Editing Flatworm (section)

===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" />