Digenea (Gr. ''Dis'' – double, ''Genos'' – race) is a class of trematodes in the Platyhelminthes

phylum In biology, a phylum (; plural: phyla) is a level of classification or taxonomic rank below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature f ...

, consisting of parasitic

flatworm The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") are a phylum of relatively simple bilaterian, unsegment ...

s (known as ''flukes'') with a syncytial

tegument Tegument may refer to: * Integumentary system, a protective organ system forming the outermost layer of an animal's body * Tegument (helminth) Tegument is a term in helminthology for the outer body covering of members of the phylum Platyhelminthe ...

and, usually, two suckers, one ventral and one oral. Adults commonly live within the digestive tract, but occur throughout the organ systems of all classes of vertebrates. Once thought to be related to the Monogenea, it is now recognised that they are closest to the

Aspidogastrea The Aspidogastrea (Ancient Greek: ' “shield”, ' “stomach/pouch”) is a small group of flukes comprising about 80 species. It is a subclass of the trematoda, and sister group to the Digenea. Species range in length from approximately one ...

and that the Monogenea are more closely allied with the Cestoda. Around 6,000 species have been described to date.

Morphology

Key features

Characteristic features of the Digenea include a syncytial tegument; that is, a tegument where the junctions between cells are broken down and a single continuous cytoplasm surrounds the entire animal. A similar tegument is found in other members of the Neodermata; a group of platyhelminths comprising the Digenea,

, Monogenea and Cestoda. Digeneans possess a vermiform, unsegmented body-plan and have a solid

parenchyma Parenchyma () is the bulk of functional substance in an animal organ or structure such as a tumour. In zoology it is the name for the tissue that fills the interior of flatworms. Etymology The term ''parenchyma'' is New Latin from the word π ...

with no body cavity ( coelom) as in all platyhelminths. Peerj-292-fig-7 Zoogonidae

There are typically two suckers, an anterior oral sucker surrounding the

mouth In animal anatomy, the mouth, also known as the oral cavity, or in Latin cavum oris, is the opening through which many animals take in food and issue vocal sounds. It is also the cavity lying at the upper end of the alimentary canal, bounded on ...

, and a ventral sucker sometimes termed the acetabulum, on the ventral surface. The oral sucker surrounds the mouth, while the ventral sucker is a blind muscular organ with no connection to any internal structure. A ''monostome'' is a worm with one sucker (oral). Flukes with an oral sucker and an acetabulum at the posterior end of the body are called ''Amphistomes''. ''Distomes'' are flukes with an oral sucker and a ventral sucker, but the ventral sucker is somewhere other than posterior. These terms are common in older literature, when they were thought to reflect systematic relationships within the groups. They have fallen out of use in modern digenean taxonomy.

Reproductive system

The vast majority of digeneans are

hermaphrodite In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes. Many Taxonomy (biology), taxonomic groups of animals (mostly invertebrate ...

s. This is likely to be an adaptation to low abundance within hosts, allowing the life cycle to continue when only one individual successfully infects the final host.

Fertilisation Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. Proce ...

is internal, with

sperm Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum ...

being transferred via the cirrus to the Laurer's Canal or genital aperture. A key group of digeneans which are

dioecious Dioecy (; ; adj. dioecious , ) is a characteristic of a species, meaning that it has distinct individual organisms (unisexual) that produce male or female gametes, either directly (in animals) or indirectly (in seed plants). Dioecious reproductio ...

are the schistosomes. Asexual reproduction in the first larval stage is ubiquitous. While the sexual formation of the digenean eggs and asexual reproduction in the first

larva A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. Th ...

l stage (miracidium) is widely reported, the

developmental biology Developmental biology is the study of the process by which animals and plants grow and develop. Developmental biology also encompasses the biology of Regeneration (biology), regeneration, asexual reproduction, metamorphosis, and the growth and di ...

of the asexual stages remains a problem. Electron microscopic studies have shown that the light microscopically visible germ balls consist of mitotically dividing cells which give rise to

embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...

s and to a line of new germ cells that become included in these embryonic stages. Since the absence of meiotic processes is not proven, the exact definition remains doubtful.

Male organs

Protandry Sequential hermaphroditism (called dichogamy in botany) is a type of hermaphroditism that occurs in many fish, gastropods, and plants. Sequential hermaphroditism occurs when the individual changes its sex at some point in its life. In particular, ...

is the general rule among the Digenea. Usually two testes are present, but some flukes can have more than 100. Also present are

vasa efferentia The efferent ducts (or efferent ductules or ductuli efferentes or ductus efferentes or vasa efferentia) connect the rete testis with the initial section of the epididymis.Hess 2018 There are two basic designs for efferent ductule structure: * ...

, a vas deferens, seminal vesicle,

ejaculatory duct The ejaculatory ducts (''ductus ejaculatorii'') are paired structures in male anatomy. Each ejaculatory duct is formed by the union of the vas deferens with the duct of the seminal vesicle. They pass through the prostate, and open into the ureth ...

and a cirrus (analogous to a penis) usually (but not always) enclosed in a cirrus sac. The cirrus may or may not be covered in proteinaceous spines. The exact conformation of these organs within the male terminal genitalia is taxonomically important at the familial and generic levels.

Female organs

Usually there is a single

ovary The ovary is an organ in the female reproductive system that produces an ovum. When released, this travels down the fallopian tube into the uterus, where it may become fertilized by a sperm. There is an ovary () found on each side of the body. ...

with an oviduct, a seminal receptacle, a pair of vitelline glands (involved in yolk and egg-shell production) with ducts, the ootype (a chamber where eggs are formed), a complex collection of glands cells called ''Mehlis’ gland'', which is believed to lubricate the uterus for egg passage. In addition, some digeneans possess a canal called Laurer's Canal, which leads from the oviduct to the dorsal surface of the body. The function of this canal is debated, but it may be used for insemination in some species or for disposal of waste products from reproduction in other species. Most trematodes possess an ovicapt, an enlarged portion of the oviduct where it joins the ovary. It probably controls the release of ova and spaces out their descent down the uterus. The uterus typically opens into a common genital atrium that also received the distal male copulatory organ (cirrus) before immediately opening onto the outer surface of the worm. The distal part of the uterus may be expanded into a metraterm, set off from the proximal uterus by a muscular sphincter, or it may be lined with spines, as in the Monorchiidae and some other families.

Digestive system

As adults, most digeneans possess a terminal or subterminal mouth, a muscular pharynx that provides the force for ingesting food, and a forked, blind digestive system consisting of two tubular sacs called caeca (sing. caecum). In some species the two gut caeca join posteriorly to make a ring-shaped gut or cyclocoel. In others the caeca may fuse with the body wall posteriorly to make one or more anuses, or with the excretory vesicle to form a uroproct. Digeneans are also capable of direct nutrient uptake through the tegument by pinocytosis and phagocytosis by the syncitium. Most adult digeneans occur in the vertebrate alimentary canal or its associated organs, where they most often graze on contents of the lumen (e.g., food ingested by the host, bile, mucus), but they may also feed across the mucosal wall (e.g.,

submucosa The submucosa (or tela submucosa) is a thin layer of tissue (biology), tissue in various organ (anatomy), organs of the gastrointestinal tract, gastrointestinal, respiratory tract, respiratory, and genitourinary system, genitourinary tracts. It i ...

, host blood). The blood flukes, such as schistosomes, spirorchiids and sanguinicolids, feed exclusively on blood. Asexual stages in

mollusc Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is esti ...

intermediate hosts feed mostly by direct absorption, although the redia stage found in some groups does have a mouth, pharynx and simple gut and may actively consume host tissue or even other parasites. Encysted

metacercaria Trematodes are parasitic flatworms of the class ''Trematoda'', specifically parasitic flukes with two suckers: one ventral and the other oral. Trematodes are covered by a tegument, that protects the organism from the environment by providing secr ...

l stages and free-living cercarial stages do not feed.

Nervous system

Paired

ganglia A ganglion is a group of neuron cell bodies in the peripheral nervous system. In the somatic nervous system this includes dorsal root ganglia and trigeminal ganglia among a few others. In the autonomic nervous system there are both sympatheti ...

at the anterior end of the body serve as the

brain The brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It consists of nervous tissue and is typically located in the head ( cephalization), usually near organs for special ...

. From this nerves extend anteriorly and posteriorly. Sensory receptors are, for the most part, lacking among the adults, although they do have tangoreceptor cells. Larval stages have many kinds of sensory receptors, including light receptors and chemoreceptors. Chemoreception plays an important role in the free-living miracidial larva recognising and locating its host.

Life cycles

There is a bewildering array of variation on the complex digenean life cycle, and plasticity in this trait is probably a key to the group's success. In general, the life cycles may have two, three, or four obligate (necessary) hosts, sometimes with transport or paratenic hosts in between. The three-host life cycle is probably the most common. In almost all species, the first host in the life cycle is a

. This has led to the inference that the ancestral digenean was a mollusc parasite and that vertebrate hosts were added subsequently. The alternation of sexual and asexual generations is an important feature of digeneans. This phenomenon involves the presence of several discrete generations in one life-cycle. A typical digenean trematode life cycle is as follows. Eggs leave the

vertebrate Vertebrates () comprise all animal taxon, taxa within the subphylum Vertebrata () (chordates with vertebral column, backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the ...

host in faeces and use various strategies to infect the first

intermediate host In biology and medicine, a host is a larger organism that harbours a smaller organism; whether a parasitic, a mutualistic, or a commensalist ''guest'' (symbiont). The guest is typically provided with nourishment and shelter. Examples include a ...

, in which sexual reproduction does not occur. Digeneans may infect the first intermediate host (usually a snail) by either passive or active means. The eggs of some digeneans, for example, are (passively) eaten by snails (or, rarely, by an annelid worm), in which they proceed to hatch. Alternatively, eggs may hatch in water to release an actively swimming, ciliated larva, the miracidium, which must locate and penetrate the body wall of the snail host. After post-ingestion hatching or penetration of the snail, the miracidium metamorphoses into a simple, sac-like ''mother sporocyst''. The mother sporocyst undergoes a round of internal asexual reproduction, giving rise to either ''rediae'' (sing. redia) or ''daughter sporocysts''. The second generation is thus the daughter parthenita sequence. These in turn undergo further asexual reproduction, ultimately yielding large numbers of the second free-living stage, the ''cercaria'' (pl. cercariae). Free-swimming cercariae leave the snail host and move through the aquatic or

marine Marine is an adjective meaning of or pertaining to the sea or ocean. Marine or marines may refer to: Ocean * Maritime (disambiguation) * Marine art * Marine biology * Marine debris * Marine habitats * Marine life * Marine pollution Military * ...

environment, often using a whip-like tail, though a tremendous diversity of tail morphology is seen. Cercariae are infective to the second host in the life cycle, and infection may occur passively (e.g., a fish consumes a cercaria) or actively (the cercaria penetrates the fish). The life cycles of some digeneans include only two hosts, the second being a vertebrate. In these groups, sexual maturity occurs after the cercaria penetrates the second host, which is in this case also the definitive host. Two-host life cycles can be primary (there never was a third host) as in the Bivesiculidae, or secondary (there was at one time in evolutionary history a third host but it has been lost). In three-host life cycles, cercariae develop in the second intermediate host into a resting stage, the ''metacercaria'', which is usually encysted in a cyst of host and parasite origin, or encapsulated in a layer of tissue derived from the host only. This stage is infective to the definitive host. Transmission occurs when the definitive host preys upon an infected second intermediate host. Metacercariae excyst in the definitive host's gut in response to a variety of physical and chemical signals, such as gut pH levels, digestive

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different molecule ...

s, temperature, etc. Once excysted, adult digeneans migrate to more or less specific sites in the definitive host and the life cycle repeats.

Evolution

The evolutionary origins of the Digenea have been debated for some time, but there appears general agreement that the proto-digenean was a parasite of a mollusc, possibly of the mantle cavity. Evidence for this comes from the ubiquity of molluscs as first intermediate hosts for digeneans, and the fact that most aspidogastreans (the sister group to the Digenea) also have mollusc associations. It is thought that the early trematodes (the collective name for digeneans and aspidogastreans) likely evolved from rhabdocoel turbellarians that colonised the open mantle cavity of early molluscs. It is likely that more complex life cycles evolved through a process of terminal addition, whereby digeneans survived predation of their mollusc host, probably by a fish. Other hosts were added by the same process until the modern bewildering diversity of life cycle patterns developed.

Important families

Digenea includes about 80 families.Olson P. D., Cribb T. H., Tkach V. V., Bray R. A. & Littlewood D. T. J. (2003). "Phylogeny and classification of the Digenea (Platyhelminthes: Trematoda)". ''International Journal for Parasitology'' 33(7): 733–755. They are listed below, organised by order. Digenea * Diplostomida **Suborder Diplostomata ***Superfamily Brachylaimoidea Joyeux & Foley, 1930 **** Brachylaimidae Joyeux & Foley, 1930 **** Leucochloridiidae Poche, 1907 ***Superfamily Diplostomoidea Poirier, 1886 **** Brauninidae Wolf, 1903 **** Cyathocotylidae Mühling, 1898 **** Diplostomidae Poirier, 1886 **** Proterodiplostomidae Dubois, 1936 **** Strigeidae Railliet, 1919 ***Superfamily Schistosomatoidea Stiles & Hassall, 1898 **** Aporocotylidae Odhner, 1912 **** Schistosomatidae Stiles & Hassall, 1898 **** Spirorchiidae Stunkard, 1921 * Plagiorchiida **Apocreadiata ***Apocreadioidea Skrjabin, 1942 **** Apocreadiidae Skrjabin, 1942 **Bivesiculata ***Bivesiculoidea **** Bivesiculidae Yamaguti, 1934 **Bucephalata ***Bucephaloidea Poche, 1907 **** Bucephalidae Poche, 1907 ****

Nuitrematidae Nuitrematidae is a monotypic family of trematodes in the order Plagiorchiida Plagiorchiida is a large order of trematodes, synonymous to Echinostomida. They belong to the Digenea, a large subclass of flukes. This order contains relatively few ...

Kurochkin, 1975 ***Gymnophalloidea Odhner, 1905 ****

Botulisaccidae Botulisaccidae is a family of trematodes belonging to the order Plagiorchiida. Genera: * '' Botulisaccus'' Caballero y C., Bravo-Hollis & Grocott, 1955 References Plagiorchiida {{Trematoda-stub ...

Yamaguti, 1971 ****

Fellodistomidae Fellodistomidae is a family of trematodes belonging to the order Plagiorchiida. References Plagiorchiida Trematode families {{Trematoda-stub ...

Nicoll, 1909 **** Gymnophallidae Odhner, 1905 **** Tandanicolidae Johnston, 1927 **Echinostomata ***Echinostomatoidea Looss, 1902 **** Calycodidae Dollfus, 1929 ****

Cyclocoelidae Cyclocoelidae is a family of trematodes in the order Plagiorchiida. Genera The genera are ordered by subfamily. *Cyclocoelinae Stossich, 1902 **'' Circumvitellatrema'' Dronen, Greiner, Ialeggio & Nolan, 2009Dronen, N. O., Greiner, E. C., Ialeggi ...

Stossich, 1902 ****

Echinochasmidae Echinochasmidae is a family of trematodes in the order Plagiorchiida. Genera *''Echinochasmus ''Echinochasmus'' is a genus of trematodes in the family Echinochasmidae. Species *''Echinochasmus cohensi'' Rao, 1951Rao, N. S. K. (1951). ''Ec ...

Odhner, 1910 **** Echinostomatidae Looss, 1899 ****

Eucotylidae Eucotylidae is a family of trematodes belonging to the order Plagiorchiida. Genera: * '' Eucotyle'' Cohn, 1904 * '' Neoeucotyle'' Kanev, Radev & Fried, 2002 * '' Paratanaisia'' Teixeira de Freitas, 1959 * '' Tamerlania'' Skrjabin, 1924 * '' Tan ...

Cohn, 1904 ****

Fasciolidae Fasciolidae is a family of trematodes and includes several parasites involved in the veterinary and medical sciences, which cause the disease Fasciolosis. Fasciolidae is divided into five genera by Olson et al. 2003. The family's various species ...

Railliet, 1895 ****

Himasthlidae Himasthlidae is a family of trematodes in the order Plagiorchiida Plagiorchiida is a large order of trematodes, synonymous to Echinostomida. They belong to the Digenea, a large subclass of flukes. This order contains relatively few significa ...

Odhner, 1910 **** Philophthalmidae Looss, 1899 ****

Psilostomidae Psilostomidae is a family of trematodes belonging to the order Plagiorchiida Plagiorchiida is a large order of trematodes, synonymous to Echinostomida. They belong to the Digenea, a large subclass of flukes. This order contains relatively few ...

Looss, 1900 ****

Rhytidodidae Rhytidodidae is a family of trematodes belonging to the order Plagiorchiida Plagiorchiida is a large order of trematodes, synonymous to Echinostomida. They belong to the Digenea, a large subclass of flukes. This order contains relatively few s ...

Odhner, 1926 ****

Typhlocoelidae Typhlocoelidae is a family of trematodes belonging to the order Plagiorchiida Plagiorchiida is a large order of trematodes, synonymous to Echinostomida. They belong to the Digenea, a large subclass of flukes. This order contains relatively few ...

Harrah, 1922 **Haplosplanchnata ***Haplosplanchnoidea Poche, 1925 ****

Haplosplanchnidae Haplosplanchnidae is a family of flatworms belonging to the order Plagiorchiida. Genera Genera: * '' Discocephalotrema'' Machida, 1993 * '' Haplosplanchnoides'' Nahhas & Cable, 1964 * '' Haplosplanchnus'' Looss, 1902 References {{Taxonbar, ...

Poche, 1926 **Hemiurata ***Azygioidea Lühe, 1909 **** Azygiidae Lühe, 1909 ***Hemiuroidea Looss, 1899 ****

Accacoeliidae Accacoeliidae is a family of trematodes belonging to the order Plagiorchiida. Genera: * ''Accacladium'' Odhner, 1928 * ''Accacladocoelium'' Odhner, 1928 * ''Accacoelium'' Monticelli, 1893 * ''Odhnerium'' Yamaguti, 1934 * ''Orophocotyle'' Looss, ...

Odhner, 1911 ****

Bathycotylidae Bathcotylidae is a family of trematodes belonging to the order Plagiorchiida Plagiorchiida is a large order of trematodes, synonymous to Echinostomida. They belong to the Digenea, a large subclass of flukes. This order contains relatively few ...

Dollfus, 1932 ****

Derogenidae Derogenidae is a family of trematodes belonging to the order Plagiorchiida. Subdivisions Derogenidae contains 22 genera arranged in two subfamilies, with one being unassigned. * Subfamily Derogeninae Nicoll, 1910 ** ''Derogenes'' Lühe, 1900 ** ' ...

Nicoll, 1910 **** Dictysarcidae Skrjabin & Guschanskaja, 1955 **** Didymozoidae Monticelli, 1888 **** Gonocercidae Skrjabin & Guschanskaja, 1955 **** Hemiuridae Looss, 1899 ****

Hirudinellidae Hirudinellidae is a family of flatworms belonging to the order Plagiorchiida. Genera: * ''Botula ''Botula'' is a genus of mussels in the family Mytilidae Mytilidae are a family of small to large marine and brackish-water bivalve mollusc ...

Dollfus, 1932 ****

Isoparorchiidae Isoparorchiidae is a family of trematodes belonging to the order Plagiorchiida. Genera: * '' Isoparorchis'' Southwell, 1913 References Plagiorchiida {{Trematoda-stub ...

Travassos, 1922 ****

Lecithasteridae Lecithasteridae is a family of trematodes belonging to the order Plagiorchiida. Genera Genera: * '' Acanthuritrema'' Yamaguti, 1970 * '' Aponurus'' Looss, 1907 * '' Assitrema'' Parukhin, 1976 References {{Taxonbar, from=Q10556135 Plagior ...

Odhner, 1905 ****

Ptychogonimidae Ptychogonimidae is a family of trematode Trematoda is a class of flatworms known as flukes. They are obligate internal parasites with a complex life cycle requiring at least two hosts. The intermediate host, in which asexual reproduction occu ...

Dollfus, 1937 **** Sclerodistomidae Odhner, 1927 ****

Sclerodistomoididae Sclerodistomoididae is a family of trematodes belonging to the order Plagiorchiida. Genera: * '' Sclerodistomoides'' Kamegai, 1971 References Plagiorchiida Trematode families {{trematoda-stub ...

Gibson & Bray, 1979 ****

Syncoeliidae Syncoeliidae is a family of trematodes belonging to the order Plagiorchiida. Genera: * '' Copiatestes'' Crowcroft, 1948 * '' Otiotrema'' Setti, 1897 * '' Paronatrema'' Dollfus, 1937 * '' Syncoelium'' Looss, 1899 References {{Taxonbar, from= ...

Looss, 1899 **Heronimata ***Heronimoidea Ward, 1918 ****

Heronimidae Heronimidae is a family of digenean trematode parasites consisting of a single genus with a single species.Pearson, J. C. (1992). On the position of the digenean family Heronimidae: an inquiry into a cladistic classification of the Digenea. ''Sys ...

Ward, 1918 **Lepocreadiata ***Lepocreadioidea Odhner, 1905 ****

Aephnidiogenidae Aephnidiogenidae is a family of trematodes in the order Plagiorchiida. Genera *''Aephnidiogenes'' Nicoll, 1915Nicoll, W. (1915). The trematode parasites of north Queensland. III. Parasites of fishes. ''Parasitology'', 8(1), 22–41. *''Austroh ...

Yamaguti, 1934 ****

Deropristidae Deropristidae is a family of trematodes belonging to the order Plagiorchiida Plagiorchiida is a large order of trematodes, synonymous to Echinostomida. They belong to the Digenea, a large subclass of flukes. This order contains relatively few ...

Cable & Hunninen, 1942 ****

Enenteridae Enenteridae is a family of trematodes belonging to the order Plagiorchiida. Genera: * '' Enenterageitus'' Huston, Cutmore & Cribb, 2019 * '' Enenterum'' Linton, 1910 * '' Koseiria'' Nagaty, 1942 * '' Neoenenterum'' Bilqees & Khatoon, 2004 * '' ...

Yamaguti, 1958 ****

Gorgocephalidae Gorgocephalidae is a family of trematode Trematoda is a class of flatworms known as flukes. They are obligate internal parasites with a complex life cycle requiring at least two hosts. The intermediate host, in which asexual reproduction occu ...

Manter, 1966 **** Gyliauchenidae Fukui, 1929 ****

Lepidapedidae Lepidapedidae is a family of trematodes belonging to the order Plagiorchiida. Genera Genera: * '' Allolepidapedon'' Yamaguti, 1940 * '' Bulbocirrus'' Yamaguti, 1965 * '' Gibsonia'' Gaevskaya & Rodyuk, 1988 References {{Taxonbar, from=Q2132 ...

Yamaguti, 1958 **** Lepocreadiidae Odhner, 1905 ****

Liliatrematidae Liliatrematidae is a family of trematodes belonging to the order Plagiorchiida. Genera: * '' Liliatrema'' Gubanov, 1953 References Plagiorchiida {{Trematoda-stub ...

Gubanov, 1953 **Monorchiata ***Monorchioidea Odhner, 1911 ****

Lissorchiidae Lissorchiidae is a family of flatworms belonging to the order Plagiorchiida. Genera Genera: * '' Alloplagiorchis'' Simer, 1931 * '' Asaccotrema'' Sokolov & Gordeev, 2019 * '' Asymphylodora'' Looss, 1899 References {{Taxonbar, from=Q5032528 ...

Magath, 1917 **** Monorchiidae Odhner, 1911 **Opisthorchiata ***Opisthorchioidea Braun, 1901 **** Cryptogonimidae Ward, 1917 **** Heterophyidae Leiper, 1909 ****

Opisthorchiidae Opisthorchiidae is a family of digenean trematodes. Opisthorchiidae have cosmopolitan distribution. The most medically important species in the family Opisthorchiidae are '' Clonorchis sinensis'', '' Opisthorchis viverrini'', and '' Opisthorchis ...

Looss, 1899 **Pronocephalata ***Paramphistomoidea Fischoeder, 1901 ****

Cladorchiidae Cladorchiidae is a family of trematodes belonging to the order Plagiorchiida. Genera Genera: * '' Allassostoma'' Stunkard, 1916 * '' Allassostomoides'' Stunkard, 1924 * '' Alphamphistoma'' Thatcher & Jégu, 1996 References {{Taxonbar, fro ...

Fischoeder, 1901 **** Mesometridae Poche, 1926 ****

Microscaphidiidae Microscaphidiidae is a family of flatworms belonging to the order Plagiorchiida. Genera Genera: * '' Angiodictyum'' Looss, 1902 * '' Curumai'' Travassos, 1961 * '' Denticauda'' Fukui, 1929 References {{Taxonbar, from=Q5032148 Platyhelmin ...

Looss, 1900 ****

Paramphistomidae Paramphistomidae is a family of trematodes in the order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherei ...

Fischoeder, 1901 ***Pronocephaloidea Looss, 1899 ****

Labicolidae Labicolidae is a family of trematodes belonging to the order Plagiorchiida. Genera: * '' Labicola'' Blair, 1979 References Plagiorchiida {{trematoda-stub ...

Blair, 1979 ****

Notocotylidae Notocotylidae is a family of trematodes in the order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein t ...

Lühe, 1909 ****

Nudacotylidae Nudacotylidae is a family of trematodes belonging to the order Plagiorchiida Plagiorchiida is a large order of trematodes, synonymous to Echinostomida. They belong to the Digenea, a large subclass of flukes. This order contains relatively few ...

Barker, 1916 ****

Opisthotrematidae Opisthotrematidae is a family of trematodes belonging to the order Plagiorchiida. Genera: * '' Folitrema'' Blair, 1981 * '' Lankatrema'' Crusz & Fernand, 1954 * '' Lankatrematoides'' Blair, 1981 * '' Moniligerum'' Dailey, Vogelbein & Forrester, ...

Poche, 1926 ****

Pronocephalidae Pronocephalidae is a family of trematodes belonging to the order Plagiorchiida. Genera Genera: * '' Adenogaster'' Looss, 1901 * '' Astrochis'' Poche, 1925 * '' Buckarootrema'' Platt & Brooks, 2001 References {{Taxonbar, from=Q5033367 Pla ...

Looss, 1899 ****

Rhabdiopoeidae Rhabdiopoeidae is a family of trematodes belonging to the order Plagiorchiida. Genera: * '' Faredifex'' Blair, 1981 * '' Haerator'' Blair, 1981 * '' Rhabdiopoeus'' Johnston, 1913 * '' Taprobanella'' Crusz & Fernand, 1954 References {{Taxonba ...

Poche, 1926 **Transversotremata ***Transversotrematoidea Witenberg, 1944 ****

Transversotrematidae Transversotrematidae is a family of trematodes in the order Plagiorchiida. It is the only family in the superfamily Transversotrematoidea, which is the only superfmaily in the suborder Order ( la, ordo) is one of the eight major hierarchical ...

Witenberg, 1944 **Xiphidiata ***Allocreadioidea Looss, 1902 ****

Acanthocolpidae Acanthocolpidae is a family of trematodes belonging to the order Plagiorchiida Plagiorchiida is a large order of trematodes, synonymous to Echinostomida. They belong to the Digenea, a large subclass of flukes. This order contains relatively fe ...

Lühe, 1906 **** Allocreadiidae Looss, 1902 ****

Batrachotrematidae Batrachotrematidae is a family of trematodes in the order Plagiorchiida. Genera The following genera are described severally, and by Rizvi, ''et al.'' (2012):Rizvi, A. N., Bursey, C. R. & Bhutia, P. T. (2012). Three new species of Digenea (Ba ...

Dollfus & Williams, 1966 ****

Brachycladiidae Brachycladiidae is a family of trematodes belonging to the order Plagiorchiida. Genera: * ''Balanorchis'' Fischoeder, 1901 * ''Brachycladium'' Looss, 1899 * ''Campula'' Cobbold, 1858 * ''Cetitrema'' A.S.Skrjabin, 1970 * ''Hunterotrema'' Mclntosh, ...

Odhner, 1905 **** Opecoelidae Ozaki, 1925 ***Gorgoderoidea Looss, 1901 ****

Callodistomidae Callodistomidae is a family of trematode Trematoda is a class of flatworms known as flukes. They are obligate internal parasites with a complex life cycle requiring at least two hosts. The intermediate host, in which asexual reproduction occ ...

Odhner, 1910 **** Dicrocoeliidae Looss, 1899 ****

Gorgoderidae Gorgoderidae is a family of trematodes in the order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein t ...

Looss, 1899 ***Haploporoidea Nicoll, 1914 **** Atractotrematidae Yamaguti, 1939 ****

Haploporidae Haploporidae is a family of trematodes in the order Plagiorchiida. Genera The genera are organised by their subfamily. *Chalcinotrematinae Overstreet & Curran, 2005 **'' Chalicinotrema'' Texeira de Freitas, 1947 **'' Paralecithobotrys'' Teixe ...

Nicoll, 1914 ***Microphalloidea Ward, 1901 ****

Diplangidae Diplangidae is a family of trematode Trematoda is a class of flatworms known as flukes. They are obligate internal parasites with a complex life cycle requiring at least two hosts. The intermediate host, in which asexual reproduction occurs, ...

Yamaguti, 1971 ****

Exotidendriidae Exotidendriidae is a family of trematode Trematoda is a class of flatworms known as flukes. They are obligate internal parasites with a complex life cycle requiring at least two hosts. The intermediate host, in which asexual reproduction occu ...

Mehra, 1935 ****

Faustulidae Faustulidae is a family of trematodes belonging to the order Plagiorchiida. Genera Genera: * '' Allofellodistomum'' Yamaguti, 1971 * '' Antorchis'' Linton, 1911 * '' Bacciger'' Nicoll, 1914 References {{Taxonbar, from=Q10494373 Plagiorc ...

Poche, 1926 **** Microphallidae Ward, 1901 **** Pachypsolidae Yamaguti, 1958 ****

Phaneropsolidae Phaneropsolidae is a family of trematodes belonging to the order Plagiorchiida. Genera: * ''Microtrema Opisthorchiidae is a family of digenean trematodes. Opisthorchiidae have cosmopolitan distribution. The most medically important species in ...

Mehra, 1935 **** Pleurogenidae Looss, 1899 ****

Prosthogonimidae The Prosthogonimidae are a family of trematodes. They are part of the huge of the suborder Xiphidiata in the order Plagiorchiida. The adults of these parasites occur in the bursa of Fabricius, caecum, cloaca, liver, oviduct and sometimes eve ...

Lühe, 1909 ****

Renicolidae Renicolidae is a family of trematodes belonging to the order Plagiorchiida. Genera: * '' Nephromonorcha'' Leonov, 1958 * '' Renicola'' Cohn, 1904 References Plagiorchiida {{Trematoda-stub ...

Dollfus, 1939 **** Zoogonidae Odhner, 1902 ***Plagiorchioidea Lühe, 1901 ****

Auridistomidae Auridistomidae is a family of trematodes belonging to the order Plagiorchiida. Genera: * '' Auridistomum'' Stafford, 1905 * ''Patagium The patagium (plural: patagia) is a membranous body part that assists an animal in obtaining lift when gl ...

Lühe, 1901 ****

Brachycoeliidae Brachycoeliidae is a family of trematodes belonging to the order Plagiorchiida. Genera: * '' Brachycoelium'' Dujardin, 1845 * '' Brachycoelium'' Stiles & Hassall, 1898 * '' Cymatocarpus'' Looss, 1899 * '' Parabrachycoelium'' Pérez-Ponce de Leo ...

Looss, 1899 ****

Cephalogonimidae Cephalogonimidae is a family of trematode parasites characterized by a genital pore located anterior to the oral sucker, at the apex of the body.Bray, R. A., Gibson, D. I., & Jones, A. (2008). ''Keys to the Trematoda, Volume 3''. CAB Internation ...

Looss, 1899 ****

Choanocotylidae Choanocotylidae is a family of trematodes belonging to the order Plagiorchiida. Genera: * '' Auriculotrema'' Platt, 2003 References Plagiorchiida {{Trematoda-stub ...

Jue Sue & Platt, 1998 ****

Echinoporidae Echinoporidae is a family of trematodes belonging to the order Plagiorchiida. Genera: * '' Echinoporus'' References {{trematoda-stub Plagiorchiida ...

Krasnolobova & Timofeeva, 1965 ****

Encyclometridae Encyclometridae is a family of trematodes belonging to the order Plagiorchiida Plagiorchiida is a large order of trematodes, synonymous to Echinostomida. They belong to the Digenea, a large subclass of flukes. This order contains relatively fe ...

Mehra, 1931 **** Leptophallidae Dayal, 1938 ****

Macroderoididae Macroderoididae is a family of trematodes belonging to the order Plagiorchiida. Genera Genera: * ''Alloglossidium'' Simer, 1929 * ''Alloglyptus'' Byrd, 1950 * ''Allomacroderoides'' Watson, 1976 References

{{Taxonbar, from=Q5014116 Plag ...

McMullen, 1937 ****

Meristocotylidae Meristocotylidae is a family of trematodes belonging to the order Plagiorchiida. Genera: * '' Meristocotyle'' Fischthal & Kuntz, 1964 References Plagiorchiida {{Trematoda-stub ...

Fischthal & Kuntz, 1981 ****

Ocadiatrematidae Ocadiatrematidae is a family of trematodes belonging to the order Plagiorchiida. Genera: * '' Ocadiatrema'' Fischthal & Kuntz, 1981 References Plagiorchiida Trematode families {{Trematoda-stub ...

Fischthal & Kuntz, 1981 ****

Orientocreadiidae Orientocreadiidae is a family of trematodes belonging to the order Plagiorchiida. Genera: * ''Macrotrema ''Macrotrema caligans'' is a species of swamp eel native to Peninsular Malaysia and the Mae Nam Chao Praya basin in Thailand. The male ...

Yamaguti, 1958 ****

Plagiorchiidae Plagiorchiidae is a family of parasitic trematodes (flukes) in the order Plagiorchiida Plagiorchiida is a large order of trematodes, synonymous to Echinostomida. They belong to the Digenea, a large subclass of flukes. This order contains rela ...

Lühe, 1901 **** Styphlotrematidae Baer, 1924 **** Telorchiidae Looss, 1899 **** Thrinascotrematidae Jue Sue & Platt, 1999 ****

Urotrematidae Pleurogenidae is a family of trematodes belonging to the order Plagiorchiida. Genera: * '' Cortrema'' Tang, 1951 * '' Langeronia'' Caballero & Bravo-Hollis, 1949 * '' Nenimandijea'' Kaw, 1950 * '' Pleurogenes'' Looss, 1896 * '' Pleurogenoides'' T ...

Poche, 1926

Human digenean infections

Only about 12 of the 6,000 known species are infectious to humans, but some of these species are important

disease A disease is a particular abnormal condition that negatively affects the structure or function of all or part of an organism, and that is not immediately due to any external injury. Diseases are often known to be medical conditions that a ...

s afflicting over 200 million people. The species that infect humans can be divided into groups, the schistosomes and the non-schistosomes.

Schistosomes

The Schistosomes occur in the circulatory system of the definitive host. Humans become infected after free-swimming cercaria liberated from infected snails penetrate the skin. These dioecious worms are long and thin, ranging in size from 10 to 30 mm in length to 0.2 to 1.0 mm in diameter. Adult males are shorter and thicker than females, and have a long groove along one side of the body in which the female is clasped. Females reach sexual maturity after they have been united with a male. After mating the two remain locked together for the rest of their lives. They can live for several years and produce many thousands of eggs. The four species of schistosomes that infect humans are members of the

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial n ...

Schistosoma ''Schistosoma'' is a genus of trematodes, commonly known as blood flukes. They are parasitic flatworms responsible for a highly significant group of infections in humans termed '' schistosomiasis'', which is considered by the World Health Organ ...

''.

Non-schistosomes

The seven major species of non-schistosomes that infect humans are listed below. People become infected after ingesting metacercarial cysts on plants or in undercooked animal flesh. Most species inhabit the human gastrointestinal tract, where they shed eggs along with host feces. ''Paragonimus westermani'', which colonizes the lungs, can also pass its eggs in

saliva Saliva (commonly referred to as spit) is an extracellular fluid produced and secreted by salivary glands in the mouth. In humans, saliva is around 99% water, plus electrolytes, mucus, white blood cells, epithelial cells (from which DNA can be ...

. These flukes generally cause mild pathology in humans, but more serious effects may also occur.

References

Notes

* Gibson, D.I., Jones, A. & Bray, R.A. (2002). ''Key to the Trematoda, vol.1'' * Littlewood D.T.J. & Bray R.A. (2001) ''Interrelationships of the Platyhelminthes.'' * Yamaguti, S. (1971). ''Synopsis of digenetic trematodes of vertebrates.'' Keigaku Publishing Co., Tokyo.

External links

Cambridge University Schistosome Research GroupFishdisease.net
{{Taxonbar, from=Q132650 Protostome subclasses