Editing Flatworm

{{Short description|Phylum of soft-bodied invertebrates}}
{{Good article}}
{{Automatic taxobox
| name = Flatworm
| fossil_range = <br />[[Permian]]–[[Holocene|Present]], <br />{{fossilrange|270|0|earliest = 508|ref=<ref name="Dentzien-Dias2013" />}} <small>Possible Cambrian, Ordovician and Devonian records</small><ref name=Rugosusivitta /><ref name=GRKnaustDesrochers />
| image = Platyhelminthes diversity.jpg
| image_caption = In a clockwise spiral, starting from top left: ''[[Diplozoidae|Eudiplozoon]] nipponicum'' ([[monogenean]]s), [[pork tapeworm]] ([[tapeworms]]), [[common liver fluke]] ([[trematodes]]), ''[[Pseudobiceros hancockanus]]'' ([[Turbellaria]])
| display_parents = 7
| taxon = Platyhelminthes
| authority = Claus, 1887
| subdivision_ranks = Classes
| subdivision = '''Traditional:'''
* [[Turbellaria]]
* [[Trematoda]]
* [[Monogenea]]
* [[Cestoda]]
'''Phylogenetic:'''
* [[Catenulida]]
* [[Rhabditophora]]
| synonyms = * Plathelminthes <small>Schneider, 1873</small><ref name="Ehlers1995">{{Cite journal |last1=Ehlers |first1=U. |last2=Sopott-Ehlers |first2=B. |doi=10.1007/BF00036354 |title=Plathelminthes or Platyhelminthes? |journal=Hydrobiologia |volume=305 |pages=1–2 |date=June 1995 |url=https://books.google.com/books?id=_lLtCAAAQBAJ&pg=PA3 |isbn=9789401100458 |s2cid=45170603 |access-date=2020-11-11 |archive-date=2023-03-23 |archive-url=https://web.archive.org/web/20230323154425/https://books.google.com/books?id=_lLtCAAAQBAJ&pg=PA3 |url-status=live }}</ref>
}}

'''Platyhelminthes''' (from the [[Greek language|Greek]] πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "[[worm]]")<ref name="Ehlers1995" /> is a [[Phylum (biology)|phylum]] of relatively simple [[bilaterian]], [[Segmentation (biology)|unsegmented]], soft-bodied [[invertebrate]]s commonly called '''flatworms''' or '''flat worms'''. Being [[acoelomate]]s (having no [[coelom|body cavity]]), and having no specialised [[circulatory system|circulatory]] and [[respiratory system|respiratory]] [[organ (anatomy)|organ]]s, they are restricted to having flattened shapes that allow [[oxygen]] and nutrients to pass through their bodies by [[diffusion]]. The digestive cavity has only one opening for both ingestion (intake of nutrients) and egestion (removal of undigested wastes); as a result, the [[food]] can not be processed continuously.

In traditional medicinal texts, Platyhelminthes are divided into [[Turbellaria]], which are mostly non-[[parasitic]] [[animal]]s such as [[planarian]]s, and three entirely parasitic groups: [[Cestoda]], [[Trematoda]] and [[Monogenea]]; however, since the turbellarians have since been proven not to be [[monophyletic]], this classification is now deprecated. Free-living flatworms are mostly predators, and live in [[water]] or in shaded, humid terrestrial environments, such as [[leaf litter]]. Cestodes (tapeworms) and trematodes (flukes) have complex life-cycles, with mature stages that live as parasites in the digestive systems of [[fish]] or land [[vertebrate]]s, and intermediate stages that infest secondary hosts. The eggs of trematodes are excreted from their main hosts, whereas adult cestodes generate vast numbers of [[hermaphroditic]], segment-like [[proglottid]]s that detach when mature, are excreted, and then release eggs. Unlike the other parasitic groups, the monogeneans are external parasites infesting [[aquatic animal]]s, and their [[larva]]e metamorphose into the adult form after attaching to a suitable host.

Because they do not have internal body [[Body cavity|cavities]], Platyhelminthes were regarded as a primitive stage in the [[evolution]] of bilaterians (animals with [[bilateral symmetry]] and hence with distinct front and rear ends). However, analyses since the mid-1980s have separated out one subgroup, the [[Acoelomorpha]], as [[Basal (phylogenetics)|basal]] bilaterians – closer to the original [[bilateria]]ns than to any other modern groups. The remaining Platyhelminthes form a [[monophyletic]] group, one that contains all and only descendants of a [[common ancestor]] that is itself a member of the group. The redefined Platyhelminthes is part of the [[Spiralia]], one of the two main groups of [[Protostomia]]. These analyses had concluded the redefined Platyhelminthes, excluding Acoelomorpha, consists of two monophyletic subgroups, [[Catenulida]] and [[Rhabditophora]], with Cestoda, Trematoda and Monogenea forming a monophyletic subgroup within one branch of the Rhabditophora. Hence, the traditional platyhelminth subgroup "Turbellaria" is now regarded as [[paraphyletic]], since it excludes the wholly parasitic groups, although these are descended from one group of "turbellarians".

A planarian species has been used in the [[Philippines]] and [[Maldives]] in an attempt to [[biological pest control|control]] populations of the [[introduced species|imported]] giant African snail (''[[Achatina fulica]]''), which was eating agricultural crops. Success was initially reported for the Maldives but this was only temporary and the role of flatworms has been questioned.<ref>{{Cite journal |last=Gerlach |first=Justin |last2=Barker |first2=Gary M. |last3=Bick |first3=Cindy S. |last4=Bouchet |first4=Philippe |last5=Brodie |first5=Gilianne |last6=Christensen |first6=Carl C. |last7=Collins |first7=Timothy |last8=Coote |first8=Trevor |last9=Cowie |first9=Robert H. |last10=Fiedler |first10=G. Curt |last11=Griffiths |first11=Owen L. |last12=Florens |first12=F. B. Vincent |last13=Hayes |first13=Kenneth A. |last14=Kim |first14=Jaynee |last15=Meyer |first15=Jean-Yves |date=2021-04-01 |title=Negative impacts of invasive predators used as biological control agents against the pest snail Lissachatina fulica: the snail Euglandina ‘rosea’ and the flatworm Platydemus manokwari |url=https://link.springer.com/article/10.1007/s10530-020-02436-w |journal=Biological Invasions |language=en |volume=23 |issue=4 |pages=997–1031 |doi=10.1007/s10530-020-02436-w |issn=1573-1464|doi-access=free }}</ref> These planarians have now spread very widely throughout the tropics and are themselves a serious threat to native snails, and should not be used for biological control. In [[Northwestern Europe|northwest Europe]], there are concerns about the spread of the [[New Zealand]] planarian ''[[Arthurdendyus triangulatus]]'', which preys on [[earthworm]]s.

==Description==

[[File:Haeckel Platodes.jpg|thumb|Various parasitic flatworms from Haeckel's ''[[Kunstformen der Natur]]'' (1904)]]

===Distinguishing features===

Platyhelminthes are [[Symmetry (biology)#Bilateral symmetry|bilaterally symmetrical]] [[animal]]s: their left and right sides are mirror images of each other; this also implies they have distinct top and bottom surfaces and distinct head and tail ends. Like other [[bilateria]]ns, they have three main [[cell (biology)|cell]] layers (endoderm, [[mesoderm]], and [[ectoderm]]),<ref name="WalkerAnderson2001PlatyhelminthesInAnderson">{{cite book
|author1=Walker, J.C. |author2=Anderson, D.T. |year=2001 |chapter=The Platyhelminthes |pages=58–80
|editor=Anderson, D.T. |title=Invertebrate Zoology |publisher=Oxford University Press
|isbn=978-0-19-551368-4
}}</ref> while the [[Symmetry (biology)#Radial symmetry|radially symmetrical]] [[cnidarian]]s and [[ctenophore]]s (comb jellies) have only two cell layers.<ref name="Hinde2001CnidariaAndCtenophoraInAnderson">{{cite book |last=Hinde |first=R.T. |year=2001 |chapter=The Cnidaria and Ctenophora |pages=28–57 |editor=Anderson, D.T. |title=Invertebrate Zoology |publisher=Oxford University Press |isbn=978-0-19-551368-4}}</ref> Beyond that, they are "defined more by what they do not have than by any particular series of specializations."<ref name="Barnes1998DiversityOfLifePlatyhelminthes">{{cite book |title=The Diversity of Living Organisms |author=Barnes, R.S.K. |publisher=Blackwell Publishing |year=1998 |isbn=978-0-632-04917-2 |pages=194–195 |url=https://books.google.com/books?id=Xvk-yrjXH70C&q=Platyhelminthes&pg=PA194 |access-date=2008-12-21 |archive-date=2023-03-23 |archive-url=https://web.archive.org/web/20230323154437/https://books.google.com/books?id=Xvk-yrjXH70C&q=Platyhelminthes&pg=PA194 |url-status=live }}</ref> Unlike most other bilaterians, Platyhelminthes have no internal body cavity, so are described as [[acoelomate]]s. Although the absence of a [[coelom]] also occurs in other bilaterians: [[gnathostomulid]]s, [[gastrotrich]]s, [[Xenacoelomorpha|xenacoelomorphs]], [[Symbion|cycliophorans]], [[entoprocta]]ns and the parastic [[mesozoa]]ns.<ref>{{Cite journal |title=Gnathostomulid phylogeny inferred from a combined approach of four molecular loci and morphology |year=2006 |doi=10.1111/j.1096-0031.2006.00085.x |last1=Sorensen |first1=Martin V. |last2=Sterrer |first2=Wolfgang |last3=Giribet |first3=Gonzalo |journal=Cladistics |volume=22 |issue=1 |pages=32–58 |pmid=34892893 |s2cid=85570246 |doi-access=free }}</ref><ref>{{Cite journal |title=An Introduction to the Study of Gastrotricha, with a Taxonomic Key to Families and Genera of the Group |year=2019 |doi=10.3390/d11070117 |doi-access=free |last1=Todaro |first1=M. Antonio |last2=Sibaja-Cordero |first2=Jeffrey Alejandro |last3=Segura-Bermúdez |first3=Oscar A. |last4=Coto-Delgado |first4=Génesis |last5=Goebel-Otárola |first5=Nathalie |last6=Barquero |first6=Juan D. |last7=Cullell-Delgado |first7=Mariana |last8=Dal Zotto |first8=Matteo |journal=Diversity |volume=11 |issue=7 |page=117 |hdl=11380/1179356 |hdl-access=free }}</ref><ref>{{Cite journal |title=Xenacoelomorpha: a case of independent nervous system centralization? |year=2016 |pmc=4685578 |last1=Gavilán |first1=B. |last2=Perea-Atienza |first2=E. |last3=Martínez |first3=P. |journal=Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences |volume=371 |issue=1685 |doi=10.1098/rstb.2015.0039 |pmid=26598722 }}</ref><ref>{{Cite journal |title=Muscular anatomy of an entoproct creeping-type larva reveals extraordinary high complexity and potential shared characters with mollusks |year=2015 |doi=10.1186/s12862-015-0394-1 |last1=Merkel |first1=Julia |last2=Lieb |first2=Bernhard |last3=Wanninger |first3=Andreas |journal=BMC Evolutionary Biology |volume=15 |issue=1 |page=130 |pmid=26138503 |pmc=4490756 |doi-access=free |bibcode=2015BMCEE..15..130M }}</ref><ref>{{Cite journal |title=Dicyemida and Orthonectida: Two Stories of Body Plan Simplification |year=2019 |doi=10.3389/fgene.2019.00443 |doi-access=free |last1=Zverkov |first1=Oleg A. |last2=Mikhailov |first2=Kirill V. |last3=Isaev |first3=Sergey V. |last4=Rusin |first4=Leonid Y. |last5=Popova |first5=Olga V. |last6=Logacheva |first6=Maria D. |last7=Penin |first7=Alexey A. |last8=Moroz |first8=Leonid L. |last9=Panchin |first9=Yuri V. |last10=Lyubetsky |first10=Vassily A. |last11=Aleoshin |first11=Vladimir V. |journal=Frontiers in Genetics |volume=10 |page=443 |pmid=31178892 |pmc=6543705 }}</ref> They also lack specialized [[circulatory system|circulatory]] and [[respiratory system|respiratory]] organs, both of these facts are defining features when classifying a flatworm's [[organ (anatomy)|anatomy]].<ref name="WalkerAnderson2001PlatyhelminthesInAnderson" /><ref name="RuppertBarnes2004Platyhelminthes">{{cite book |author1=Ruppert, E.E. |author2=Fox, R.S. |author3=Barnes, R.D. |name-list-style=amp |title=Invertebrate Zoology |publisher=Brooks / Cole |edition=7 |isbn=978-0-03-025982-1 |year=2004 |pages=[https://archive.org/details/isbn_9780030259821/page/226 226–269] |url-access=registration |url=https://archive.org/details/isbn_9780030259821/page/226 }}</ref> Their bodies are soft and unsegmented.<ref name="Rhode2001PlatyhelminthesInEncLifeSci" />

{|class="wikitable" style="margin-left:4px"
! Attribute !! [[Cnidarian]]s and [[Ctenophore]]s<ref name="Hinde2001CnidariaAndCtenophoraInAnderson" /> !! Platyhelminthes (flatworms)<ref name="WalkerAnderson2001PlatyhelminthesInAnderson" /><ref name="RuppertBarnes2004Platyhelminthes"/> ||More "advanced" [[bilateria]]ns<ref name="RuppertBarnes2004Bilateria">{{cite book |author1=Ruppert, E.E. |author2=Fox, R.S. |author3=Barnes, R.D. |name-list-style=amp |title=Invertebrate Zoology |publisher=Brooks / Cole |edition=7 |isbn=978-0-03-025982-1 |year=2004 |pages=[https://archive.org/details/isbn_9780030259821/page/196 196–224] |url-access=registration |url=https://archive.org/details/isbn_9780030259821/page/196 }}</ref>
|- align="center"
! Bilateral symmetry
|No ||colspan="2" |Yes
|- align="center"
! Number of main cell layers
|[[Diploblasty|Two]], with jelly-like layer between them ([[mesoglea]]) ||colspan="2" |[[Triploblasty|Three]]
|- align="center"
! Distinct [[brain]]
|No ||colspan="2" |Yes
|- align="center"
! Specialized [[digestive system]]
|No ||colspan="2" |Yes
|- align="center"
! Specialized [[excretory system]]
|No ||colspan="2" |Yes
|- align="center"
! Body cavity containing internal organs
|colspan="2" |No ||Yes
|- align="center"
! Specialized [[circulatory system|circulatory]] and [[respiratory system|respiratory]] [[organ (anatomy)|organ]]s
|colspan="2" |No ||Yes
|}

===Features common to all subgroups===

The lack of circulatory and respiratory organs limits platyhelminths to sizes and shapes that enable [[oxygen]] to reach and [[carbon dioxide]] to leave all parts of their bodies by simple [[diffusion]]. Hence, many are microscopic, and the large species have flat ribbon-like or leaf-like shapes. Because there is no circulatory system which can transport nutrients around, the guts of large species have many branches, allowing the nutrients to diffuse to all parts of the body.<ref name="Barnes1998DiversityOfLifePlatyhelminthes" /> Respiration through the whole surface of the body makes them vulnerable to fluid loss, and restricts them to environments where [[dehydration]] is unlikely: sea and freshwater, moist terrestrial environments such as [[leaf litter]] or between grains of soil, and as [[parasite]]s within other animals.<ref name="WalkerAnderson2001PlatyhelminthesInAnderson" />

The space between the skin and gut is filled with [[mesenchyme]], also known as [[Parenchyma#Flatworms|parenchyma]], a [[connective tissue]] made of [[cell (biology)|cells]] and reinforced by [[collagen]] fibers that act as a type of [[skeleton]], providing attachment points for [[muscle]]s. The mesenchyme contains all the internal organs and allows the passage of oxygen, nutrients and waste products. It consists of two main types of cell: fixed cells, some of which have fluid-filled [[vacuole]]s; and [[stem cell]]s, which can transform into any other type of cell, and are used in regenerating tissues after injury or [[asexual reproduction]].<ref name="WalkerAnderson2001PlatyhelminthesInAnderson" />

Most platyhelminths have no [[anus]] and regurgitate undigested material through the mouth. The genus ''[[Paracatenula]]'', whose members include tiny flatworms living in symbiosis with bacteria, is even missing a mouth and a gut.<ref>{{Cite web |url=https://www.iflscience.com/this-seabed-flatworm-got-rid-of-its-mouth-and-anus-replacing-its-entire-digestive-system-with-bacteria-52075 |title=This Seabed Flatworm Got Rid Of Its Mouth And Anus, Replacing Its Entire Digestive System With Bacteria |date=8 April 2019 |access-date=2022-12-11 |archive-date=2022-12-11 |archive-url=https://web.archive.org/web/20221211200016/https://www.iflscience.com/this-seabed-flatworm-got-rid-of-its-mouth-and-anus-replacing-its-entire-digestive-system-with-bacteria-52075 |url-status=live }}</ref> However, some long species have an anus and some with complex, branched guts have more than one anus, since excretion only through the mouth would be difficult for them.<ref name="RuppertBarnes2004Platyhelminthes" /> The gut is lined with a single layer of [[endoderm]]al cells that absorb and digest food. Some species break up and soften food first by secreting [[enzyme]]s in the gut or [[pharynx]] (throat).<ref name="WalkerAnderson2001PlatyhelminthesInAnderson"/>

All animals need to keep the [[Concentration (chemistry)|concentration]] of dissolved substances in their body fluids at a fairly constant level. Internal parasites and free-living marine animals live in environments with high concentrations of dissolved material, and generally let their tissues have the same level of concentration as the environment, while freshwater animals need to prevent their body fluids from becoming too dilute. Despite this difference in environments, most platyhelminths use the same system to [[osmoregulation|control the concentration]] of their body fluids. [[Flame cell]]s, so called because the beating of their [[flagella]] looks like a flickering candle flame, extract from the mesenchyme water that contains wastes and some reusable material, and drive it into networks of tube cells which are lined with flagella and [[microvilli]]. The tube cells' flagella drive the water towards exits called [[nephridiopore]]s, while their microvilli reabsorb reusable materials and as much water as is needed to keep the body fluids at the right concentration. These combinations of flame cells and tube cells are called [[protonephridia]].<ref name="WalkerAnderson2001PlatyhelminthesInAnderson" /><ref name="RuppertBarnes2004Bilateria" />

In all platyhelminths, the [[nervous system]] is concentrated at the head end. Other platyhelminths have rings of [[Ganglion|ganglia]] in the head and main nerve trunks running along their bodies.<ref name="WalkerAnderson2001PlatyhelminthesInAnderson" /><ref name="RuppertBarnes2004Platyhelminthes" />

==Major subgroups==

Early classification divided the flatworms in four groups: Turbellaria, Trematoda, Monogenea and Cestoda. This classification had long been recognized to be artificial, and in 1985, Ehlers<ref>Ehlers U. (1985). "Phylogenetic relationships within the Plathelminthes", pp 143–158
in ''The Origins and Relationships of Lower Invertebrates''. S Conway Morris, JD George, R Gibson, HM Platt (eds.). Clarendon Press, Oxford.</ref> proposed a [[phylogenetic]]ally more correct classification, where the massively [[polyphyletic]] "Turbellaria" was split into a dozen orders, and Trematoda, Monogenea and Cestoda were joined in the new order [[Neodermata]]. However, the classification presented here is the early, traditional, classification, as it still is the one used everywhere except in scientific articles.<ref name="WalkerAnderson2001PlatyhelminthesInAnderson"/><ref>{{Cite book |url=https://www.worldcat.org/oclc/988304549 |title=Zoology 2016 |date=2015 |publisher=Macmillan/McGraw-Hill School Div |others=Stephen Miller, John Harley |isbn=978-0-07-667895-2 |oclc=988304549 |access-date=2021-03-27 |archive-date=2023-03-23 |archive-url=https://web.archive.org/web/20230323154501/https://www.worldcat.org/title/988304549 |url-status=live}}</ref>

===Turbellaria===

{{main |Turbellaria}}

[[File:Pseudoceros dimidiatus.jpg|thumb |The turbellarian ''[[Pseudoceros dimidiatus]]'']]

[[File:Mating Pseudobiceros bedfordi.png|thumb |Two [[turbellaria]]ns are mating by [[penis fencing]]. Each has two penises, the white spikes on the undersides of their heads.]]

These have about 4,500 species,<ref name="RuppertBarnes2004Platyhelminthes"/> are mostly free-living, and range from {{convert|1|mm|in|2|abbr=on}} to {{convert|600|mm|in|abbr=on}} in length. Most are predators or scavengers, and terrestrial species are mostly nocturnal and live in shaded, humid locations, such as leaf litter or rotting wood. However, some are [[symbiosis|symbiotes]] of other animals, such as [[crustacea]]ns, and some are [[parasite]]s. Free-living turbellarians are mostly black, brown or gray, but some larger ones are brightly colored.<ref name="WalkerAnderson2001PlatyhelminthesInAnderson" /> The [[Acoela]] and [[Nemertodermatida]] were traditionally regarded as turbellarians,<ref name="RuppertBarnes2004Platyhelminthes" /><ref name="JondeliusEtAl2002Nemertodermatida" /> but are now regarded as members of a separate phylum, the [[Acoelomorpha]],<ref name="Halanych2004NewViewOfAnimalPhylogeny" /><ref name="LarssonJondelius2008PhylogenyOfCatenulida" /> or as two separate phyla.<ref name="Wallberg2007EtAlDismissalOfAcoelomorpha" /> ''[[Xenoturbella]]'', a [[genus]] of very simple animals,<ref name="Westblad1949Xenoturbella" /> has also been reclassified as a separate phylum.<ref name="BourlatEtAl2003XenoturbellaPhylum" />

Some turbellarians have a simple [[pharynx]] lined with [[Cilium|cilia]] and generally feed by using cilia to sweep food particles and small prey into their mouths, which are usually in the middle of their undersides. Most other turbellarians have a pharynx that is eversible (can be extended by being turned inside-out), and the mouths of different species can be anywhere along the underside.<ref name="WalkerAnderson2001PlatyhelminthesInAnderson" /> The freshwater species ''[[Microstomum caudatum]]'' can open its mouth almost as wide as its body is long, to swallow prey about as large as itself.<ref name="RuppertBarnes2004Platyhelminthes" /> Predatory species in suborder [[Kalyptorhynchia]] often have a muscular pharynx equipped with hooks or teeth used for seizing prey.<ref>{{cite journal | pmc=4612338 | date=2015 | author1=Smith JP III | last2=Litvaitis | first2=M. K. | last3=Gobert | first3=S. | last4=Uyeno | first4=T. | last5=Artois | first5=T. | title=Evolution and Functional Morphology of the Proboscis in Kalyptorhynchia (Platyhelminthes) | journal=Integrative and Comparative Biology | volume=55 | issue=2 | pages=205–216 | doi=10.1093/icb/icv056 | pmid=26002347 }}</ref>

Most turbellarians have pigment-cup [[ocelli]] ("little eyes"); one pair in most species, but two or even three pairs in others. A few large species have many eyes in clusters over the brain, mounted on tentacles, or spaced uniformly around the edge of the body. The ocelli can only distinguish the direction from which light is coming to enable the animals to avoid it. A few groups have [[statocyst]]s - fluid-filled chambers containing a small, solid particle or, in a few groups, two. These statocysts are thought to function as balance and acceleration sensors, as they perform the same way in [[cnidaria]]n [[medusae]] and in [[ctenophore]]s. However, turbellarian statocysts have no sensory cilia, so the way they sense the movements and positions of solid particles is unknown. On the other hand, most have ciliated touch-sensor cells scattered over their bodies, especially on tentacles and around the edges. Specialized cells in pits or grooves on the head are most likely smell sensors.<ref name="RuppertBarnes2004Platyhelminthes" />

[[Planarian]]s, a subgroup of seriates, are famous for their ability to regenerate if divided by cuts across their bodies. Experiments show that (in fragments that do not already have a head) a new head grows most quickly on those fragments which were originally located closest to the original head. This suggests the growth of a head is controlled by a chemical whose concentration diminishes throughout the organism, from head to tail. Many turbellarians [[cloning|clone]] themselves by transverse or longitudinal division, whilst others, reproduce by [[budding]].<ref name="RuppertBarnes2004Platyhelminthes" />

The vast majority of turbellarians are [[hermaphrodite]]s (they have both female and male reproductive cells) which fertilize eggs [[internal fertilization|internally]] by [[copulation (zoology)|copulation]].<ref name="RuppertBarnes2004Platyhelminthes" /> Some of the larger aquatic species mate by [[penis fencing]] – a duel in which each tries to impregnate the other, and the loser adopts the female role of developing the eggs.<ref>{{cite web |url=https://www.pbs.org/kcet/shapeoflife/episodes/hunt_explo2.html |access-date=2008-12-21 |title=Fighting to mate: flatworm penis fencing |author=Newman, Leslie |publisher=[[Public Broadcasting Service|PBS]] |archive-date=2013-11-14 |archive-url=https://web.archive.org/web/20131114002014/http://www.pbs.org/kcet/shapeoflife/episodes/hunt_explo2.html |url-status=live }}</ref> In most species, "miniature adults" emerge when the eggs hatch, but a few large species produce [[plankton]]-like [[larva]]e.<ref name="RuppertBarnes2004Platyhelminthes" />
{{clear}}

===Trematoda===

{{main|Trematoda}}

These parasites' name refers to the cavities in their [[holdfast (biology)|holdfast]]s (Greek τρῆμα, hole),<ref name="WalkerAnderson2001PlatyhelminthesInAnderson" /> which resemble suckers and anchor them within their hosts.<ref name="Rhode2001PlatyhelminthesInEncLifeSci">{{Cite book|author=Klaus Rohde|chapter=Platyhelminthes (Flatworms)|title=Encyclopedia of Life Sciences (eLS)|doi=10.1038/npg.els.0001585|year=2001|isbn=978-0470016176|publisher=John Wiley & Sons, Ltd}}</ref> The skin of all species is a [[syncitium]], which is a layer of cells that shares a single external [[Biological membrane|membrane]]. Trematodes are divided into two groups, Digenea and Aspidogastrea (also known as Aspodibothrea).<ref name="RuppertBarnes2004Platyhelminthes" />

====Digenea====
{{main|Digenea}}

[[File:Metagonimus LifeCycle.gif|thumb|300px|Life cycle of the digenean ''[[Metagonimus]]'']]

These are often called flukes, as most have flat [[rhomboid]] shapes like that of a [[flounder]] (Old English {{Lang|ang|flóc}}). There are about 11,000 species, more than all other platyhelminthes combined, and second only to [[roundworm]]s among parasites on [[metazoan]]s.<ref name="RuppertBarnes2004Platyhelminthes" /> Adults usually have two holdfasts: a ring around the mouth and a larger sucker midway along what would be the underside in a free-living flatworm.<ref name="WalkerAnderson2001PlatyhelminthesInAnderson" />
Although the name "Digeneans" means "two generations", most have very complex life cycles with up to seven stages, depending on what combinations of environments the early stages encounter – the most important factor being whether the eggs are deposited on land or in water. The intermediate stages transfer the parasites from one host to another. The [[definitive host]] in which adults develop is a land vertebrate; the earliest host of juvenile stages is usually a snail that may live on land or in water, whilst in many cases, a fish or arthropod is the second host.<ref name="RuppertBarnes2004Platyhelminthes" /> For example, the adjoining illustration shows the life cycle of the intestinal fluke ''[[metagonimus]]'', which hatches in the intestine of a snail, then moves to a fish where it penetrates the body and encysts in the flesh, then migrating to the small intestine of a land animal that eats the fish raw, finally generating eggs that are excreted and ingested by snails, thereby completing the cycle. A similar life cycle occurs with ''[[Opisthorchis viverrini]]'', which is found in South East Asia and can infect the liver of humans, causing [[Cholangiocarcinoma]] (bile duct cancer). Schistosomes, which cause the devastating tropical disease [[bilharzia]], also belong to this group.<ref name="SCP" />

Adults range between {{convert|0.2|mm|in|abbr=on}} and {{convert|6|mm|in|abbr=on}} in length. Individual adult digeneans are of a single sex, and in some species slender females live in enclosed grooves that run along the bodies of the males, partially emerging to lay eggs. In all species the adults have complex reproductive systems, capable of producing between 10,000 and 100,000 times as many eggs as a free-living flatworm. In addition, the intermediate stages that live in snails reproduce asexually.<ref name="RuppertBarnes2004Platyhelminthes" />

Adults of different species infest different parts of the definitive host - for example the [[intestine]], [[lung]]s, large blood vessels,<ref name="WalkerAnderson2001PlatyhelminthesInAnderson" /> and liver.<ref name="RuppertBarnes2004Platyhelminthes" /> The adults use a relatively large, muscular [[pharynx]] to ingest cells, cell fragments, [[mucus]], body fluids or blood. In both the adult and snail-inhabiting stages, the external syncytium absorbs dissolved nutrients from the host. Adult digeneans can live without oxygen for long periods.<ref name="RuppertBarnes2004Platyhelminthes" />

====Aspidogastrea====

{{main|Aspidogastrea}}

Members of this small group have either a single divided sucker or a row of suckers that cover the underside.<ref name="RuppertBarnes2004Platyhelminthes" /> They infest the guts of [[teleost|bony]] or [[elasmobranch|cartilaginous]] fish, turtles, or the body cavities of marine and freshwater [[bivalve]]s and [[gastropod]]s.<ref name="WalkerAnderson2001PlatyhelminthesInAnderson" /> Their eggs produce [[cilia]]ted swimming larvae, and the life cycle has one or two hosts.<ref name="RuppertBarnes2004Platyhelminthes" />

===Cercomeromorpha===

[[Cercomeromorpha]] contains parasites attach themselves to their hosts by means of disks that bear crescent-shaped hooks. They are divided into the Monogenea and Cestoda groupings.<ref name="RuppertBarnes2004Platyhelminthes" />

====Monogenea====

[[File:Journal.pone.0079155.g004 Only silhouettes of bodies.svg|thumb|left|150 px|Silhouettes of bodies of various [[polyopisthocotylea]]n [[Monogenea]]ns<ref name=Justine2013>{{Cite journal|vauthors=Justine JL, Rahmouni C, Gey D, Schoelinck C, Hoberg EP |title=The Monogenean which lost its clamps|doi=10.1371/journal.pone.0079155|journal=PLOS ONE|year=2013|pmid=24278118 |pmc=3838368|volume=8|issue=11|pages=e79155|bibcode=2013PLoSO...879155J|doi-access=free}}</ref>]]

{{main|Monogenea}}

Of about 1,100 species of [[monogeneans]], most are external parasites that require particular host species - mainly fish, but in some cases amphibians or aquatic reptiles. However, a few are internal parasites. Adult monogeneans have large attachment organs at the rear, known as [[haptor]]s (Greek ἅπτειν, ''haptein'', means "catch"), which have [[Sucker (parasitic worm anatomy)|suckers]], [[clamp (zoology)|clamps]], and hooks. They often have flattened bodies. In some species, the [[pharynx]] secretes enzymes to digest the host's skin, allowing the parasite to feed on blood and cellular debris. Others graze externally on mucus and flakes of the hosts' skins. The name "Monogenea" is based on the fact that these parasites have only one nonlarval generation.<ref name="RuppertBarnes2004Platyhelminthes" />

====Cestoda====

{{main|Cestoda}}

[[File:Taenia solium Life cycle.tif|thumb|300px |Life cycle of the eucestode ''[[Taenia (flatworm)|Taenia]]'': Inset 5 shows the scolex, which has four ''[[Taenia solium]]'', a disk with hooks on the end. Inset 6 shows the tapeworm's whole body, in which the scolex is the tiny, round tip in the top left corner, and a mature proglottid has just detached.]]

These are often called tapeworms because of their flat, slender but very long bodies – the name "[[cestoda|cestode]]" is derived from the [[Latin]] word ''cestus'', which means "tape". The adults of all 3,400 cestode species are internal parasites. Cestodes have no mouths or guts, and the [[syncitium|syncitial]] skin absorbs nutrients – mainly [[carbohydrate]]s and [[amino acid]]s – from the host, and also disguises it chemically to avoid attacks by the host's [[immune system]].<ref name="RuppertBarnes2004Platyhelminthes" /> Shortage of carbohydrates in the host's diet stunts the growth of parasites and may even kill them. Their [[metabolism]]s generally use simple but inefficient chemical processes, compensating for this inefficiency by consuming large amounts of food relative to their physical size.<ref name="WalkerAnderson2001PlatyhelminthesInAnderson" />

In the majority of species, known as eucestodes ("true tapeworms"), the neck produces a chain of segments called proglottids via a process known as [[strobilation]]. As a result, the most mature proglottids are furthest from the scolex. Adults of ''[[Taenia saginata]]'', which infests humans, can form proglottid chains over {{convert|20|m|ft}} long, although {{convert|4|m|ft}} is more typical. Each proglottid has both male and female reproductive organs. If the host's gut contains two or more adults of the same cestode species they generally fertilize each other, however, proglottids of the same worm can fertilize each other and even themselves. When the eggs are fully developed, the proglottids separate and are excreted by the host. The eucestode life cycle is less complex than that of [[digenea]]ns, but varies depending on the species. For example:
* Adults of ''[[Diphyllobothrium]]'' infest fish, and the juveniles use [[copepod]] crustaceans as intermediate hosts. Excreted proglottids release their eggs into the water where the eggs hatch into [[cilia]]ted, swimming larvae. If a larva is swallowed by a copepod, it sheds the cilia and the skin becomes a syncitium; the larva then makes its way into the copepod's [[hemocoel]] (an internal cavity which is the central part of the [[circulatory system]]) where it attaches itself using three small hooks. If the copepod is eaten by a fish, the larva [[metamorphosis|metamorphoses]] into a small, unsegmented tapeworm, drills through to the gut and grows into an adult.<ref name="RuppertBarnes2004Platyhelminthes" />
* Various species of ''[[Taenia (flatworm)|Taenia]]'' infest the guts of humans, cats and dogs. The juveniles use herbivores – such as pigs, cattle and rabbits – as intermediate hosts. Excreted proglottids release eggs that stick to grass leaves and hatch after being swallowed by a herbivore. The larva then makes its way to the herbivore's muscle tissue, where it metamorphoses into an oval worm about {{convert|10|mm|in}} long, with a scolex that is kept internally. When the definitive host eats infested raw or undercooked meat from an intermediate host, the worm's scolex pops out and attaches itself to the gut, when the adult tapeworm develops.<ref name="RuppertBarnes2004Platyhelminthes" />

Members of the smaller group known as [[Cestodaria]] have no scolex, do not produce proglottids, and have body shapes similar to those of diageneans. Cestodarians parasitize fish and turtles.<ref name="WalkerAnderson2001PlatyhelminthesInAnderson" />
{{Clear}}

==Classification and evolutionary relationships==

{{See also|List of bilateral animal orders}}

The relationships of Platyhelminthes to other [[Bilateria]] are shown in the [[phylogenetic tree]]:<ref name="Halanych2004NewViewOfAnimalPhylogeny" />

{{clade
|label1=[[Bilateria]]
|1={{clade
   |1=[[Acoelomorpha]] [[File:Proporus sp. (no background).png|55 px]]
   |2={{clade
      |1=[[Deuterostomia]] [[File:Cyprin carpi 090613-0329 tdp.png|60 px]]
      |label2=[[Protostomia]]
      |2={{clade
         |1=[[Ecdysozoa]] [[File:Aptostichus simus Monterey County.jpg|60px]]
         |label2=[[Spiralia]]
         |2={{clade
            |1=[[Gnathifera (clade)|Gnathifera]] <span style="{{MirrorH}}">[[File:Pseudosagitta maxima 31349361.png|70px]]</span>
            |label2=[[Platytrochozoa]]
            |sublabel2=580 mya
            |2={{clade
               |label1=[[Rouphozoa]]
               |1={{clade
                  |1=[[Gastrotricha]] [[File:Diuronotus aspetos 1.png|60px]]
                  |sublabel2=270 mya
                  |2='''Platyhelminthes''' [[File:Pseudobiceros bedfordi 13376124.png|70px]]
                  }}
               |label2=[[Lophotrochozoa]]
               |sublabel2=550 mya
               |2={{clade
                  |1=[[Mollusca]] <span style="{{MirrorH}}">[[File:Grapevinesnail 01a.jpg|65px]]</span>
                  |2=[[Annelida]] [[File:Polychaeta (no) 2.jpg|60px]]
                  }}
               }}
            }}
         }}
      }}
   }}
}}

The internal relationships of Platyhelminthes are shown below. The tree is not fully resolved.<ref name="TimothyEtAl2004ProtostomesAndPlatyhelminthes" /><ref name=Boll2013>{{Cite journal |doi=10.4013/nbc.2013.81.06 |last1=Boll |first1=P. K. |last2=Rossi |first2=I. |last3=Amaral |first3=S. V. |last4=Oliveira |first4=S. M. |last5=Müller |first5=E. S. |last6=Lemos |first6=V. S. |last7=Leal-Zanchet |first7=A. M. |date=2013 |title=Platyhelminthes ou apenas semelhantes a Platyhelminthes? Relações filogenéticas dos principais grupos de turbelários |journal=Neotropical Biology and Conservation |volume=8 |issue=1 |pages=41–52|doi-access=free }}</ref><ref name=Egger2015>{{Cite journal |doi=10.1016/j.cub.2015.03.034 |last1=Egger |first1=B. |last2=Lapraz |first2=F. |last3=Tomiczek |first3=B. |last4=Müller |first4=S. |last5=Dessimoz |first5=C. |last6=Girstmair |first6=J. |last7=Škunca |first7=N. |last8=Rawlinson |first8=K. A. |last9=Cameron |first9=C. B. |last10=Beli |first10=E. |last11=Todaro |first11=M. A. |last12=Gammoudi |first12=M. |last13=Noreña |first13=C. |last14=Telford |first14=M. I. |date=18 May 2015 |title=A Transcriptomic-Phylogenomic Analysis of the Evolutionary Relationships of Flatworms |journal=Current Biology |volume=25 |issue=10 |pages=1347–1353 |pmid=25866392 |pmc=4446793}}</ref>

{{clade
|label1='''Platyhelminthes'''
|1={{clade
  |1=[[Catenulida]]
  |label2=[[Rhabditophora]]
  |2={{clade
    |1=[[Macrostomorpha]]
    |label2=[[Trepaxonemata]]
    |2={{clade
      |label1=[[Amplimatricata]]
      |1={{clade
        |1=[[Prorhynchidae|Prorhynchida]]
        |2=[[Polycladida]]
         }}
      |2={{clade
        |1=[[Gnosonesimidae|Gnosonesimida]]
        |label2=[[Euneoophora]]
        |2={{clade
          |label1=[[Rhabdocoela]]
          |1={{clade
            |1=[[Kalyptorhynchia]]
            |2=[[Dalytyphloplanida]]
             }}
          |2={{clade
            |1=[[Proseriata]]
            |label2=Acentrosomata
            |2={{clade
              |label1=[[Adiaphanida]]
              |1={{clade
                |1=[[Prolecithophora]]
                |2={{clade
                  |1=[[Fecampiida]]
                  |2=[[Tricladida]] (planarians)
                   }}
                 }}
              |label2=[[Bothrioneodermata]]
              |2={{clade
                |label1=Bothrioplanata
                |1=[[Bothrioplanida]] (freshwater)
                |sublabel2=''parasitic''
                |2=[[Neodermata]] ([[fluke (parasite)|fluke]]s, [[tapeworms]])
                 }}
               }}
             }}
           }}
         }}
       }}
     }}
   }}
}}

The oldest confidently identified parasitic flatworm fossils are [[cestode]] eggs found in a [[Permian]] shark [[coprolite]], but helminth hooks still attached to [[Devonian]] [[acanthodians]] and [[placoderms]] might also represent parasitic flatworms with simple life cycles.<ref>{{cite book |isbn=978-0-12-804001-0 |author=De Baets, K., P. Dentzien-Dias, I. Upeniece, O. Verneau and P. C. J. Donoghue |doi=10.1016/bs.apar.2015.06.002 |pmid=26597066 |title=Advances in Parasitology |chapter=Chapter Three – Constraining the Deep Origin of Parasitic Flatworms and Host-Interactions with Fossil Evidence |editor=Kenneth De Baets and D. Timothy J Littlewood |volume=90 |pages=93–135 |date=2015-12-15 |s2cid=7278956 |url=https://hal-univ-perp.archives-ouvertes.fr/hal-01257074/file/debaets2015.pdf |access-date=2022-07-15 |archive-date=2022-07-30 |archive-url=https://web.archive.org/web/20220730040408/https://hal-univ-perp.archives-ouvertes.fr/hal-01257074/file/debaets2015.pdf |url-status=live }}</ref> The oldest known free-living platyhelminth specimen is a fossil preserved in [[Eocene]] age [[Baltic amber]] and placed in the monotypic species ''[[Micropalaeosoma|Micropalaeosoma balticus]]'',<ref name="Poinar2003">{{cite journal |doi=10.1111/j.1744-7410.2003.tb00095.x |last1=Poinar |first1=G. |year=2003 |title=A Rhabdocoel Turbellarian (Platyhelminthes, Typhloplanoida) in Baltic Amber with a Review of Fossil and Sub-Fossil Platyhelminths |journal= Invertebrate Biology |volume=122 |issue=4|pages=308–312 |jstor=3227067}}</ref> whilst the oldest subfossil specimens are [[Schistosoma|schistosome]] eggs discovered in ancient Egyptian [[Mummy|mummies]].<ref name="Rhode2001PlatyhelminthesInEncLifeSci" /> The Platyhelminthes have very few [[synapomorphy|synapomorphies]] - distinguishing features that all Platyhelminthes (but no other animals) exhibit. This makes it difficult to work out their relationships with other groups of animals, as well as the relationships between different groups that are described as members of the Platyhelminthes.<ref>{{cite journal |author1=Carranza, S. |author2=Baguñà, J. |author3=Riutort, M.  |name-list-style=amp |title=Are the Platyhelminthes a monophyletic primitive group?
|journal=Molecular Biology and Evolution |volume=14|pages=485–497 |pmid=9159926 |issue=5 |date= May 1, 1997 |doi=10.1093/oxfordjournals.molbev.a025785 |doi-access=free }}</ref>

The "traditional" view before the 1990s was that Platyhelminthes formed the [[sister group]] to all the other bilaterians, which include, for instance, [[arthropod]]s, [[mollusc]]s, [[annelid]]s and [[chordate]]s. Since then, [[molecular phylogenetics]], which aims to work out evolutionary "family trees" by comparing different organisms' [[biochemistry|biochemicals]] such as [[DNA]], [[RNA]] and [[protein]]s, has radically changed scientists' view of evolutionary relationships between animals.<ref name="Halanych2004NewViewOfAnimalPhylogeny" /> Flatworms are now recognized as secondarily simplified bilaterians.<ref>{{Cite journal|title=A global metabarcoding analysis expands molecular diversity of Platyhelminthes and reveals novel early-branching clades|first1=Konstantina|last1=Mitsi|first2=Alicia S.|last2=Arroyo|first3=Iñaki|last3=Ruiz-Trillo|date=September 27, 2019|journal=Biology Letters|volume=15|issue=9|pages=20190182|doi=10.1098/rsbl.2019.0182|pmid=31506037|pmc=6769146}}</ref>

Detailed [[morphology (biology)|morphological]] analyses of anatomical features in the mid-1980s, as well as molecular phylogenetics analyses since 2000 using different sections of DNA, agree that [[Acoelomorpha]], consisting of [[Acoela]] (traditionally regarded as very simple "[[turbellaria]]ns"<ref name="RuppertBarnes2004Platyhelminthes" />) and [[Nemertodermatida]] (another small group previously classified as "turbellarians"<ref name="JondeliusEtAl2002Nemertodermatida">{{cite journal |author1=Jondelius, U. |author2=Ruiz-Trillo, I. |author3=Baguñà, J. |author4=Riutort, M. |title=The Nemertodermatida are basal bilaterians and not members of the Platyhelminthes |journal=Zoologica Scripta |volume=31 |issue=2 |pages=201–215 |date=April 2002 |doi=10.1046/j.1463-6409.2002.00090.x |s2cid=84015834 }}</ref>) are the sister group to all other bilaterians.<ref name="Halanych2004NewViewOfAnimalPhylogeny" /><ref name="LarssonJondelius2008PhylogenyOfCatenulida">{{cite journal |author1=Larsson, K. |author2=Jondelius, U. |title=Phylogeny of Catenulida and support for Platyhelminthes |journal=Organisms Diversity & Evolution |volume=8 |date=20 December 2008 |pages=378–387 |doi=10.1016/j.ode.2008.09.002 |issue=5 }}</ref> However, a 2007 study concluded that Acoela and Nemertodermatida were two distinct groups of bilaterians.<ref name="Wallberg2007EtAlDismissalOfAcoelomorpha">{{cite journal |author1=Wallberg, A. |author2=Curini-Galletti, M. |author3=Ahmadzadeh, A. |author4=Jondelius, U.  |name-list-style=amp |title=Dismissal of Acoelomorpha: Acoela and Nemertodermatida are separate early bilaterian clades |journal=Zoologica Scripta |volume=36 |issue=5 |pages=509–523 |date= September 2007 |doi=10.1111/j.1463-6409.2007.00295.x
|s2cid=85599100 }}</ref>

''[[Xenoturbella]]'', a bilaterian whose only well-defined organ is a [[statocyst]], was originally classified as a "primitive turbellarian".<ref name="Westblad1949Xenoturbella">{{cite journal |author=Westblad, E. |title=''Xenoturbella bocki'' n.g., n.sp., a peculiar, primitive turbellarian type |journal=Arkiv för Zoologi |year=1949 |volume=1 |pages=3–29 }}</ref> Later studies suggested it may instead be a [[deuterostome]],<ref name="BourlatEtAl2003XenoturbellaPhylum">{{cite journal|title=''Xenoturbella'' is a deuterostome that eats molluscs |journal=Nature |date=21 August 2003 |volume=424 |pages=925–928 |doi=10.1038/nature01851 |vauthors=Bourlat SJ, Nielsen C, Lockyer AE, Littlewood DT, Telford MJ |pmid=12931184
|issue=6951 |bibcode=2003Natur.424..925B |s2cid=4413357 }}</ref><ref>{{cite journal|title=Deuterostome phylogeny reveals monophyletic chordates and the new phylum Xenoturbellida |journal=Nature |year=2006 |volume=444 |pages=85–88 |doi=10.1038/nature05241 |pmid=17051155 |issue=7115 |vauthors=Bourlat SJ, Juliusdottir T, Lowe CJ, Freeman R, Aronowicz J, Kirschner M, Lander ES, Thorndyke M, Nakano H, Kohn AB, Heyland A, Moroz LL, Copley RR, Telford MJ |bibcode=2006Natur.444...85B |s2cid=4366885 }}</ref> but more detailed molecular phylogenetics have led to its classification as sister-group to the Acoelomorpha.<ref name="Cannon2016">{{cite journal|last1=Cannon |first1=J.T. |last2=Vellutini |first2=B.C. |last3=Smith |first3=J. |last4=Ronquist |first4=F. |last5=Jondelius |first5=U. |last6=Hejnol |first6=A. |title=Xenacoelomorpha is the sister group to Nephrozoa |journal=Nature |volume=530 |issue=7588 |date=4 February 2016 |pages=89–93 |pmid=26842059 |doi=10.1038/nature16520|url=http://urn.kb.se/resolve?urn=urn:nbn:se:nrm:diva-1844 |bibcode=2016Natur.530...89C |s2cid=205247296 }}</ref>

The Platyhelminthes excluding Acoelomorpha contain two main groups - [[Catenulida]] and [[Rhabditophora]] - both of which are generally agreed to be monophyletic (each contains all and only the descendants of an ancestor that is a member of the same group).<ref name="LarssonJondelius2008PhylogenyOfCatenulida" /><ref name="TimothyEtAl2004ProtostomesAndPlatyhelminthes">{{cite book |author1=Timothy, D. |author2=Littlewood, J. |author3=Telford, M. J. |author4=Bray, R. A. |name-list-style=amp |chapter=Protostomes and Platyhelminthes |pages=209–223 |editor1=Cracraft, J. |editor2=Donoghue, M. J. |title=Assembling the Tree of Life |publisher=Oxford University Press US |year=2004 |isbn=978-0-19-517234-8 |chapter-url=https://books.google.com/books?id=_jLl8zIRzucC&q=neodermata+phylogeny&pg=PA217 |access-date=2020-11-11 |archive-date=2023-03-23 |archive-url=https://web.archive.org/web/20230323154500/https://books.google.com/books?id=_jLl8zIRzucC&q=neodermata+phylogeny&pg=PA217 |url-status=live }}</ref> Early molecular phylogenetics analyses of the Catenulida and Rhabditophora left uncertainties about whether these could be combined in a single monophyletic group; a study in 2008 concluded that they could, therefore Platyhelminthes could be redefined as Catenulida plus Rhabditophora, excluding the Acoelomorpha.<ref name="LarssonJondelius2008PhylogenyOfCatenulida" />

Other molecular phylogenetics analyses agree the redefined Platyhelminthes are most closely related to [[Gastrotricha]], and both are part of a grouping known as [[Platyzoa]]. Platyzoa are generally agreed to be at least closely related to the [[Lophotrochozoa]], a super[[phylum]] that includes molluscs and [[annelid]] worms. The majority view is that Platyzoa are part of Lophotrochozoa, but a significant minority of researchers regard Platyzoa as a sister group of Lophotrochozoa.<ref name="Halanych2004NewViewOfAnimalPhylogeny">{{cite journal |author=Halanych, K. M. |year=2004 |title=The New View of Animal Phylogeny |url=http://mugue.narod.ru/supporting_materials/Halanych_2004.pdf |journal=Annual Review of Ecology, Evolution, and Systematics |volume=35 |pages=229–256 |doi=10.1146/annurev.ecolsys.35.112202.130124 |access-date=2010-10-28 |archive-date=2017-06-11 |archive-url=https://web.archive.org/web/20170611044006/http://mugue.narod.ru/supporting_materials/Halanych_2004.pdf |url-status=live }}</ref>

It has been agreed since 1985 that each of the wholly parasitic platyhelminth groups ([[Cestoda]], [[Monogenea]] and [[Trematoda]]) is monophyletic, and that together these form a larger monophyletic grouping, the [[Neodermata]], in which the adults of all members have [[syncytium|syncytial]] skins.<ref name="WillemsWallbergEtAl2005FillingGap">{{cite journal |author1=Willems, W.R. |author2=Wallberg, A. |author3=Jondelius, U. |title=Filling a gap in the phylogeny of flatworms: relationships within the Rhabdocoela (Platyhelminthes), inferred from 18S ribosomal DNA sequences |journal=Zoologica Scripta |volume=35 |issue=1 |pages=1–17 |year=2005 |doi=10.1111/j.1463-6409.2005.00216.x |url=https://doclib.uhasselt.be/dspace/bitstream/1942/1609/1/Filling%20the%20gap.pdf |access-date=2008-12-23 |display-authors=etal |hdl=1942/1609 |s2cid=85917387 |archive-url=https://web.archive.org/web/20111006005758/https://doclib.uhasselt.be/dspace/bitstream/1942/1609/1/Filling%20the%20gap.pdf |archive-date=2011-10-06 |url-status=dead |hdl-access=free }}</ref> However, there is debate about whether the [[Cestoda]] and [[Monogenea]] can be combined as an intermediate monophyletic group, the [[Cercomeromorpha]], within the Neodermata.<ref name="WillemsWallbergEtAl2005FillingGap" /><ref>{{cite journal |author1=Lockyer, A.E. |author2=Olson, P.D. |author3=Littlewood, D.T.J.  |name-list-style=amp |title=Utility of complete large and small subunit rRNA genes in resolving the phylogeny of the Neodermata (Platyhelminthes): implications and a review of the cercomer theory |journal=Biological Journal of the Linnean Society |volume=78 |issue=2 |pages=155–171 |doi=10.1046/j.1095-8312.2003.00141.x |year=2003|doi-access=free }}</ref> It is generally agreed that the Neodermata are a sub-group a few levels down in the "family tree" of the Rhabditophora.<ref name="LarssonJondelius2008PhylogenyOfCatenulida" /> Hence the traditional sub-phylum "[[Turbellaria]]" is [[paraphyletic]], since it does not include the Neodermata although these are descendants of a sub-group of "turbellarians".<ref>{{cite journal |author=Ehlers, U. |title=Comments on a phylogenetic system of the Platyhelminthes |journal=Hydrobiologia |volume=132 |issue=1 |pages=1–12 |year= 1986 |doi=10.1007/BF00046222  |s2cid=6018712 }}</ref>

==Evolution==

An outline of the origins of the parasitic lifestyle has been proposed;<ref name=Perkins2010>{{cite journal |last1=Perkins |first1=EM |last2=Donnellan |first2=SC |last3=Bertozzi |first3=T |last4=Whittington |first4=ID |year=2010 |title=Closing the mitochondrial circle on paraphyly of the Monogenea (Platyhelminthes) infers evolution in the diet of parasitic flatworms |journal=Int J Parasitol |volume=40 |issue=11|pages=1237–45 |doi=10.1016/j.ijpara.2010.02.017 |pmid=20493870 }}</ref> epithelial feeding [[monopisthocotylea]]ns on fish hosts are basal in the Neodermata and were the first shift to parasitism from free living ancestors. The next evolutionary step was a dietary change from [[epithelium]] to [[blood]]. The last common ancestor of Digenea + Cestoda was monogenean and most likely sanguinivorous.

In several members of the order [[Rhabdocoela]] an [[Endosymbiont|endosymbiotic]] relationship with [[microalgae]] has evolved. Some species in the same order has also evolved [[kleptoplasty]].<ref>{{Cite journal|title=A new case of kleptoplasty in animals: Marine flatworms steal functional plastids from diatoms|first1=Niels W. L.|last1=Van Steenkiste|first2=India|last2=Stephenson|first3=María|last3=Herranz|first4=Filip|last4=Husnik|first5=Patrick J.|last5=Keeling|first6=Brian S.|last6=Leander|date=July 5, 2019|journal=Science Advances|volume=5|issue=7|doi=10.1126/sciadv.aaw4337|pmid=31328166|pmc=6636991}}</ref>

The earliest known fossils confidently classified as tapeworms have been dated to {{Ma|270}}, after being found in [[coprolite]]s (fossilised faeces) from an [[elasmobranch]].<ref name="Dentzien-Dias2013">{{cite journal |last1=Dentzien-Dias |first1=PC |last2=Poinar |first2=G Jr |last3=de Figueiredo |first3=AE |last4=Pacheco |first4=AC |last5=Horn |first5=BL |last6=Schultz |first6=CL |date=30 January 2013 |title=Tapeworm eggs in a 270 million-year-old shark coprolite |journal=PLOS ONE |volume=8 |issue=1|page=e55007 |doi=10.1371/journal.pone.0055007 |pmid=23383033 |pmc=3559381|bibcode=2013PLoSO...855007D |doi-access=free }}</ref> Putative older fossils include a ribbon-shaped, bilaterally symmetrical organism named ''[[Rugosusivitta]] orthogonia'' from the Early Cambrian of [[China]],<ref name=Rugosusivitta>{{Cite journal|last1=Tang |first1=F. |last2=Song |first2=S. |last3=Zhang |first3=G. |last4=Chen |first4=A. |last5=Liu |first5=J. |title=Enigmatic ribbon-like fossil from Early Cambrian of Yunnan, China |year=2021 |journal=China Geology |volume=4 |issue=2 |pages=205–214 |doi=10.31035/cg2020056 |bibcode=2021CGeo....4..205T |doi-access=free }}</ref> brownish bodies on the bedding planes reported from the Late [[Ordovician]] ([[Katian]]) [[Vaureal Formation|Vauréal Formation]] ([[Canada]]) by Knaust & Desrochers (2019), tentatively interpreted as turbellarians (though the authors cautioned that they might ultimately turn out to be fossils of [[Acoelomorpha|acoelomorphs]] or [[nemertea]]ns)<ref name=GRKnaustDesrochers>{{Cite journal|author1=Dirk Knaust |author2=André Desrochers |year=2019 |title=Exceptionally preserved soft-bodied assemblage in Ordovician carbonates of Anticosti Island, eastern Canada |journal=Gondwana Research |volume=71 |pages=117–128 |doi=10.1016/j.gr.2019.01.016 |bibcode=2019GondR..71..117K |s2cid=134814852 }}</ref> and circlets of fossil hooks preserved with [[Placodermi|placoderm]] and [[Acanthodii|acanthodian]] fossils from the [[Devonian]] of [[Latvia]], at least some of which might represent parasitic monogeneans.<ref>{{cite book |author1=Kenneth De Baets |author2=Paula Dentzien-Dias |author3=Ieva Upeniece |author4=Olivier Verneau |author5=Philip C.J. Donoghue |year=2015 |chapter=Constraining the deep origin of parasitic flatworms and host-interactions with fossil evidence |editor1=Kenneth De Baets |editor2=D. Timothy J Littlewood |title=Fossil parasites |series=Advances in Parasitology |volume=90 |pages=93–135 |doi=10.1016/bs.apar.2015.06.002 |pmid=26597066 |isbn=9780128040010 |s2cid=7278956 |issn=0065-308X |url=https://hal-univ-perp.archives-ouvertes.fr/hal-01257074/file/debaets2015.pdf |access-date=2022-07-15 |archive-date=2022-07-30 |archive-url=https://web.archive.org/web/20220730040408/https://hal-univ-perp.archives-ouvertes.fr/hal-01257074/file/debaets2015.pdf |url-status=live }}</ref>

==Interaction with humans==

===Parasitism===

[[File:Neurocysticercosis.gif|thumb |Magnetic resonance image of a patient with [[neurocysticercosis]] demonstrating multiple [[Cestoda|cysticerci]] of the pork tapeworm ''[[Taenia solium]]'' within the brain]]

[[Cestode]]s (tapeworms) and [[digenea]]ns (flukes) cause diseases in humans and their [[livestock]], whilst [[monogenea]]ns can cause serious losses of stocks in [[fish farm]]s.<ref name="NorthropClewesShaw2000Parasites" /> [[Schistosomiasis]], also known as bilharzia or snail fever, is the second-most devastating parasitic disease in tropical countries, behind [[malaria]]. The [[Carter Center]] estimated 200&nbsp;million people in 74 countries are infected with the disease, and half the victims live in Africa. The condition has a low [[mortality rate]], but usually presents as a [[chronic illness]] that can damage internal organs. It can impair the growth and [[cognitive development]] of children, increasing the risk of [[bladder cancer]] in adults. The disease is caused by several flukes of the genus ''[[Schistosoma]]'', which can bore through human skin; those most at risk use infected bodies of water for recreation or [[laundry]].<ref name="SCP">{{Cite web|author=The Carter Center|title=Schistosomiasis Control Program|url=http://www.cartercenter.org/health/schistosomiasis/index.html|access-date=2008-07-17|archive-date=2008-07-20|archive-url=https://web.archive.org/web/20080720091015/http://cartercenter.org/health/schistosomiasis/index.html|url-status=live}}</ref>

In 2000, an estimated 45&nbsp;million people were infected with the beef tapeworm ''[[Taenia saginata]]'' and 3&nbsp;million with the pork tapeworm ''[[Taenia solium]]''.<ref name="NorthropClewesShaw2000Parasites" /> Infection of the digestive system by adult tapeworms causes abdominal symptoms that, whilst unpleasant, are seldom disabling or life-threatening.<ref>{{cite journal |author1=García, H.H. |author2=Gonzalez, A.E. |author3=Evans, C.A.W. |author4=Gilman, R.H. |title=''Taenia solium'' cysticercosis |journal=The Lancet |volume=362 |issue=9383|year=2003|pages=547–556 |doi=10.1016/S0140-6736(03)14117-7 |pmid=12932389 |pmc=3103219 }}</ref><ref>{{cite journal |author=WHO Expert Committee |title=Public health significance of intestinal parasitic infections |journal=Bulletin of the World Health Organization |volume=65 |issue=5 |pages=575–588 |year=1987 |url=http://whqlibdoc.who.int/bulletin/1987/Vol65-No5/bulletin_1987_65(5)_575-588.pdf |access-date=2008-12-24 |pmid=3501340 |pmc=2491073 |archive-url=https://web.archive.org/web/20090816023000/http://whqlibdoc.who.int/bulletin/1987/Vol65-No5/bulletin_1987_65(5)_575-588.pdf |archive-date=2009-08-16 |url-status=dead }}</ref> However, [[neurocysticercosis]] resulting from penetration of ''T. solium'' larvae into the [[central nervous system]] is the major cause of acquired [[epilepsy]] worldwide.<ref>{{cite journal |author=Commission on Tropical Diseases of the International League Against Epilepsy |title=Relationship Between Epilepsy and Tropical Diseases |journal=Epilepsia |volume=35 |issue=1 |pages=89–93 |year=1994 |doi=10.1111/j.1528-1157.1994.tb02916.x |pmid=8112262 |s2cid=221733822 }}</ref> In 2000, about 39&nbsp;million people were infected with [[trematode]]s (flukes) that naturally parasitize fish and crustaceans, but can pass to humans who eat raw or lightly cooked seafood. Infection of humans by the broad fish tapeworm ''[[Diphyllobothrium latum]]'' occasionally causes [[vitamin B12|vitamin B<sub>12</sub>]] deficiency and, in severe cases, [[megaloblastic anemia]].<ref name="NorthropClewesShaw2000Parasites" />

The threat to humans in developed countries is rising as a result of social trends: the increase in [[organic farming]], which uses [[manure]] and [[sewage sludge]] rather than artificial [[fertilizer]]s, spreads parasites both directly and via the droppings of [[seagull]]s which feed on manure and sludge; the increasing popularity of raw or lightly cooked foods; imports of meat, [[seafood]] and [[salad]] vegetables from high-risk areas; and, as an underlying cause, reduced awareness of parasites compared with other [[public health]] issues such as [[pollution]]. In less-developed countries, inadequate sanitation and the use of human [[feces]] (night soil) as fertilizer or to enrich fish farm ponds continues to spread parasitic platyhelminths, whilst poorly designed water-supply and [[irrigation]] projects have provided additional channels for their spread. People in these countries usually cannot afford the cost of fuel required to cook food thoroughly enough to kill parasites. Controlling parasites that infect humans and livestock has become more difficult, as many species have become [[Drug resistance|resistant]] to drugs that used to be effective, mainly for killing juveniles in meat.<ref name="NorthropClewesShaw2000Parasites">{{cite journal |author1=Northrop-Clewes, C.A. |author2=Shaw, C. |title=Parasites |journal=British Medical Bulletin |volume=56 |pages=193–208 |year=2000 |doi=10.1258/0007142001902897 |pmid=10885116 |issue=1 |doi-access=free }}</ref> While poorer countries still struggle with unintentional infection, cases have been reported of intentional infection in the US by dieters who are desperate for rapid weight-loss.<ref>{{cite news |url=http://www.today.com/health/iowa-woman-tries-tapeworm-diet-prompts-doctor-warning-6C10935746 |title=Iowa woman tries 'tapeworm diet', prompts doctor warning |work=[[Today (U.S. TV program)]] |date=2013-08-16 |access-date=2013-09-22 |archive-date=2017-11-07 |archive-url=https://web.archive.org/web/20171107112623/https://www.today.com/health/iowa-woman-tries-tapeworm-diet-prompts-doctor-warning-6C10935746 |url-status=live}}</ref>

===Pests===

There is concern in northwest Europe (including the British Isles) regarding the possible proliferation of the New Zealand [[planarian]] ''[[Arthurdendyus triangulatus]]'' and the Australian flatworm ''[[Australoplana sanguinea]]'', both of which prey on earthworms.<ref>{{Cite web |url=http://www.gov.im/lib/docs/daff/informationsheetupdatedflatworms.pdf |title=Flatworm information sheet – Isle of Man Government |access-date=2014-05-26 |archive-url=https://web.archive.org/web/20130501025632/http://www.gov.im/lib/docs/daff/informationsheetupdatedflatworms.pdf |archive-date=2013-05-01 |url-status=dead }}</ref> ''A. triangulatus'' is thought to have reached Europe in containers of plants imported by [[botanical garden]]s.<ref>{{cite journal |author1=Boag, B. |author2=Yeates, G.W. |title=The Potential Impact of the New Zealand Flatworm, a Predator of Earthworms, in Western Europe |journal=Ecological Applications |volume=11|issue=5|pages=1276–1286 |doi=10.1890/1051-0761(2001)011[1276:TPIOTN]2.0.CO;2 |year=2001 |issn=1051-0761}}</ref>

===Benefits===

The planarian ''[[Platydemus manokwari]]'' has been deliberately released in an attempt at control of the gaint African [[snail]] ''[[Achatina fulica]]'' which damages agricultural plants. It was first observed as an invasive species on [[Guam]] and was then released in the [[Philippines]] and [[Maldives]]. It has now spread to many islands in the Pacific and Caribbean and is now spreading across the southern [[USA]] and south-east Asia, including mainland [[China]]. It was initially claimed that ''P.&nbsp;manokwari'' severely reduced, and in places exterminated, ''A.&nbsp;fulica'' – achieving much greater success than most [[biological pest control]] programs, which generally aim for a low, stable population of the pest species. However, the decline of ''A.&nbsp;fulica'' is no longer thought to have been due to flatworm predation.<ref>Gerlach et al. 2021, ''[https://doi.org/10.1007/s10530-020-02436-w Negative impacts of invasive predators used as biological control agents against the pest snail Lissachatina fulica: the snail Euglandina ‘rosea’ and the flatworm Platydemus manokwari]''. Biological Invasions 23, 997–1031 </ref> These planarians are a serious threat to native snails and should never be used for biological control.<ref>{{cite book |title=Natural Enemies of Terrestrial Molluscs |author=Barker, G.M. |chapter=Terrestrial planarians |pages=261–263 |publisher=CABI Publishing |year=2004 |isbn=978-0-85199-319-5 |chapter-url=https://books.google.com/books?id=bjAh_Gszsy8C&pg=PA227 |access-date=2020-11-11 |archive-date=2023-03-23 |archive-url=https://web.archive.org/web/20230323154504/https://books.google.com/books?id=bjAh_Gszsy8C&pg=PA227 |url-status=live }}</ref><ref name=Justine2014>{{Cite journal |last1=Justine |first1=Jean-Lou |last2=Winsor |first2=Leigh |last3=Gey |first3=Delphine |last4=Gros |first4=Pierre |last5=Thévenot |first5=Jessica |title=The invasive New Guinea flatworm ''Platydemus manokwari'' in France, the first record for Europe: time for action is now. |journal=PeerJ |volume=2 |pages=e297 |year=2014 |doi=10.7717/peerj.297 |pmid=24688873 |pmc=3961122 |doi-access=free }}</ref>

A study in Argentina shows the potential for planarians such as ''[[Girardia]] anceps'', ''[[Mesostoma ehrenbergii]]'', and ''Bothromesostoma evelinae'' to reduce populations of the mosquito species ''[[Aedes aegypti]]'' and ''[[Culex pipiens]]''. The experiment showed that ''G.&nbsp;anceps'' can prey on all [[instars]] of both mosquito species, yet maintain a steady predation rate over time. The ability of these flatworms to live in artificial containers demonstrated the potential of placing these species in popular mosquito breeding sites, which might reduce the amount of [[mosquito-borne disease]].<ref>{{cite journal |doi=10.1016/j.biocontrol.2008.12.010 |volume=49 |issue=3 |title=Predation potential of three flatworm species (Platyhelminthes: Turbellaria) on mosquitoes (Diptera: Culicidae) |journal=Biological Control |pages=270–276 |year=2009 |last1=Tranchida |first1=María C. |last2=MacIá |first2=Arnaldo |last3=Brusa |first3=Francisco |last4=Micieli |first4=María V. |last5=García |first5=Juan J. |bibcode=2009BiolC..49..270T }}</ref>

== See also ==
* [[Miracidium]]
* [[Regenerative medicine]]
* [[Schistosoma]]

==References==
{{Reflist|25em}}

==Further reading==
{{refbegin}}
* {{cite book |author=Campbell, Neil A. |title=Biology |edition=Fourth |publisher=Benjamin/Cummings Publishing |place=New York |year=1996 |page=599 |isbn=0-8053-1957-3}}
* {{cite book |editor1=Crawley, John L. |editor2=van de Graff, Kent M. |title=A Photographic Atlas for the Zoology Laboratory |edition=Fourth |publisher=Morton Publishing Company |place=Colorado |year=2002 |isbn=0-89582-613-5}}
* {{cite book |title=The Columbia Electronic Encyclopedia |edition=6th |publisher=Columbia University Press |year=2004 |access-date=8 February 2005 |url=http://www.infoplease.com/ce6/sci/A0839338.html}}
* {{cite book |author1=Evers, Christine A. |author2=Starr, Lisa |title=Biology: Concepts and Applications |edition=6th |place=United States |publisher=Thomson |year=2006 |isbn=0-534-46224-3 |url-access=registration |url=https://archive.org/details/biologyconceptsa06edstar }}
* {{cite journal |doi=10.1016/S0378-1119(01)00863-0 |title=Genetic network of the eye in Platyhelminthes: Expression and functional analysis of some players during planarian regeneration |year=2002 |journal=Gene |volume=287 |issue=1–2 |pages=67–74 |pmid=11992724 |last1=Saló |first1=E. |last2=Pineda |first2=D. |last3=Marsal |first3=M. |last4=Gonzalez |first4=J. |last5=Gremigni |first5=V. |last6=Batistoni |first6=R.}}
{{refend}}

==External links==

{{Commons category|Platyhelminthes}}
{{Wikispecies|Platyhelminthes}}
* {{cite web |url=http://www.rzuser.uni-heidelberg.de/~bu6/index.html |title=Marine flatworms of the world}}

{{Animalia}}
{{Life on Earth}}
{{Helminthiases}}
{{Taxonbar|from=Q124900}}
{{Authority control}}

[[Category:Platyhelminthes|Platyhelminthes]]
[[Category:Late Ordovician first appearances]]
[[Category:Extant Ordovician first appearances]]