members of the phylum Nematoda through comparative genetic analyses.<ref name=":1">{{Cite journal |last1=Khadka |first1=Bijendra |last2=Chatterjee |first2=Tonuka |last3=Gupta |first3=Bhagwati P. |last4=Gupta |first4=Radhey S. |date=2019-09-24 |title=Genomic Analyses Identify Novel Molecular Signatures Specific for the ''Caenorhabditis'' and other Nematode Taxa Providing Novel Means for Genetic and Biochemical Studies |journal=Genes |volume=10 |issue=10 |pages=739 |doi=10.3390/genes10100739 |pmid=31554175 |pmc=6826867 |doi-access=free}}</ref> 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.<ref name=":1" /> An analysis of the mitochondrial DNA suggests that the following groupings are valid<ref name=Liu2013>{{cite journal |last1=Liu |first1=Guo-Hua |last2=Shao |first2=Renfu |last3=Li |first3=Jia-Yuan |last4=Zhou |first4=Dong-Hui |last5=Li |first5=Hu |last6=Zhu |first6=Xing-Quan |title=The complete mitochondrial genomes of three parasitic nematodes of birds: a unique gene order and insights into nematode phylogeny |journal=BMC Genomics |volume=14 |issue=1 |date=2013 |page=414 |pmid=23800363 |pmc=3693896 |doi=10.1186/1471-2164-14-414 |doi-access=free}}</ref>

* subclass [[Dorylaimia]]
* orders [[Rhabditida]], [[Trichinellida]] and [[Mermithida]]
* suborder [[Rhabditina]]
* infraorders [[Spiruromorpha]] and [[Oxyuridomorpha]]

In 2022 a new classification of the entire phylum Nematoda was presented by M. Hodda. It was based on current molecular, developmental and morphological evidence.<ref>{{cite journal |last=Hodda |first=M. |year=2022 |title=Phylum Nematoda: a classification, catalogue and index of valid genera, with a census of valid species |journal=Zootaxa |volume=5114 |issue=1 |pages=1–289 |doi=10.11646/zootaxa.5114.1.1 |pmid=35391386 |doi-access=free}}</ref> Under this classification, the classes and subclasses are:

* Class [[Enoplea]]
** Subclass [[Enoplia]]
** Subclass [[Oncholaimia]]
** Subclass [[Triplonchia]]
* Class [[Dorylaimida]]
** Subclass [[Dorylaimia]]
** Subclass [[Bathyodontia]]
** Subclass [[Trichocephalia]]
* Class [[Chromadorea]]
** Subclass [[Chromadoria]]
** Subclass [[Plectia]]

== Fossil record==
Nematode eggs from the [[clade]]s Ascaridina, Spirurina, and Trichocephalida have been discovered in [[coprolite]]s from the [[Oligocene]]-aged Tremembé Formation, which represented a [[Paleolake|palaeolake]] in present-day [[São Paulo (state)|São Paulo]] with a diverse [[fossil]] assemblage of birds, fish, and arthropods that lent itself to fostering high nematode diversity.<ref>{{Cite journal |last1=Macêdo do Carmo |first1=Gustavo |last2=Garcia |first2=Renato Araujo |last3=Vieira |first3=Fabiano Matos |last4=de Souza Lima |first4=Sueli |last5=Ismael de Araújo-Júnior |first5=Hermínio |last6=Pinheiro |first6=Ralph Maturano |date=May 2023 |title=Paleoparasitological study of avian trace fossils from the Tremembé Formation (Oligocene of the Taubaté Basin), São Paulo, Brazil |url=https://linkinghub.elsevier.com/retrieve/pii/S089598112300130X |journal=[[Journal of South American Earth Sciences]] |language=en |volume=125 |pages=104319 |doi=10.1016/j.jsames.2023.104319 |bibcode=2023JSAES.12504319M |access-date=12 April 2024 |via=Elsevier Science Direct|url-access=subscription }}</ref> Nematodes have also been found in various [[lagerstätte]]n, such as [[Burmese amber]], the [[Moltrasio Formation]], and the [[Rhynie chert]], where the earliest known fossils are known from.

== Anatomy==
[[File:C elegans male.svg|thumb|Internal anatomy of a male ''C.&nbsp;elegans'' nematode]]
[[File:Ascaris female 200x section.jpg|thumb|Cross-section of female ''[[Ascaris]]''. The large circles filled with small green circles are the uterus and eggs. The long narrow feature is the [[#Digestive system|digestive tract]]. The smaller red and orange circles are the [[Ovary|ovaries]] and [[oviduct]]s. The cluster of green and black blobs in the upper right and lower left are the nerve cords ([[Ventral nerve cord|ventral]] and [[Dorsal nerve cord|dorsal]]). Surrounding the internal organs are the frilly green longitudinal muscles, the dark hypodermis, and the green outer [[Cuticle#Cuticle of invertebrates|cuticle]].]]

Nematodes are very small, slender worms. Most are free-living, often less than 2.5&nbsp;mm long and some only about 1&nbsp;mm. Many nematodes are microscopic. Some soil nematodes can reach up to 7&nbsp;mm in length, and some marine species can reach up to 5&nbsp;cm. Some are parasitic and can reach lengths of 50&nbsp;cm or more.<ref name="Ruppert7th">{{cite book |last1=Ruppert |first1=Edward E. |last2=Barnes |first2=Robert D. |last3=Fox |first3=Richard S. |title=Invertebrate zoology: a functional evolutionary approach |date=2019 |publisher=Cengage Learning |location=Delhi, India |isbn=9788131501047 |pages=757–770 |edition=Seventh, Seventh Indian Reprint}}</ref>

The body is often ornamented with ridges, rings, bristles, or other distinctive structures.<ref name="Weischer 2000">{{cite book |last1=Weischer |first1=B. |last2=Brown |first2=D.J. |year=2000 |title=An Introduction to Nematodes: General Nematology |publisher=Pensoft |location=Sofia, Bulgaria |isbn=978-954-642-087-9 |pages=75–76}}</ref>

The head 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.<ref name="Barnes_1980"/> The [[Epidermis (zoology)|epidermis]] is either a [[syncytium]] or a single layer of cells, and is covered by a thick [[collagen]]ous [[cuticle]]. 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]] 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 [[Ventral nerve cord|nerve cord]]s; this is a unique arrangement in the animal kingdom, in which nerve cells normally extend fibers into the muscles rather than ''vice versa''.<ref name="Barnes_1980"/>

=== Digestive system===
The oral cavity is lined with cuticles, which are often strengthened with structures, such as ridges, especially in carnivorous species, which may bear several teeth. The mouth often includes a sharp [[stylet (zoology)|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.<ref name="Barnes_1980"/> The oral cavity opens into a muscular, sucking [[pharynx]], also lined with cuticle. Digestive glands are found in this region of the gut, producing [[digestive enzyme|enzyme]]s that start to break down the food. In stylet-bearing species, these may even be injected into the prey.<ref name="Barnes_1980"/>

No [[stomach]] is present, with the pharynx connecting directly to a muscleless [[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 a cuticle, forming a [[rectum]], which expels waste through the [[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]]s at either end to help control food movement through the body.<ref name="Barnes_1980"/>

=== 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.<ref name="Barnes_1980"/>

There is an excretory gland, also known as a ''ventral cell'', or ''renette cell'' in all species of Adenophorea. In Secernentia there is an excretory canal system that may or may not use a gland cell.<ref name="Ruppert7th"/>

=== Nervous system===
{{See also |Muscle arms}}

At the anterior end of the animal a dense, circular [[Circumesophageal nerve ring|nerve ring]] which serves as the [[brain]] surrounds the pharynx.<ref name="Barnes_1980"/> From this ring six labial papillary nerve cords extend anteriorly, while six nerve cords; a large ventral, a smaller dorsal and two pairs of sublateral cords extend posteriorly.<ref>{{Cite book |url=https://books.google.com/books?id=qXRREAAAQBAJ&dq=nematodes+nerve+ring+pharynx+ventral+dorsal+sublateral+cords&pg=PA10 |title=Free-living Marine Nematodes from the East China Sea |first1=Yong |last1=Huang |first2=Yuqing |last2=Guo |date=27 November 2021 |publisher=Springer Nature |isbn=978-981-16-3836-7 |via=Google Books}}</ref> Each nerve lies within a cord of connective tissue lying beneath the cuticle and between the muscle cells. The [[ventral nerve cord|ventral nerve]] is the largest, and has a double structure forward of the [[excretion|excretory]] [[wikt:pore|pore]]. The dorsal nerve is responsible for motor control, while the lateral nerves are sensory, and the ventral combines both functions.<ref name="Barnes_1980"/>

The nervous system is the only place in the body that contains [[cilia]]; these are all nonmotile and with a sensory function.<ref>{{cite web |url=http://www.wormbook.org/chapters/www_ciliumbiogenesis.2/ciliumbiogenesis.html |title=The sensory cilia of ''Caenorhabditis elegans'' |website=www.wormbook.org}}</ref><ref>{{cite journal |pmc=2870953 |pmid=20215474 |doi=10.1534/genetics.110.114009 |volume=185 |issue=1 |title=Hearing in Drosophila requires TilB, a conserved protein associated with ciliary motility |journal=Genetics |date=May 2010 |pages=177–88 |last1=Kavlie |first1=RG |last2=Kernan |first2=MJ |last3=Eberl |first3=DF}}</ref>

The body is covered in numerous sensory [[bristle]]s and [[Papilla (worms)|papillae]] that together provide a sense of touch. Behind the sensory bristles on the head lie two small pits, or '[[amphid]]s'. These are well supplied with nerve cells and are probably [[chemoreceptor|chemoreception]] organs. A few aquatic nematodes possess what appear to be [[pigment]]ed eye-spots, but whether or not these are actually sensory in nature is unclear.<ref name="Barnes_1980"/>

== Reproduction==
[[File:Eucoleus aerophilus male spicule.jpg|thumb|Extremity of a male nematode showing the [[Spicule (nematode)|spicule]], used for copulation, bar=100 μm<ref>{{Cite journal |last1=Lalošević |first1=V. |last2=Lalošević |first2=D. |last3=Capo |first3=I. |last4=Simin |first4=V. |last5=Galfi |first5=A. |last6=Traversa |first6=D. |title=High infection rate of zoonotic ''Eucoleus aerophilus'' infection in foxes from Serbia. |journal=Parasite |volume=20 |pages=3 |year=2013 |doi=10.1051/parasite/2012003 |pmid=23340229 |pmc=3718516}}</ref>]]

Most nematode species are [[dioecious]], with separate male and female individuals, though some, such as ''[[Caenorhabditis elegans]]'', are [[androdioecious]], consisting of [[hermaphrodite]]s and rare males. Both sexes possess one or two tubular [[gonad]]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]] and then during intercourse into a glandular and muscular ejaculatory duct associated with the [[vas deferens]] and [[cloaca]]. In females, the ovaries each open into an [[oviduct]] (in hermaphrodites, the eggs enter a [[spermatheca]] first) and then a glandular [[uterus]]. The uteri both open into a common vulva/vagina, usually located in the middle of the morphologically ventral surface.<ref name="Barnes_1980"/>

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 (zoology)|copulation]], one or more [[chitin]]ized [[Spicule (nematode)|spicule]]s move out of the cloaca and are inserted into the genital pore of the female. [[Amoeboid]] [[sperm]] crawl along the spicule into the female worm. Nematode sperm is thought to be the only [[eukaryotic cell]] without the globular protein [[G-actin]].<ref>{{Cite journal |last1=Nelson |first1=G A |last2=Roberts |first2=T M |last3=Ward |first3=S |date=1982-01-01 |title=Caenorhabditis elegans spermatozoan locomotion: amoeboid movement with almost no actin. |url=https://rupress.org/jcb/article/92/1/121/19566/Caenorhabditis-elegans-spermatozoan-locomotion |journal=The Journal of Cell Biology |volume=92 |issue=1 |pages=121–131 |doi=10.1083/jcb.92.1.121 |pmc=2111997 |pmid=7199049}}</ref>

Eggs may be [[embryonated]] or unembryonated when passed by the female, meaning their fertilized eggs may not yet be developed. A few species are known to be [[ovoviviparous]]. The eggs are protected by an outer shell, secreted by the uterus. In free-living roundworms, the eggs hatch into [[larva]]e, which appear essentially identical to the adults, except for an underdeveloped reproductive system; in parasitic roundworms, the lifecycle is often much more complicated.<ref name="Barnes_1980"/> The structure of the eggshell is complicated and includes several layers; a detailed anatomical and terminological framework has been proposed for these layers in 2023.<ref name="Bond&Huffman2023">{{cite journal |last1=Bond |first1=Alan Thomas |last2=Huffman |first2=David George |title=Nematode eggshells: A new anatomical and terminological framework, with a critical review of relevant literature and suggested guidelines for the interpretation and reporting of eggshell imagery |journal=Parasite |volume=30 |year=2023 |doi=10.1051/parasite/2023007 |page=6 |pmid=36920277 |pmc=10016204 }} {{open access}}</ref>

Nematodes as a whole possess a wide range of modes of reproduction.<ref name="Bell_1982">{{cite book |last=Bell |first=G. |title=The masterpiece of nature: the evolution and genetics of sexuality |publisher=University of California Press |location=Berkeley |year=1982 |isbn=978-0-520-04583-5}}</ref> Some nematodes, such as ''[[Heterorhabditis]]'' spp., undergo a process called [[endotokia matricida]]: intrauterine birth causing maternal death.<ref name="Johnigk_Ehlers_1999">{{cite journal |last1=Johnigk |first1=Stefan-Andreas |last2=Ehlers |first2=Ralf-Udo |year=1999 |title=''Endotokia matricida'' in hermaphrodites of ''Heterorhabditis'' spp. and the effect of the food supply |journal=Nematology |issn=1388-5545 |volume=1 |issue=7–8 |pages=717–726 |doi=10.1163/156854199508748 |s2cid=85279418 }}</ref> Some nematodes are [[hermaphrodite|hermaphroditic]], and keep their self-fertilized eggs inside the [[uterus]] until they hatch. The juvenile nematodes then ingest the parent nematode. This process is significantly promoted in environments with a low food supply.<ref name="Johnigk_Ehlers_1999"/>

The nematode model species ''C. elegans'', ''[[Caenorhabditis briggsae|C. briggsae]]'', and ''[[Pristionchus pacificus]]'', among other species, exhibit [[androdioecy]],<ref name="Haag_et_al_2018">{{cite book |last1=Haag |first1=Eric S. |last2=Helder |first2=Johannes |last3=Mooijman |first3=Paul J. W. |last4=Yin |first4=Da |last5=Hu |first5=Shuang |year=2018 |chapter=The evolution of uniparental reproduction in Rhabditina nematodes: Phylogenetic patterns, developmental causes, and surprising consequences |title=Transitions Between Sexual Systems |editor=Leonard, J.L. |publisher=Springer |pages=99–122 |doi=10.1007/978-3-319-94139-4_4 |isbn=978-3-319-94137-0 |chapter-url=https://link.springer.com/chapter/10.1007/978-3-319-94139-4_4}}</ref> 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]].{{citation needed |date=December 2024}}

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.<ref name="Barnes_1980">{{cite book |last= Barnes |first=R.G. |title=Invertebrate zoology |publisher=Sanders College |location=Philadelphia |year=1980 |isbn=978-0-03-056747-6}}</ref>

==Aging==
The nematode ''[[Caenorhabditis elegans]]'' is often used as a model organism for studying [[ageing|aging]] at the molecular level. For example, in ''C. elegans'' aging negatively impacts [[DNA repair]], and mutants of ''C. elegans'' that are long-lived were shown to have increased DNA repair capability.<ref name="Hyun2008">{{cite journal |last1=Hyun |first1=Moonjung |last2=Lee |first2=Jihyun |last3=Lee |first3=Kyungjin |last4=May |first4=Alfred |last5=Bohr |first5=Vilhelm A. |last6=Ahn |first6=Byungchan |title=Longevity and resistance to stress correlate with DNA repair capacity in ''Caenorhabditis elegans'' |journal=Nucleic Acids Res |volume=36 |issue=4 |pages=1380–9 |date=March 2008 |pmid=18203746 |pmc=2275101 |doi=10.1093/nar/gkm1161 }}</ref> These findings suggest a genetically determined correlation between DNA repair capacity and lifespan.<ref name=Hyun2008/> In female ''C. elegans'', [[germline]] processes that control DNA repair and formation of [[chromosomal crossover]]s during [[meiosis]] were shown to progressively deteriorate with age.<ref>{{cite journal |last1=Raices |first1=Marilina |last2=Bowman |first2=Richard |last3=Smolikove |first3=Sarit |last4=Yanowitz |first4=Judith L. |title=Aging Negatively Impacts DNA Repair and Bivalent Formation in the ''C. elegans'' Germ Line |journal=Front Cell Dev Biol |volume=9 |issue= |pages=695333 |date=2021 |pmid=34422819 |pmc=8371636 |doi=10.3389/fcell.2021.695333 |doi-access=free}}</ref>

== 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,<ref>{{cite journal | url=https://pubmed.ncbi.nlm.nih.gov/9851916/ | pmid=9851916 | date=1998 | last1=c. Elegans Sequencing | first1=Consortium | title=Genome sequence of the nematode C. Elegans: A platform for investigating biology | journal=Science | volume=282 | issue=5396 | pages=2012–2018 | doi=10.1126/science.282.5396.2012 }}</ref> the developmental fate of every cell determined, and every neuron mapped.<ref>{{cite web | url=https://www.wormatlas.org/neuronalwiring.html | title=Neuronal Wiring }}</ref>

== Parasitic species==
[[File:Parasite140080-fig3 Gastrointestinal parasites in seven primates of the Taï National Park - Helminths.png|thumb|left |[[Fecal]] parasitic (mostly) nematodes from stools of [[Old World monkey]]s]]

Nematodes that commonly parasitise humans include [[ascarid]]s (''Ascaris''), [[filaria]]s, [[hookworm]]s, [[pinworm (parasite)|pinworm]]s (''Enterobius''), and [[whipworm]]s (''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 nematode]]s parasitize insects and are mostly considered beneficial by humans, but some attack beneficial insects.{{citation needed |date=December 2024}}

One form of nematode is entirely dependent upon [[fig wasp]]s, 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.{{citation needed |date=December 2024}}

[[File:Soybean cyst nematode and egg SEM.jpg|thumb|Colorized [[electron micrograph]] of [[soybean cyst nematode]] (''Heterodera glycines'') and egg]]

A parasitic [[Tetradonematidae|tetradonematid]] nematode discovered in 2005, ''[[Myrmeconema neotropicum]]'', induces fruit mimicry in the tropical ant ''[[Cephalotes atratus]]''. Infected ants develop bright red [[gaster (insect anatomy)|gaster]]s (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]]e, thus completing the lifecycle of ''M. neotropicum''.<ref name="pmid18279076">{{cite journal |last1=Yanoviak |first1=S. P. |last2=Kaspari |first2=M. |last3=Dudley |first3=R. |last4=Poinar |first4=G. |title=Parasite-induced fruit mimicry in a tropical canopy ant |journal=Am. Nat. |volume=171 |issue=4 |pages=536–44 |date=April 2008 |pmid=18279076 |doi=10.1086/528968 |bibcode=2008ANat..171..536Y |s2cid=23857167 }}</ref>

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.<ref>{{Cite journal |title=Organisms associated with ''Lasioglossum zephyrum'' (Hymenoptera: Halictidae) |jstor=25083474 |journal=Journal of the Kansas Entomological Society |date=1965-10-01 |pages=367–389 |volume=38 |issue=4 |first=Suzanne W. T. |last=Batra}}</ref>

=== 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; and the pest nematodes, which attack plants, or act as [[vector (epidemiology)|vectors]] spreading [[plant virus]]es between crop plants.<ref>{{cite journal |last1=Purcell |first1=Mary |last2=Johnson |first2=Marshall W. |last3=Lebeck |first3=Lynn M. |last4=Hara |first4=Arnold H. |title=Biological Control of Helicoverpa zea (Lepidoptera: Noctuidae) with Steinernema carpocapsae (Rhabditida: Steinernematidae) in Corn Used as a Trap Crop |journal=Environmental Entomology |date=1992 |volume=21 |issue=6 |pages=1441–7 |doi=10.1093/ee/21.6.1441}}</ref> Predatory nematodes include ''[[Phasmarhabditis hermaphrodita]]'' which is a lethal parasite of gastropods such as [[slug]]s and [[snail]]s.<ref>{{cite journal |last1=Wilson |first1=M. J. |last2=Glen |first2=D. M. |last3=George |first3=S. K. |title=The rhabditid nematode Phasmarhabditis hermaphrodita as a potential biological control agent for slugs |journal=Biocontrol Science and Technology |date=January 1993 |volume=3 |issue=4 |pages=503–511 |doi=10.1080/09583159309355306 |bibcode=1993BioST...3..503W }}</ref> Some members of the genus ''[[Steinernema]]'' such as ''[[Steinernema carpocapsae]]'' and ''[[Steinernema riobrave]]'' are generalist parasites of [[webworm]]s, [[cutworm]]s, armyworms, [[Oncideres cingulata|girdlers]], some [[weevil]]s, [[Woodboring beetle|wood-borers]] and [[Helicoverpa zea|corn earworm moths]].<ref>{{cite book |last1=Rajamani |first1=Meenatchi |last2=Negi |first2=Aditi |title=Sustainable Bioeconomy |chapter=Biopesticides for Pest Management |date=2021 |pages=239–266 |doi=10.1007/978-981-15-7321-7_11 |isbn=978-981-15-7320-0 |s2cid=228845133 }}</ref> These organisms are grown commercially as [[biological pest control]] agents which can be used as an alternative to [[pesticides]]; their use is considered very safe.<ref>{{cite journal |last1=Ehlers |first1=R.-U. |last2=Hokkanen |first2=H. M. T. |title=Insect Biocontrol with Non-endemic Entomopathogenic Nematodes (Steinernema and Heterorhabditis spp.): Conclusions and Recommendations of a Combined OECD and COST Workshop on Scientific and Regulatory Policy Issues |journal=Biocontrol Science and Technology |date=September 1996 |volume=6 |issue=3 |pages=295–302 |doi=10.1080/09583159631280 |bibcode=1996BioST...6..295E }}</ref> Plant-parasitic nematodes include several groups causing severe crop losses, taking 10% of crops worldwide every year.<ref name="Smiley-et-al-2017">{{cite journal |last1=Smiley |first1=Richard W. |last2=Dababat |first2=Abdelfattah A. |last3=Iqbal |first3=Sadia |last4=Jones |first4=Michael G. K. |last5=Maafi |first5=Zahra Tanha |last6=Peng |first6=Deliang |last7=Subbotin |first7=Sergei A. |last8=Waeyenberge |first8=Lieven |title=Cereal Cyst Nematodes: A Complex and Destructive Group of ''Heterodera'' Species |journal=[[Plant Disease (journal)|Plant Disease]] |publisher=[[American Phytopathological Society]] |volume=101 |issue=10 |year=2017 |issn=0191-2917 |doi=10.1094/pdis-03-17-0355-fe |pages=1692–1720 |pmid=30676930 |doi-access=free |bibcode=2017PlDis.101.1692S }}</ref> The most common genera are ''[[Aphelenchoides]]'' ([[foliar nematode]]s), ''[[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. This nematode is transmitted from tree to tree by sawyer beetles (''[[Monochamus]]'').<ref>{{Citation |last1=Gibbs |first1=J.N. |title=PATHOLOGY {{!}} Insect Associated Tree Diseases |date=2004 |url=https://linkinghub.elsevier.com/retrieve/pii/B0121451607000703 |encyclopedia=Encyclopedia of Forest Sciences |pages=802–8 |access-date=2023-03-21 |publisher=Elsevier |language=en |doi=10.1016/b0-12-145160-7/00070-3 |isbn=978-0-12-145160-8 |last2=Webber |first2=J.F.|url-access=subscription }}</ref>

Greenhouse growers use [[entomopathogenic nematode]]s as beneficial agents to control [[fungus gnat]]s. The nematodes enter the larvae of the gnats by way of their anus, mouth, and [[Spiracle (arthropods)|spiracle]]s (breathing pores) and then release [[bacteria]] which kills the gnat larvae. Commonly used nematode species to control pests on greenhouse crops include ''[[Steinernema]] feltiae'' for fungus gnats and [[western flower thrips]], ''[[Steinernema carpocapsae]]'' used to control shore flies, ''Steinernema kraussei'' for control of [[Vine weevil|black vine weevil]]s, and ''[[Heterorhabditis bacteriophora]]'' to control beetle larvae.<ref>{{Cite magazine |last=Kloosterman |first=Stephen |date=April 2022 |title=Small Soldiers |magazine=Green House Product News |volume=32 |issue=4 |pages=26–29}}</ref>

Rotations of plants with nematode-resistant species or varieties is one means of managing parasitic nematode infestations. For example, planting [[Tagetes |Tagetes marigolds]] as a cover crop just prior to planting a nematode-susceptible plant, has been shown to suppress nematodes.<ref>{{Cite web |title=ENY-056/NG045: Marigolds (Tagetes spp.) for Nematode Management |access-date=2023-11-20 |url=https://edis.ifas.ufl.edu/publication/NG045 |first1=R. |last1=Krueger |first2=K. E. |last2=Dover |first3=R. |last3=McSorley |first4=K-H. |last4=Wang |website=[[Institute of Food and Agricultural Sciences]]}}</ref> Another approach involves using natural antagonists, particularly bacteria and fungi, which have proven effective in suppressing plant-parasitic nematodes,<ref>{{cite journal |last1=Pires |first1=David |last2=Vicente |first2=Cláudia S. L. |last3=Menéndez |first3=Esther |last4=Faria |first4=Jorge M. S. |last5=Rusinque |first5=Leidy |last6=Camacho |first6=Maria J. |last7=Inácio |first7=Maria L. |title=The Fight against Plant-Parasitic Nematodes: Current Status of Bacterial and Fungal Biocontrol Agents |journal=Pathogens |volume=11 |issue=10 |pages=1178 |date=October 2022 |doi=10.3390/pathogens11101178 |doi-access=free |pmid=36231510 |pmc=9566127 |hdl=10174/32705 |hdl-access=free }}</ref> 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.<ref name="US-2008/072494">{{ cite patent |country=US |number=2008072494 |status=application |title=Micronutrient elicitor for treating nematodes in field crops |pubdate=2008-03-27 |fdate=2006-09-07 |inventor=Stoner, R.J.; Linden, J.C. }}</ref> [[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 pest that has resulted in quarantines and crop failures worldwide. It can be controlled, however. [[CSIRO]], the scientific research body of the Australian government, found a 13- to 14-fold reduction of nematode population densities in plots having [[Chinese mustard]] ''[[Brassica juncea]]'' green manure or seed meal in the soil.<ref name=CSIRO>{{cite journal |last1=Loothfar |first1=R. |last2=Tony |first2=S. |date=2005-03-22 |title=Suppression of root knot nematode (''Meloidogyne javanica'') after incorporation of Indian mustard cv. Nemfix as green manure and seed meal in vineyards |journal=[[Australasian Plant Pathology]] |volume=34 |issue=1 |pages=77–83 |doi=10.1071/AP04081 |bibcode=2005AuPP...34...77R |s2cid=24299033 |url=http://www.publish.csiro.au/paper/AP04081 |access-date=2010-06-14 |df=dmy-all|url-access=subscription }}</ref>

=== Disease in humans===
[[File:Intestinal nematode infections world map - DALY - WHO2002.svg|thumb|upright=1.23 |[[Disability-adjusted life year]] for intestinal nematode infections per 100,000 in 2002.{{div col |colwidth=6em}} {{legend |#ffff65 |&nbsp;&nbsp;<&nbsp; 25}} {{legend |#fff200 |25–50}} {{legend |#ffdc00 |50–75}} {{legend |#ffc600 |75–100}} {{legend |#ffb000 |100–120}} {{legend |#ff9a00 |120–140}} {{legend |#ff8400 |140–160}} {{legend |#ff6e00 |160–180}} {{legend |#ff5800 |180–200}} {{legend |#ff4200 |200–220}} {{legend |#ff2c00 |220–240}} {{legend |#cb0000 |&nbsp;&nbsp;>&nbsp; 240}} {{legend |#b3b3b3 |no data}} {{div col end}}]]
[[File:Anthelmintic effect of papain on Heligmosomoides bakeri.ogv|thumb|[[Anthelmintic]] effect of [[papain]] on ''[[Heligmosomoides bakeri]]'']]

A number of pathogenic intestinal nematodes cause diseases in humans, including [[ascariasis]], [[trichuriasis]], and [[hookworm disease]]. ''[[Anisakis]]'' species parasitise fish, and [[marine mammal]]s and when consumed by humans can cause [[anisakiasis]] a [[gastric]] or gastroallergic disease.<ref name="Patiño">{{cite journal |last1=Patiño |first1=JA |last2=Olivera |first2=MJ |title=Gastro-allergic anisakiasis: The first case reported in Colombia and a literature review. |journal=Biomedica: Revista del Instituto Nacional de Salud |date=15 June 2019 |volume=39 |issue=2 |pages=241–246 |doi=10.7705/biomedica.v39i2.3936 |pmid=31529811|doi-access=free }}</ref>  Gastrointestinal nematode infections in humans are common, with approximately 50% of the global population being affected. Developing countries are most heavily impacted, in part due to lack of access to medical care.<ref>{{cite journal |last1=Stepek |first1=Gillian |last2=Buttle |first2=David J. |last3=Duce |first3=Ian R. |last4=Behnke |first4=Jerzy M. |title=Human gastrointestinal nematode infections: are new control methods required? |journal=International Journal of Experimental Pathology |date=October 2006 |volume=87 |issue=5 |pages=325–341 |doi=10.1111/j.1365-2613.2006.00495.x |pmid=16965561 |pmc=2517378 }}</ref>

[[Trichinosis]] starts in the intestines but larvae can migrate to muscle. ''[[Filaria]]l'' nematodes cause [[filariasis|filariases]].

[[Toxocariasis]] is a [[zoonotic]] infection caused by roundworms passed from dogs, and sometimes cats. It can give rise to different types of ''larva migrans'' such as [[visceral larva migrans]], and [[ocular larva migrans]].

Studies have shown that parasitic nematodes infect [[American eel]]s causing damage to the eel's swim bladder,<ref>{{cite journal | doi=10.1139/cjfas-2018-0136 | title=Temporal, spatial, and biological variation of nematode epidemiology in American eels | date=2019 | last1=Warshafsky | first1=Zoemma T. | last2=Tuckey | first2=Troy D. | last3=Vogelbein | first3=Wolfgang K. | last4=Latour | first4=Robert J. | last5=Wargo | first5=Andrew R. | journal=Canadian Journal of Fisheries and Aquatic Sciences | volume=76 | issue=10 | pages=1808–1818 | bibcode=2019CJFAS..76.1808W | hdl=1807/95295 | url=https://scholarworks.wm.edu/vimsarticles/1780 | hdl-access=free }}</ref> dairy animals like cattle and buffalo,<ref>{{cite journal | doi=10.1023/A:1005263009921 | date=1999 | last1=Jithendran | first1=K.P. | last2=Bhat | first2=T.K. | title=Epidemiology of Parasitoses in Dairy Animals in the North West Humid Himalayan Region of India with Particular Reference to Gastrointestinal Nematodes | journal=Tropical Animal Health and Production | volume=31 | issue=4 | pages=205–214 | pmid=10504100 }}</ref> and all species of sheep.<ref>{{cite journal |last1=Morgan |first1=E.R. |last2=van Dijk |first2=J. |title=Climate and the epidemiology of gastrointestinal nematode infections of sheep in Europe |journal=Vet Parasitol |volume=189 |issue=1 |pages=8–14 |date=September 2012 |pmid=22494941 |doi=10.1016/j.vetpar.2012.03.028 }}</ref>

== Soil ecosystems==
{{further |Soil ecology}}

About 90% of nematodes reside in the top 15&nbsp;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.<ref name=brady>{{cite book |last1=Brady |first1=Nyle C. |last2=Weil |first2=Ray R. |year=2009 |title=Elements of the Nature and Properties of Soils |edition=3rd |publisher=Prentice Hall |isbn=978-0-13-501433-2 |oclc=1015309711}}</ref> But plant parasitic nematodes cause billions of dollars in annual crop damage worldwide.<ref>{{cite journal | url=http://www.annualreviews.org/doi/full/10.1146/annurev.phyto.41.052102.104023 | doi=10.1146/annurev.phyto.41.052102.104023 | title=Parasitic nematode interactions with mammals and plants | date=2003 | last1=Jasmer | first1=Douglas P. | last2=Goverse | first2=Aska | last3=Smant | first3=Geert | journal=Annual Review of Phytopathology | volume=41 | issue=1 | pages=245–270 | pmid=14527330 | bibcode=2003AnRvP..41..245J | url-access=subscription }}</ref>

One group of [[carnivorous fungus|carnivorous fungi]], the [[Nematophagous fungus|nematophagous fungi]], are predators of soil nematodes.<ref>{{Cite news |last=Nosowitz |first=Fan |date=2021-02-08 |title=How California Crops Fought Off a Pest Without Using Pesticide |url=https://modernfarmer.com/2021/02/how-california-crops-fought-off-a-pest-without-using-pesticide/ |access-date=2021-02-15 |work=Modern Farmer}}</ref> They can set enticements for the nematodes in the form of lassos or adhesive structures.<ref name="Pramer 1964">{{cite journal |last=Pramer |first=C. |year=1964 |title=Nematode-trapping fungi |journal=Science |volume=144 |issue=3617 |pages=382–388 |doi=10.1126/science.144.3617.382 |pmid=14169325 |bibcode=1964Sci...144..382P}}</ref><ref name="Hauser_1985">{{cite journal |date=December 1985 |title=Nematode-trapping fungi |url=http://www.carnivorousplants.org/cpn/articles/CPNv14n1p8_11.pdf |journal=Carnivorous Plant Newsletter |volume=14 |issue=1 |pages=8–11 |last=Hauser |first=J.T. |doi=10.55360/cpn141.jh945 }}</ref><ref name="Ahrén 1998">{{cite journal |last1=Ahrén |first1=D. |last2=Ursing |first2=B.M. |last3=Tunlid |first3=A. |year=1998 |title=Phylogeny of nematode-trapping fungi based on 18S rDNA sequences |journal=FEMS Microbiology Letters |volume=158 |issue=2 |pages=179–184 |pmid=9465391 |doi=10.1111/j.1574-6968.1998.tb12817.x}}</ref> They can also release powerful toxins when in contact with nematodes.<ref name="Lee2023">{{cite journal |last1=Lee |first1=CH |last2=Lee |first2=YY |last3=Chang |first3=YC |last4=Pon |first4=WL |last5=Lee |first5=SP |last6=Wali |first6=N |last7=Nakazawa |first7=T |last8=Honda |first8=Y |last9=Shie |first9=JJ |last10=Hsueh |first10=YP |title=A carnivorous mushroom paralyzes and kills nematodes via a volatile ketone. |journal=Science Advances |date=18 January 2023 |volume=9 |issue=3 |pages=eade4809 |doi=10.1126/sciadv.ade4809 |pmid=36652525|pmc=9848476 |bibcode=2023SciA....9E4809L }}</ref>

== Survivability==
{{See also|List of longest-living organisms#Revived into activity after stasis}}

The nematode ''[[Caenorhabditis elegans]]'' an important [[model organism]], was used  as part of an ongoing research project conducted on the 2003 [[Space Shuttle Columbia|Space Shuttle ''Columbia'']] mission [[STS-107]], and survived the [[Space Shuttle Columbia disaster#Recovery of debris|re-entry breakup]]. It is believed to be the first known species to survive a virtually unprotected atmospheric descent to Earth's surface.<ref>{{cite magazine |magazine=Astrobiology Magazine |url=http://www.astrobio.net/topic/origins/extreme-life/columbia-survivors/ |title=Columbia Survivors |date=Jan 1, 2006 |access-date=January 12, 2016 |archive-url=https://web.archive.org/web/20160304201612/http://www.astrobio.net/topic/origins/extreme-life/columbia-survivors/ |archive-date=March 4, 2016 |url-status=usurped}}</ref><ref>{{cite journal |last1=Szewczyk |first1=Nathaniel J. |last2=Mancinelli |first2=Rocco L. |last3=McLamb |first3=William |last4=Reed |first4=David |last5=Blumberg |first5=Baruch S. |last6=Conley |first6=Catharine A. |title=''Caenorhabditis elegans'' Survives Atmospheric Breakup of STS–107, Space Shuttle Columbia |journal=Astrobiology |date=December 2005 |volume=5 |issue=6 |pages=690–705 |doi=10.1089/ast.2005.5.690 |bibcode=2005AsBio...5..690S |pmid=16379525}}</ref> The Antarctic nematode ''[[Panagrolaimus]] davidi'' was able to withstand intracellular freezing depending on how well it had been fed.<ref>{{cite journal |last1=Raymond |first1=Mélianie R. |last2=Wharton |first2=David A. |date=February 2013 |title=The ability of the Antarctic nematode ''Panagrolaimus davidi'' to survive intracellular freezing is dependent upon nutritional status |journal=Journal of Comparative Physiology B |volume=183 |issue=2 |pages=181–8 |doi=10.1007/s00360-012-0697-0 |pmid=22836298 |s2cid=17294698 |url=http://link.springer.com/10.1007/s00360-012-0697-0|url-access=subscription }}</ref> In 2023 an individual of ''[[Panagrolaimus kolymaensis]]'' was revived after 46,000 years in Siberian permafrost.<ref>{{cite journal |last1=Shatilovich |first1=Anastasia |last2=Gade |first2=Vamshidhar R. |date=27 July 2023 |title=A novel nematode species from the Siberian permafrost shares adaptive mechanisms for cryptobiotic survival with ''C. elegans'' dauer larva |journal=PLOS Genetics |volume=19 |issue=7 |pages=e1010798 |doi=10.1371/journal.pgen.1010798 |pmid=37498820 |pmc=10374039 |doi-access=free }}</ref>

== See also==
* {{annotated link |Biological pest control}}
* {{annotated link |List of organic gardening and farming topics}}
* {{annotated link |List of parasites of humans}}
* {{annotated link |Nematode.net}}
* [[Soil food web]]
* {{annotated link |Worm bagging}}

==References==
{{Reflist}}

== External links==
{{Commons category |Nematoda}}
* [https://web.archive.org/web/20101230000456/http://www.harper-adams.ac.uk/groups/crops/nematology/ Harper Adams University College Nematology Research]
* [https://wayback.archive-it.org/all/20110721030344/http://knol.google.com/k/nematodes-roundworms-of-man#view Nematodes/roundworms of man]
* [http://www.ucmp.berkeley.edu/phyla/ecdysozoa/nematoda.html Introduction to the Nematoda]
* [http://www.esn-online.org/ European Society of Nematologists]
* http://webarchive.loc.gov/all/20020914155908/http://www.nematodes.org/
* [https://web.archive.org/web/20061027152335/http://intramar.ugent.be/nemys/start.asp?group=2 NeMys World free-living Marine Nematodes database]
* [http://faculty.ucr.edu/%7Epdeley/lab/taxonomy.html Nematode Virtual Library]
* [http://nematologists.org/ Society of Nematologists]
* [http://nematologists.org.au/ Australasian Association of Nematologists] {{Webarchive |url=https://web.archive.org/web/20150226031051/http://nematologists.org.au/ |date=26 February 2015 }}
* [http://entnemdept.ufl.edu/creatures/main/search_higher_nematodes.htm Phylum Nematoda – nematodes] on the [[University of Florida|UF]] 
* [https://entnemdept.ufl.edu/creatures/ Featured Creatures Web site]—University of Florida  Institute of Food and Agricultural Sciences (IFAS)

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[[Category:Cosmopolitan animals]]
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[[Category:Nematoida]]
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[[Category:Pest control]]