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{{Taxobox
| name = Nematodes
| image = roundworm.jpg
| regnum = [[Animal]]ia
| phylum = '''Nematoda'''
| phylum_authority = [[Karl Rudolphi|Rudolphi]], 1808
| subdivision_ranks = Classes
| subdivision = 
[[Adenophorea]]<br />
&nbsp;&nbsp;&nbsp;Subclass [[Enoplia]]<br />
&nbsp;&nbsp;&nbsp;Subclass [[Chromadoria]]<br />
[[Secernentea]]<br />
&nbsp;&nbsp;&nbsp;Subclass [[Rhabditia]]<br />
&nbsp;&nbsp;&nbsp;Subclass [[Spiruria]]<br />
&nbsp;&nbsp;&nbsp;Subclass [[Diplogasteria]]<br />
&nbsp;&nbsp;&nbsp;Subclass [[Tylenchia]]
}}
{{SI}}
{{CMG}}
__NOTOC__


==Overview==

The '''nematodes''' or '''roundworms''' ([[phylum (biology)|Phylum]] '''Nematoda''' from [[Greek language|Greek]] {{polytonic|νῆμα}} (nema): "thread" + {{polytonic|-ώδη}} -ode "like") are one of the most common phyla of [[animal]]s, with over 80,000 different described species (over 15,000 are [[parasite|parasitic]]). 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 [[Antarctica]] and [[oceanic trench|oceanic trenches]]. Further, there are a great many parasitic forms, including [[pathogen]]s in most plants, animals, and also in humans.

The nematodes were originally named nematoidea by [[Karl Rudolphi|Rudolphi]] (1808). They were renamed nematodes by Burmeister 1837 (as a family; Leuckart 1848 and von Siebold 1848 both promoted them to the rank of order), then nematoda ([[K. M. Diesing|Diesing]] 1861), though [[Nathan Cobb]] (1919) argued that they should be called nemata or nemates (and in English 'nemas' rather than 'nematodes'). After some confusion which saw the nematodes placed (often together with the horsehair worms, [[nematomorpha]]) as a class or order in various groups such as [[Aschelminthes]], [[Ray Lankester|Lankester]] (1877) definitively promoted them to the level of [[phylum]].

==Morphology==
Nematodes are unsegmented, [[bilateria|bilaterally symmetric]] and [[triploblastic]] [[protostome]]s with a complete [[digestive system]]. Roundworms have no circulatory or respiratory systems so they use [[diffusion]] to breathe.<ref>ATKINSON,H.J. (1973) "THE RESPIRATORY PHYSIOLOGY OF THE MARINE NEMATODES ENOPLUS BREVIS(BASTIAN) AND E. COMMUNIS (BASTIAN) ''Journal of Experimental Biology'' '''59''': 355-266[http://jeb.biologists.org/cgi/reprint/59/1/255.pdf]</ref> Although they lack a circulatory system, nutrients are transported throughout the body via fluid in the pseudocoelom. They are thin and are round in cross section. Nematodes are one of the simplest animal groups to have a complete digestive system, with a separate orifice for food intake and waste excretion, a pattern followed by all subsequent, more complex animals. The body cavity is a [[body cavity|pseudocoelom]] (persistent [[blastula]]), which lacks the muscles of coelomate animals used to force food down the digestive tract. Nematodes thus depend on internal/external pressures and body movement to move food through their digestive tracts. The mouth is often surrounded by various flaps or projections used in feeding and sensation. The portion of the body past the [[anus]] or [[cloaca]] is called the "tail." As they grow, their cells get larger, but the total number is constant, called [[eutely]]. The epidermis secretes a layered cuticle made of three layers of [[collagen]]<ref name="hickman"/> that protects the body from drying out, from digestive juices, or from other harsh environments. Although this cuticle allows movement and shape changes via a [[hydrostatic skeleton|hydrostatic skeletal system]], it is very inelastic so does not allow the volume of the worm to increase. Therefore, as the worm grows, it has to [[ecdysis|molt]] and form new cuticles. The cuticles don't allow volume to increase so as to keep hydrostatic pressure inside the organism very high. For this reason, the roundworms do not possess circular muscles (just longitudinal ones) as they're not required. This hydrostatic pressure is the reason the roundworms are round.

Nematodes have a simple nervous system, with a main [[ventral nerve cord]] and a smaller [[dorsal nerve cord]]<ref>
White ''et al.'' (1976) ''The Structure of the Ventral Nerve Cord of Caenorhabditis elegans''[http://links.jstor.org/sici?sici=0080-4622%2819760810%29275%3A938%3C327%3ATSOTVN%3E2.0.CO%3B2-E]</ref>. Sensory structures at the anterior end are called [[amphid]]s, while sensory structures at the posterior end are called [[phasmid]]s.

Most free-living nematodes are microscopic, though a few parasitic forms can grow to over a meter in length (typically as parasites of very large animals such as whales). There are no circular [[muscle]]s, so the body can only undulate from side to side. Contact with solid objects is necessary for locomotion; its thrashing motions vary from mostly to completely ineffective at swimming. 

Nematodes generally eat [[bacteria]], [[fungi]] and [[protozoans]], although some are [[filter feeders]]. [[Excretion]] is through a separate excretory pore. Nematodes also contract bacterial infections within excretion pores.

==Reproduction==
Reproduction is usually sexual. Males are usually smaller than females (often much smaller) and often have a characteristically bent tail for holding the female for [[copulation]].  During copulation, one or more [[chitin|chitinized]] [[spicule]]s move out of the cloaca and are inserted into 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]].

Eggs may be embryonated or unembryonated when passed by the female, meaning that their fertilized eggs may not yet be developed. In free-living roundworms, the eggs hatch into larva, which eventually grow into adults; in parasitic roundworms, the life cycle is often much more complicated.

Some nematodes, specifically ''[[Heterorhabditis]]'' spp., undergo a process called ''endotokia matricida''; intra-uterine birth causing maternal death.<ref name=Endotokia>{{cite journal|author=Johnigk, Stefan-Andreas; Ehlers, Ralf-Udo|title=Endotokia matricida in hermaphrodites of Heterorhabditis spp. and the effect of the food supply |journal=Nematology|volume=1|issue=|pages=717-726|year=1999|url=http://www.ingentaconnect.com/content/brill/nemy/1999/00000001/F0020007/art00006 | doi = 10.1163/156854199508748}}</ref> The hermaphroditic nematode keeps its self-fertilized eggs inside its uterus until they hatch. The juvenile nematodes will then ingest the parent nematode. This process is significantly promoted in environments with a low or reducing food supply.<ref name=Endotokia/>

==Free-living species==
In free-living species, development usually consists of four molts of the cuticle during growth.  Different species feed on materials as varied as 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 is ''[[Caenorhabditis elegans]]'', which lives in the soil and has found much use as a [[model organism]].  ''C. elegans'' has had its entire genome sequenced, as well as the developmental fate of every cell determined, and every neuron mapped.

Some Nematodes can undergo [[cryptobiosis]].

==Parasitic species==
Parasitic forms often have quite complicated life cycles, moving between several different hosts or locations in the host's body.  Infection occurs variously by eating uncooked meat with larvae in it, by entrance into unprotected cuts or directly through the skin, by transfer via [[blood-sucking]] insects, and so forth.

Nematodes commonly parasitic on humans include [[whipworm]]s, [[hookworm]]s, [[pinworm]]s, [[ascarid]]s, and [[filarid]]s. The species ''[[Trichinella spiralis]]'', commonly known as the trichina worm, occurs in rats, pigs, and humans, and is responsible for the disease [[trichinosis]]. ''[[Baylisascaris]]'' usually infests wild animals but can be deadly to humans as well. ''[[Haemonchus contortus]]'' is one of the most abundant infectious agents in sheep around the world, causing great economic damage to sheep farms.
In contrast, [[entomopathogenic nematode]]s parasitize insects and are considered by humans to be beneficial.

One form of nematode is entirely dependent upon the wasps which are the sole source of [[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. 

Plant parasitic nematodes include several groups causing severe crop losses. The most common genera are: ''Aphelenchoides'' ([[foliar nematodes]]), ''Meloidogyne'' (root-knot nematodes), ''Heterodera'', ''Globodera'' (cyst nematodes) such as the [[potato cyst nematode]], ''Nacobbus'', ''Pratylenchus'' (lesion nematodes), ''Ditylenchus'', ''Xiphinema'', ''Longidorus'', ''Trichodorus''. Several phytoparasitic nematode species cause histological damages to roots, including the formation of visible galls (''Meloidogyne'') 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 GFLV (Grapevine Fanleaf Virus), an important disease of grapes.

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.

The largest nematode ever recorded, ''Placentonema gigantissima'', was discovered parasitizing the placenta of a sperm whale, measuring 8.5 m in length with a diameter of 0.3 mm, and containing 32 ovaries.<ref name=Gubanov/>

Other large nematodes include: ''[[Dioctophyma renale]]'', the giant kidney worm, a parasite most commonly found in [[mink]] but also in dogs and humans, that can reach up to 103 cm in length.<ref>{{cite web | title = Giant Kidney Worm Infection in Mink and Dogs | work = The Merck Veterinary Manual | date = 2006 | url = http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/130506.htm | accessdate = 2007-02-10 }}</ref><ref name=Gubanov>Gubanov, N.M. 1951. Dokl Akad Nauk SSSR. 1951 Apr 21;77(6):1123-5. Translated: [Giant nematoda from the placenta of Cetacea; Placentonema gigantissima nov. gen., nov. sp.]</ref>

==Abundance==
The nematode species are very difficult to distinguish.  Of the [[body cavity|pseudocoelomates]], the nematodes are the most common. Nematodes have successfully adapted to nearly every [[Ecological_niche|niche]] from marine to fresh water, from the polar regions to the tropics, as well as the highest to the lowest of elevations. They represent, for example, 90% of all life on the seafloor of the Earth. [http://www.eurekalert.org/pub_releases/2007-12/cp-dsl122007.php]  Though 20,000 species have been classified it is estimated that this number might be upwards of 500,000 if all were known.<ref>{{cite book |last= Hyman |first= L.H. |title=The invertebrates, vol. III. Acanthocephala, Aschelminthes, and Entoprocta, the pseudocoelomate Bilateria. |publisher= Mc Graw Hill |location= New York|isbn=0070316619}}</ref>. In certain fertile areas the [[topsoil]] is estimated to contain in the [[1000000000 (number)|billion]]s of nematodes per [[acre]].<ref name="hickman">{{cite book |last= Hickman |first= Cleveland  |coauthors= Roberts L. Keen S. Larson A. Eisenhour D|title= Animal Diversity |edition= 4th |publisher= Mc Graw Hill |location= New York|language= English |isbn= 978-0-07-252844-2}}</ref> In the 1914 edition of the Yearbook of the [[United States Department of Agriculture]] N.A.Cobb wrote on the abundance of nematodes<ref name="hickman"/>: {{cquote2|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 thin 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.|N.A.Cobb}}

==Gardening==
[[image:Soybean cyst nematode and egg SEM.jpg|thumb|200px|[[Soybean cyst nematode]] and egg]]
Depending on the species, a nematode may be beneficial or detrimental to a gardener's cause.

From a gardening perspective, there are two categories of nematode: predatory ones, which will kill garden pests like [[cutworm]]s, and pest nematodes, like the [[root-knot nematode]], which attack garden plants.

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]].

==Phylogeny==
{{refimprove-section|date=January 2007}}
While it has recently been suggested that nematodes are related to the [[arthropod]]s and [[priapulid]]s and should be grouped with them in the [[Ecdysozoa]] (molting animals), there is substantial resistance within the nematology community. Grouping organisms based on behaviors is not generally accepted. While there seems to be ''some'' [[evolution]]ary connection between these phyla, the exact nature of their relationship is still being debated.<ref>{{cite journal | author=Pilato G, Binda MG, Biondi O, D’Urso V, Lisi O, Marletta A, Maugeri S, Nobile V, Rappazzo G, Sabella G, Sammartano F, Turrisi G, Viglianisi F | title=The clade Ecdysozoa, perplexities and questions| journal=Zoologischer Anzeiger| year=2005| volume=244| issue=1| pages=43-50 | doi = 10.1016/j.jcz.2005.04.001}}</ref>

That the roundworms have a large number of peculiar [[apomorph]]ies and in many cases a parasitic lifestyle confounds analyses; the [[DNA sequence]] data hitherto analyzed is equivocal on ecdysozoan [[monophyly]]. Genetic analyses of roundworms suggest that - as is also indicated by their unique morphological features - the group has been under intense [[Natural selection|selective]] pressure during its early radiation, resulting apparently in accelerated rates of both morphological and molecular evolution. Until a strong [[phylogenetic]] tree based on combined evidence is produced, most agree that the Nematoda should simply be referred to as part of the [[Metazoa]].

==Cultural references==
*The first episode of the children's television show [[Doug]] involves a nematode, though it is posited that the animal is purely fictional. See [[List of Doug episodes]], #1.

*In the [[SpongeBob SquarePants]] episode [[Home Sweet Pineapple]], nematodes eat Spongebob's pineapple house, and he is forced to find a new home.  In [[Best Day Ever]], SpongeBob leads hungry nematodes away from the Krusty Krab by playing the song "Best Day Ever" with his hands and [[proboscis]]. In the episode [[Sailor Mouth]] SpongeBob reads graffiti that says "Nematoads are people too."

==Robustness==
*Hundreds of nematode worms (''C. elegans''), featured in a research project on mission [[STS-107]], survived the [[Space Shuttle Columbia Disaster]].<ref>{{cite news |title=Worms survived Columbia disaster |url=http://news.bbc.co.uk/1/hi/sci/tech/2992123.stm |publisher=[[BBC News]] |date=[[2003-05-01]]}}</ref>

==See also==
*[[Caenorhabditis elegans]]: An important [[model organism]] often used to study [[cellular differentiation]], sometimes referred to as "worm" by scientists.
*[[Ascariasis]]: A human disease caused by the parasitic roundworm ''Ascaris lumbricoides''
*[[Toxocariasis]]: A [[helminth]] infection of humans caused by the [[dog]] or [[cat]] [[roundworm]], ''[[Toxocara canis]]'' or ''[[Toxocara cati]]''
*[[Xiphinema|Xiphinema: List of species]]

==References==
{{reflist|2}}

==External links==
{{commonscat|Nematoda}}
* http://www.ucmp.berkeley.edu/phyla/ecdysozoa/nematoda.html
* [http://www.nematode.net/ Nematode.net: Repository of parasitic nematode sequences.]
* http://www.nematodes.org/
*[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://www.ifns.org/ International Federation of Nematology Societies]
*[http://nematologists.org/ Society of Nematologists]
*[http://nematologists.org.au/ Australasian Association of Nematologists]
*[http://news.bbc.co.uk/2/hi/science/nature/6612411.stm Research on nematodes and longevity]
<br>

[[Category:Parasites]]


[[ar:ديدان اسطوانية]]
[[bg:Кръгли червеи]]
[[ca:Nematode]]
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[[da:Rundorme]]
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[[et:Ümarussid]]
[[es:Nematoda]]
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[[ko:선형동물]]
[[hr:Oblići]]
[[io:Nematodo]]
[[is:Þráðormar]]
[[it:Nematoda]]
[[la:Nematoda]]
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[[mk:Цевчести црви]]
[[ms:Cacing Gelang]]
[[nl:Rondwormen]]
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