Editing Flatworm (section)

==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" />
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===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" />
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