Monogenea

Monogenea (adj. monogenean) are a group of largely ectoparasitic members of the flatworm phylum Platyhelminthes. They are also known by the name monogenetic trematodes.

Characteristics
Monogenea are small parasitic flatworms. The body is usually flat and oval, and rarely longer than about 2cm. Like other flatworms, Monogenea have no true body cavity (coelom). They have a simple digestive system consisting of a mouth opening with a muscular pharynx and an intestine with no terminal opening (anus).

Monogeneans have a collection of various attachment structures. The anterior structures are collectively termed the prohaptor, while the posterior ones are collectively termed the opisthaptor. The posterior end evolved into a fancy holdfast structure.

Systematics and evolution
The ancestors of Monogenea were probably free-living flatworms similar to modern Turbellaria. According to molecular studies, their closest relatives among Platyhelminthes are the tapeworms. There are about 50 families and thousands of described and undescribed species.

Some parasitologists divide Monogenea into two (or three) subclasses based on the complexity of their haptor: Monopisthocotylea have one main part to the haptor, often with hooks or a large attachment disc, whereas 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.

Monopistocotylea include: All of which can cause epizootics in freshwater fish when raised in aquaculture.
 * Genus Gyrodactylus, which has no eyespots and is viviparous.
 * Genus 'Dactylogyrus, which has four eyespots and lays eggs. This is one of the largest metazoan genera, with at least 970 species.
 * Genus Neobenedenia, which is much larger and lives on the skin of many tropical marine species, causing problematic disease in marine aquaria.

Polyopisthocotylea include:
 * Genus Diclidophora, which is primarily found in marine fish and primitive freshwater fish like sturgeons and paddlefish.

Ecology and life cycle
Monogenea are mainly parasites on the surface of fish. Monogenea are especially common on the skin, fins and gills of fishes. Less commonly, they can be found in the urinary bladder and rectum of cold-blooded vertebrates. None infect birds, but one (Oculotrema hippopotami) infects mammals, parasitizing the eye of a hippopotamus.

Monogenea are usually hermaphrodites (the male reproductive system becoming functional before the female part).They have direct life-cycles with no asexual reproduction (unlike the Digenea) and in those that lay eggs, a larval stage (generally ciliated) called an oncomiracidium that is responsible for transmission from host to host. As adults, they eat the blood, mucus, and epithelial cells of their host.

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