Digenea (Gr. ''Dis'' – double, ''Genos'' – race) is a

class Class, Classes, or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used d ...

trematodes Trematoda is a class of flatworms known as trematodes, and commonly 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, is a moll ...

in the

Platyhelminthes Platyhelminthes (from the Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") is a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrates commonly called f ...

phylum In biology, a phylum (; : phyla) is a level of classification, or taxonomic rank, that is below Kingdom (biology), kingdom and above Class (biology), class. Traditionally, in botany the term division (taxonomy), division has been used instead ...

, consisting of

parasitic Parasitism is a close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The ent ...

flatworm Platyhelminthes (from the Greek language, Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") is a Phylum (biology), phylum of relatively simple bilaterian, Segmentation (biology), ...

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

syncytial A syncytium (; : syncytia; from Greek: σύν ''syn'' "together" and κύτος ''kytos'' "box, i.e. cell") or symplasm is a multinucleate cell that can result from multiple cell fusions of uninuclear cells (i.e., cells with a single nucleus), in ...

tegument 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 Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...

. Once thought to be related to the

Monogenea Monogeneans, members of the class Monogenea, are a group of ectoparasitic flatworms commonly found on the skin, gills, or fins of fish. They have a direct lifecycle and do not require an intermediate host. Adults are hermaphrodites, meaning they ...

, 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 on ...

and that the Monogenea are more closely allied with the

Cestoda Cestoda is a class of parasitic worms in the flatworm phylum (Platyhelminthes). Most of the species—and the best-known—are those in the subclass Eucestoda; they are ribbon-like worms as adults, commonly known as tapeworms. Their bodies co ...

. 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 Neodermata is a clade of rhabditophoran flatworms containing the parasitic groups Trematoda, Monogenea and Cestoda. Description All neodermatans are parasites, in many groups having a free-swimming larval stage. The most striking feature uniti ...

; a group of

platyhelminths Platyhelminthes (from the Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") is a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrates commonly called f ...

comprising the Digenea,

and

. Digeneans possess a

vermiform Vermes (" vermin/vermes") is an obsolete taxon used by Carl Linnaeus and Jean-Baptiste Lamarck for non-arthropod invertebrate animals. Linnaeus In Linnaeus's ''Systema Naturae'', the Vermes had the rank of class, occupying the 6th (and last) ...

, unsegmented body-plan and have a solid

parenchyma upright=1.6, Lung parenchyma showing damage due to large subpleural bullae. Parenchyma () is the bulk of functional substance in an animal organ such as the brain or lungs, or a structure such as a tumour. In zoology, it is the tissue that ...

with no

body cavity A body cavity is any space or compartment, or potential space, in an animal body. Cavities accommodate organs and other structures; cavities as potential spaces contain fluid. The two largest human body cavities are the ventral body cavity, a ...

(

coelom The coelom (or celom) is the main body cavity in many animals and is positioned inside the body to surround and contain the digestive tract and other organs. In some animals, it is lined with mesothelium. In other animals, such as molluscs, i ...

) as in all platyhelminths. Peerj-292-fig-7 Zoogonidae

There are typically two suckers, an

anterior Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...

oral The word oral may refer to: Relating to the mouth * Relating to the mouth, the first portion of the alimentary canal that primarily receives food and liquid **Oral administration of medicines ** Oral examination (also known as an oral exam or ora ...

sucker surrounding the

mouth A mouth also referred to as the oral is the body orifice through which many animals ingest food and animal communication#Auditory, vocalize. The body cavity immediately behind the mouth opening, known as the oral cavity (or in Latin), is also t ...

, and a

ventral Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

sucker sometimes termed the

acetabulum The acetabulum (; : acetabula), also called the cotyloid cavity, is a wikt:concave, concave surface of the pelvis. The femur head, head of the femur meets with the pelvis at the acetabulum, forming the Hip#Articulation, hip joint. Structure The ...

, 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 A hermaphrodite () is a sexually reproducing organism that produces both male and female gametes. Animal species in which individuals are either male or female are gonochoric, which is the opposite of hermaphroditic. The individuals of many ...

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 zygote and initiate its development into a new individual organism or of ...

is internal, with

sperm Sperm (: sperm or sperms) is the male reproductive Cell (biology), 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 ...

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 certain species that have distinct unisexual individuals, each producing either male or female gametes, either directly (in animals) or indirectly (in seed plants). Dioecious reproduction is ...

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

eggs An egg is an organic vessel in which an embryo begins to develop. Egg, EGG or eggs may also refer to: Biology * Egg cell, the female reproductive cell (gamete) in oogamous organisms Food * Eggs as food Places * Egg, Austria * Egg, Switzerland ...

and asexual reproduction in the first

larva A larva (; : larvae ) is a distinct juvenile form many animals undergo before metamorphosis into their next life stage. Animals with indirect development such as insects, some arachnids, amphibians, or cnidarians typically have a larval phase ...

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 Mitosis () is a part of the cell cycle in eukaryotic cells in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis is an equational division which gives rise to genetically identical cells in which the t ...

dividing

cell Cell most often refers to: * Cell (biology), the functional basic unit of life * Cellphone, a phone connected to a cellular network * Clandestine cell, a penetration-resistant form of a secret or outlawed organization * Electrochemical cell, a de ...

s which give rise to

embryo An embryo ( ) is the initial stage of development for 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 sp ...

s and to a line of new

germ cells A germ cell is any cell that gives rise to the gametes of an organism that reproduces sexually. In many animals, the germ cells originate in the primitive streak and migrate via the gut of an embryo to the developing gonads. There, they undergo ...

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 one of the two types of hermaphroditism, the other type being simultaneous hermaphroditism. It occurs when the organism's sex changes at some point in its life. A sequential hermaphrodit ...

is the general rule among the Digenea. Usually two

testes A testicle or testis ( testes) is the gonad in all male bilaterians, including humans, and is homologous to the ovary in females. Its primary functions are the production of sperm and the secretion of androgens, primarily testosterone. The ...

are present, but some flukes can have more than 100. Also present are vasa efferentia, a

vas deferens The vas deferens (: vasa deferentia), ductus deferens (: ductūs deferentes), or sperm duct is part of the male reproductive system of many vertebrates. In mammals, spermatozoa are produced in the seminiferous tubules and flow into the epididyma ...

seminal vesicle The seminal vesicles (also called vesicular glands or seminal glands) are a pair of convoluted tubular accessory glands that lie behind the urinary bladder of male mammals. They secrete fluid that largely composes the semen. The vesicles are 5 ...

ejaculatory duct The ejaculatory ducts (''ductus ejaculatorii'') are paired structures in the male reproductive system. Each ejaculatory duct is formed by the union of the vas deferens with the Excretory duct of seminal gland, duct of the seminal vesicle. They pa ...

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 a gonad in the female reproductive system that produces ova; when released, an ovum travels through the fallopian tube/ oviduct into the uterus. There is an ovary on the left and the right side of the body. The ovaries are end ...

with an

oviduct The oviduct in vertebrates is the passageway from an ovary. In human females, this is more usually known as the fallopian tube. The eggs travel along the oviduct. These eggs will either be fertilized by spermatozoa to become a zygote, or will dege ...

, a

seminal receptacle Female sperm storage is a biological process and often a type of sexual selection in which sperm cells transferred to a female during mating are temporarily retained within a specific part of the reproductive tract before the oocyte, or egg, is fe ...

, a pair of vitelline glands (involved in

yolk Among animals which produce eggs, the yolk (; also known as the vitellus) is the nutrient-bearing portion of the egg whose primary function is to supply food for the development of the embryo. Some types of egg contain no yolk, for example bec ...

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 The cecum ( caecum, ; plural ceca or caeca, ) is a pouch within the peritoneum that is considered to be the beginning of the large intestine. It is typically located on the right side of the body (the same side of the body as the appendix, ...

). 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

anus In mammals, invertebrates and most fish, the anus (: anuses or ani; from Latin, 'ring' or 'circle') is the external body orifice at the ''exit'' end of the digestive tract (bowel), i.e. the opposite end from the mouth. Its function is to facil ...

es, or with the excretory vesicle to form a uroproct. Digeneans are also capable of direct nutrient uptake through the tegument by

pinocytosis In cellular biology, pinocytosis, otherwise known as fluid endocytosis and bulk-phase pinocytosis, is a mode of endocytosis in which small molecules dissolved in extracellular fluid are brought into the cell through an invagination of the cell me ...

and

phagocytosis Phagocytosis () is the process by which a cell (biology), cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs ph ...

by the

syncitium A syncytium (; : syncytia; from Greek: σύν ''syn'' "together" and κύτος ''kytos'' "box, i.e. cell") or symplasm is a multinucleate cell that can result from multiple cell fusions of uninuclear cells (i.e., cells with a single nucleus), ...

. Most adult digeneans occur in the vertebrate

alimentary canal The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The tract is the largest of the body's systems, after the cardiovascular system. ...

or its associated

organs In a multicellular organism, an organ is a collection of tissues joined in a structural unit to serve a common function. In the hierarchy of life, an organ lies between tissue and an organ system. Tissues are formed from same type cells to a ...

, 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 in various organs of the gastrointestinal, respiratory, and genitourinary tracts. It is the layer of dense irregular connective tissue that supports the mucosa (mucous membrane) an ...

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

mollusc Mollusca is a phylum of protostome, protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant taxon, extant species of molluscs are recognized, making it the second-largest animal phylum ...

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 metacercarial stages and free-living

cercaria A cercaria (plural cercariae) is a larval form of the trematode class of parasites. It develops within the germinal cells of the Trematode life cycle stages, sporocyst or redia. A cercaria has a tapering head with large penetration glands. It may ...

l stages do not feed.

Nervous system

Paired

ganglia A ganglion (: ganglia) 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 a ...

at the anterior end of the body serve as the

brain The brain is an organ (biology), 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 ...

. From this

nerves A nerve is an enclosed, cable-like bundle of nerve fibers (called axons). Nerves have historically been considered the basic units of the peripheral nervous system. A nerve provides a common pathway for the electrochemical nerve impulses called ...

extend anteriorly and posteriorly.

Sensory receptors Sensory neurons, also known as afferent neurons, are neurons in the nervous system, that convert a specific type of stimulus, via their receptors, into action potentials or graded receptor potentials. This process is called sensory transducti ...

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 () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...

host in

faeces Feces (also known as faeces American and British English spelling differences#ae and oe, or fæces; : faex) are the solid or semi-solid remains of food that was not digested in the small intestine, and has been broken down by bacteria in the ...

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 ...

, in which sexual reproduction does not occur. Digeneans may infect the first intermediate host (usually a

snail A snail is a shelled gastropod. The name is most often applied to land snails, terrestrial molluscs, terrestrial pulmonate gastropod molluscs. However, the common name ''snail'' is also used for most of the members of the molluscan class Gas ...

) by either passive or active means. The eggs of some digeneans, for example, are (passively) eaten by snails (or, rarely, by an

annelid The annelids (), also known as the segmented worms, are animals that comprise the phylum Annelida (; ). The phylum contains over 22,000 extant species, including ragworms, earthworms, and leeches. The species exist in and have adapted to vario ...

worm), in which they proceed to hatch. Alternatively, eggs may hatch in water to release an actively swimming, ciliated larva, the

miracidium The miracidium is the second stage in the life cycle of trematodes. When trematode eggs are laid and come into contact with fresh water, they hatch and release miracidium. In this phase, miracidia are ciliated and free-swimming. This stage is comp ...

, 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 Asexual reproduction is a type of reproduction that does not involve the fusion of gametes or change in the number of chromosomes. The offspring that arise by asexual reproduction from either unicellular or multicellular organisms inherit the f ...

, 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 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 A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...

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 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 ...

. Two-host life cycles can be primary (there never was a third host) as in the

Bivesiculidae Bivesiculidae 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 signi ...

, 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 A cyst is a closed sac, having a distinct envelope and division compared with the nearby tissue. Hence, it is a cluster of cells that have grouped together to form a sac (like the manner in which water molecules group together to form a bubb ...

of host and parasite origin, or encapsulated in a layer of tissue derived from the host only. This stage is infective to the

. 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 An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...

temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...

, 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

turbellaria The Turbellaria are one of the traditional sub-divisions of the phylum Platyhelminthes (flatworms), and include all the sub-groups that are not exclusively parasitic. There are about 4,500 species, which range from to large freshwater forms mo ...

ns 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 Diplostomida is an order of trematodes in the subclass Digenea. It is synonymous with Strigeatida Poche, 1926. Families Order Diplostomida *Suborder Diplostomata **Superfamily Brachylaimoidea Joyeux & Foley, 1930 ***Brachylaimidae Joyeux & Foley ...

**Suborder Diplostomata ***Superfamily Brachylaimoidea Joyeux & Foley, 1930 **** Brachylaimidae Joyeux & Foley, 1930 ****

Leucochloridiidae ''Leucochloridium'' is a genus of parasitic trematode worms in the order Diplostomida. It Is the sole genus in the family Leucochloridiidae.Carus, C. G. (1835). Beobachtung über einen merkwürdigen schöngefärbten Eingeweidewurm, ''Leucochlorid ...

Poche, 1907 ***Superfamily Diplostomoidea Poirier, 1886 ****

Brauninidae Brauninidae is a monotypic family of trematodes in the order Diplostomida. It consists of one genus, ''Braunina'' Heider, 1900,Heider, K. (1900). Über ''Braunina'', ein neues Genus aus der Gruppe der Hemistomidae. ''Verhandlungen der Deutschen ...

Wolf, 1903 ****

Cyathocotylidae Cyathocotylidae is a family of trematodes in the order Diplostomida Diplostomida is an order of trematodes in the subclass Digenea. It is synonymous with Strigeatida Poche, 1926. Families Order Diplostomida *Suborder Diplostomata **Superfami ...

Mühling, 1898 ****

Diplostomidae Diplostomidae is a family of trematodes in the order Diplostomida Diplostomida is an order of trematodes in the subclass Digenea. It is synonymous with Strigeatida Poche, 1926. Families Order Diplostomida *Suborder Diplostomata **Superfamily ...

Poirier, 1886 **** Proterodiplostomidae Dubois, 1936 ****

Strigeidae Strigeidae is a family of trematodes in the order Diplostomida Diplostomida is an order of trematodes in the subclass Digenea. It is synonymous with Strigeatida Poche, 1926. Families Order Diplostomida *Suborder Diplostomata **Superfamily Br ...

Railliet, 1919 ***Superfamily Schistosomatoidea Stiles & Hassall, 1898 ****

Aporocotylidae Aporocotylidae is a family of trematodes within the order Diplostomida Diplostomida is an order of trematodes in the subclass Digenea. It is synonymous with Strigeatida Poche, 1926. Families Order Diplostomida *Suborder Diplostomata **Superf ...

Odhner, 1912 ****

Schistosomatidae Schistosomatidae is a family of digenetic trematodes with complex parasitic life cycles. Immature developmental stages of schistosomes are found in molluscs and adults occur in vertebrates. The best studied group, the blood flukes of the genu ...

Stiles & Hassall, 1898 ****

Spirorchiidae Spirorchiidae is a family of digenetic trematodes. Infestation by these trematodes leads to the disease spirorchiidiosis. Spirorchiids are mainly parasites of turtles. It has been synonymised with Proparorchiidae Ward, 1921, Spirorchidae Stunk ...

Stunkard, 1921 *

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 significant parasites of humans. The following families are placed here, ...

**Apocreadiata ***Apocreadioidea Skrjabin, 1942 ****

Apocreadiidae Apocreadiidae is a family of parasitic worms in the class Trematoda. Characteristics Members of the family are characterised by having extensive vitelline (yolk producing) follicles, eye-spot pigment dispersed in the front half of the body, a r ...

Skrjabin, 1942 **Bivesiculata ***Bivesiculoidea ****

Yamaguti, 1934 **Bucephalata ***Bucephaloidea Poche, 1907 ****

Bucephalidae Bucephalidae is a family of trematodes that parasitize fish. They lack suckers, having instead a muscular organ called a "rhynchus" at the front end which they use to attach to their hosts. The characteristics of the rhynchus are used to help d ...

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 ...

Kurochkin, 1975 ***Gymnophalloidea Odhner, 1905 **** Botulisaccidae Yamaguti, 1971 **** Fellodistomidae Nicoll, 1909 **** Gymnophallidae Odhner, 1905 **** Tandanicolidae Johnston, 1927 **Echinostomata ***Echinostomatoidea Looss, 1902 ****

Calycodidae Calycodidae is a family of trematodes belonging to the order Plagiorchiida. Genera: * ''Calycodes'' Looss, 1901 References

Plagiorchiida Trematode families {{trematoda-stub ...

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 Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideal ...

Odhner, 1910 ****

Echinostomatidae Echinostomatidae is a family of trematodes in the order Plagiorchiida, first described in 1899.Fuhrmann, O. (1928). ''Zweite Klasse des Cladus Platyhelminthes: Trematoda''. Berlin & Leipzig: Kükenthal's Handbuch der Zoologie. Subdivisions The ...

Looss, 1899 ****

Eucotylidae Eucotylidae 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 f ...

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 Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, ...

Odhner, 1910 ****

Philophthalmidae Philophthalmidae is a family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideal ...

Looss, 1899 ****

Psilostomidae Psilostomidae is a family of trematode Trematoda is a Class (biology), class of flatworms known as trematodes, and commonly as flukes. They are obligate parasite, obligate Endoparasites, internal parasites with a complex biological life cycle ...

Looss, 1900 ****

Rhytidodidae Rhytidodidae is a family of trematode Trematoda is a Class (biology), class of flatworms known as trematodes, and commonly as flukes. They are obligate parasite, obligate Endoparasites, internal parasites with a complex biological life cycle, ...

Odhner, 1926 ****

Typhlocoelidae Typhlocoelidae is a family of trematode Trematoda is a Class (biology), class of flatworms known as trematodes, and commonly as flukes. They are obligate parasite, obligate Endoparasites, internal parasites with a complex biological life cycl ...

Harrah, 1922 **Haplosplanchnata ***Haplosplanchnoidea Poche, 1925 **** Haplosplanchnidae Poche, 1926 **Hemiurata ***Azygioidea Lühe, 1909 **** Azygiidae Lühe, 1909 ***Hemiuroidea Looss, 1899 **** Accacoeliidae Odhner, 1911 **** Bathycotylidae Dollfus, 1932 ****

Derogenidae Derogenidae is a family of trematode Trematoda is a Class (biology), class of flatworms known as trematodes, and commonly as flukes. They are obligate parasite, obligate Endoparasites, internal parasites with a complex biological life cycle, ...

Nicoll, 1910 **** Dictysarcidae Skrjabin & Guschanskaja, 1955 **** Didymozoidae Monticelli, 1888 **** Gonocercidae Skrjabin & Guschanskaja, 1955 **** Hemiuridae Looss, 1899 **** Hirudinellidae Dollfus, 1932 **** Isoparorchiidae Travassos, 1922 **** Lecithasteridae Odhner, 1905 **** Ptychogonimidae Dollfus, 1937 ****

Sclerodistomidae Sclerodistomidae is a family of trematodes belonging to the order Plagiorchiida. Genera: * ''Eurycoelum'' Brock, 1886 * ''Kenmackenzia'' Gibson, 1983 * ''Prosogonotrema'' Vigueras, 1940 * ''Prosorchiopsis'' Robert-Philippe Dollfus, Dollfus, 1947 ...

Odhner, 1927 **** Sclerodistomoididae Gibson & Bray, 1979 **** Syncoeliidae Looss, 1899 **Heronimata ***Heronimoidea Ward, 1918 **** Heronimidae Ward, 1918 **Lepocreadiata ***Lepocreadioidea Odhner, 1905 **** Aephnidiogenidae Yamaguti, 1934 **** Deropristidae Cable & Hunninen, 1942 **** Enenteridae Yamaguti, 1958 **** Gorgocephalidae Manter, 1966 **** Gyliauchenidae Fukui, 1929 **** Lepidapedidae Yamaguti, 1958 **** Lepocreadiidae Odhner, 1905 **** Liliatrematidae Gubanov, 1953 **Monorchiata ***Monorchioidea Odhner, 1911 **** Lissorchiidae Magath, 1917 **** Monorchiidae Odhner, 1911 **Opisthorchiata ***Opisthorchioidea Braun, 1901 **** Cryptogonimidae Ward, 1917 ****

Heterophyidae Heterophyidae is a family of intestinal trematodes in the order Plagiorchiida. Description: " Tegument covered by spines. Oral sucker not armed or armed by circumoral spines. Pharynx presented. Genital synus presented. Ventral and genital s ...

Leiper, 1909 **** Opisthorchiidae Looss, 1899 **Pronocephalata ***Paramphistomoidea Fischoeder, 1901 **** Cladorchiidae Fischoeder, 1901 **** Mesometridae Poche, 1926 **** Microscaphidiidae Looss, 1900 **** Paramphistomidae Fischoeder, 1901 ***Pronocephaloidea Looss, 1899 **** Labicolidae Blair, 1979 **** Notocotylidae Lühe, 1909 **** Nudacotylidae Barker, 1916 **** Opisthotrematidae Poche, 1926 ****

Pronocephalidae Pronocephalidae 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 relative ...

Looss, 1899 **** Rhabdiopoeidae Poche, 1926 **Transversotremata ***Transversotrematoidea Witenberg, 1944 **** Transversotrematidae Witenberg, 1944 **Xiphidiata ***Allocreadioidea Looss, 1902 **** Acanthocolpidae Lühe, 1906 **** Allocreadiidae Looss, 1902 **** Batrachotrematidae Dollfus & Williams, 1966 **** Brachycladiidae Odhner, 1905 ****

Opecoelidae Opecoelidae is a family of trematodes. It is the largest digenean family with over 90 genera and nearly 900 species, almost solely found in marine and freshwater teleost fishes.Bray, Rodney A., Cribb, Thomas H., Littlewood, D. Timothy J. & Waesch ...

Ozaki, 1925 ***Gorgoderoidea Looss, 1901 **** Callodistomidae Odhner, 1910 **** Dicrocoeliidae Looss, 1899 **** Gorgoderidae Looss, 1899 ***Haploporoidea Nicoll, 1914 **** Atractotrematidae Yamaguti, 1939 **** Haploporidae Nicoll, 1914 ***Microphalloidea Ward, 1901 **** Diplangidae Yamaguti, 1971 **** Exotidendriidae Mehra, 1935 **** Faustulidae Poche, 1926 **** Microphallidae Ward, 1901 **** Pachypsolidae Yamaguti, 1958 **** Phaneropsolidae Mehra, 1935 **** Pleurogenidae Looss, 1899 **** Prosthogonimidae Lühe, 1909 ****

Renicolidae Renicolidae 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 f ...

Dollfus, 1939 **** Zoogonidae Odhner, 1902 ***Plagiorchioidea Lühe, 1901 **** Auridistomidae Lühe, 1901 **** Brachycoeliidae Looss, 1899 **** Cephalogonimidae Looss, 1899 ****

Choanocotylidae Choanocotylidae 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 relative ...

Jue Sue & Platt, 1998 **** Echinoporidae Krasnolobova & Timofeeva, 1965 **** Encyclometridae Mehra, 1931 **** Leptophallidae Dayal, 1938 **** Macroderoididae McMullen, 1937 **** Meristocotylidae Fischthal & Kuntz, 1981 **** Ocadiatrematidae Fischthal & Kuntz, 1981 **** Orientocreadiidae Yamaguti, 1958 **** Plagiorchiidae Lühe, 1901 **** Styphlotrematidae Baer, 1924 **** Telorchiidae Looss, 1899 ****

Thrinascotrematidae Thrinascotrematidae 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 rela ...

Jue Sue & Platt, 1999 **** Urotrematidae Poche, 1926

Human digenean infections

Only about 12 of the 6,000 known species are

infectious infection is the invasion of tissues by pathogens, their multiplication, and the reaction of host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmissible disease or communicable dise ...

to humans, but some of these species are important

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

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 In vertebrates, the circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the body. It includes the cardiovascular system, or vascular system, that consists of the heart ...

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 (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

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

''.

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 as spit or drool) 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 ...

. 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
{{Authority control Protostome subclasses