Editing Monogenea (section)

==Systematics and evolution==
The ancestors of Monogenea were probably free-living flatworms similar to modern [[Turbellaria]].
According to the more widely accepted view, "rhabdocoel turbellarians gave rise to monogeneans; these, in turn, gave rise to [[digeneans]], from which the [[cestodes]] were derived. Another view is that the rhabdocoel ancestor gave rise to two lines; one gave rise to monogeneans, which gave rise to digeneans, and the other line gave rise to cestodes".<ref name="tub" />

About 50 [[family (biology)|families]] and thousands of species are described{{citation needed|date=May 2022}}.

Some parasitologists divide the Monogenea into two (or three) subclasses based on the complexity of their haptor: the [[Monopisthocotylea]] have one main part to the haptor, often with hooks or a large attachment disc, whereas the [[Polyopisthocotylea]] have multiple parts to the haptor, typically clamps.  These groups are also known as Polyonchoinea and Heteronchoinea, respectively.  Polyopisthocotyleans are almost exclusively gill-dwelling blood feeders, whereas monopisthocotyleans may live on the gills, skin, and fins.

[[Monopisthocotylea]] include:
*Genus ''[[Gyrodactylus]]'' has no eyespots and is viviparous.
*Genus ''[[Dactylogyrus]]'' has four [[eyespot (flatworm)|eyespot]]s and is oviparous.  This is one of the largest metazoan genera, with at least 970 species.
*Genus ''[[Neobenedenia]]'' much larger and lives on the skin of many tropical marine species, causing problematic infections in marine aquaria.
All of these can cause [[epizootic]]s in [[freshwater fish]] when raised in [[aquaculture]].

[[Polyopisthocotylea]] include:
*Genus ''[[Diclidophora]]'' is primarily found in [[marine fish]] and primitive freshwater fish such as [[sturgeon]]s and [[paddlefish]].
*Genus ''[[Protopolystoma]]'' is found in aquatic clawed toads (''[[Xenopus]]'' species).