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

class Class 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 differently ...

trematodes 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, is usually a snail. The definitive host ...

in the

Platyhelminthes 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, unsegm ...

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 ...

, consisting of

parasitic Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson ha ...

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, unsegmen ...

s (known as ''flukes'') with a syncytial 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 () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with ...

. Once thought to be related to the

Monogenea Monogeneans 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 have both male and female repr ...

, 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, known as tapeworms. Their bodies consist of ...

. 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 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, unsegmen ...

comprising the Digenea,

and

. 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 � ...

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, and ...

(

coelom The coelom (or celom) is the main body cavity in most 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, ...

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

There are typically two

sucker Sucker may refer to: General use * Lollipop or sucker, a type of confection * Sucker (slang), a slang term for a very gullible person * Hard candy ** Cough drop ** Mint (candy) Biology * Sucker (botany), a term for a shoot that arises undergro ...

s, an

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

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 or ...

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 Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. 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 (), also called the cotyloid cavity, is a concave surface of the pelvis. The head of the femur meets with the pelvis at the acetabulum, forming the hip joint. Structure There are three bones of the ''os coxae'' (hip bone) that c ...

, 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 taxonomic groups of animals (mostly invertebrates) do not have ...

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. Pro ...

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 reproducti ...

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

eggs Humans and human ancestors have scavenged and eaten animal eggs for millions of years. Humans in Southeast Asia had domesticated chickens and harvested their eggs for food by 1,500 BCE. The most widely consumed eggs are those of fowl, especial ...

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. ...

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, asexual reproduction, metamorphosis, and the growth and differentiation of st ...

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

cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery ...

s 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 Germ or germs may refer to: Science * Germ (microorganism), an informal word for a pathogen * Germ cell, cell that gives rise to the gametes of an organism that reproduces sexually * Germ layer, a primary layer of cells that forms during embryo ...

that become included in these embryonic stages. Since the absence of meiotic processes is not proven, the exact definition remains doubtful.

Male organs

Protandry is the general rule among the Digenea. Usually two

testes A testicle or testis (plural testes) is the male reproductive gland or gonad in all bilaterians, including humans. It is homologous to the female ovary. The functions of the testes are to produce both sperm and androgens, primarily testoste ...

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

vas deferens The vas deferens or ductus deferens is part of the male reproductive system of many vertebrates. The ducts transport sperm from the epididymis to the ejaculatory ducts in anticipation of ejaculation. The vas deferens is a partially coiled tube ...

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

, ejaculatory duct 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 The oviduct in mammals, is the passageway from an ovary. In human females this is more usually known as the Fallopian tube or uterine tube. The eggs travel along the oviduct. These eggs will either be fertilized by spermatozoa to become a zygote, o ...

, 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 f ...

, 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 ...

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 or caecum 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, to which it is joined). The wo ...

). 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 The anus (Latin, 'ring' or 'circle') is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, the residual semi-solid waste that remains after food digestion, which, ...

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 ...

and

phagocytosis Phagocytosis () is the process by which a 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 phagocytosis is ...

by the

syncitium A syncytium (; plural syncytia; from Greek: σύν ''syn'' "together" and κύτος ''kytos'' "box, i.e. cell") or symplasm is a multinucleate cell which 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 GI tract contains all the major organs of the digestive system, in humans and ...

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 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) and j ...

, 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 est ...

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

cercaria A cercaria (plural cercariae) is the larval form of the 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 w ...

l 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 sympathe ...

at the anterior end of the body serve as the

brain A brain is an organ (biology), organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as Visual perception, vision. I ...

. From this

nerves A nerve is an enclosed, cable-like bundle of nerve fibers (called axons) in the peripheral nervous system. A nerve transmits electrical impulses. It is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the e ...

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 potentials. This process is called sensory transduction. The cel ...

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 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 ...

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 taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with ...

host in

faeces Feces ( or faeces), known colloquially and in slang as poo and poop, 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 large intestine. Feces contain a relati ...

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

) 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 (Annelida , from Latin ', "little ring"), also known as the segmented worms, are a large phylum, with over 22,000 extant species including ragworms, earthworms, and leeches. The species exist in and have adapted to various ecol ...

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 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 ...

, 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 Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...

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 signifi ...

, 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 bubble) ...

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

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...

, 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 Strigeida Poche, 1926. Families Order Diplostomida *Suborder Diplostomata **Superfamily Brachylaimoidea Joyeux & Foley, 1930 ***Brachylaimidae Joyeux & Foley, ...

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

Brachylaimidae Brachylaimidae is a family of parasitic flukes in the sub-class Digenea. Adults are usually found within the digestive tracts and other organs of mammals or birds and have a complex three-stage life cycle.Olson, P. D.; Cribb, T. H.; Tkach, V. V. ...

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 Z ...

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 Strigeida Poche, 1926. Families Order Diplostomida *Suborder Diplostomata **Superfamily ...

Mühling, 1898 ****

Diplostomidae Diplostomidae is a family of trematodes in the order Diplostomida. Genera *'' Austrodiplostomum'' Szidat & Nani, 1951Szidat, L. & Nani, A. (1951). Diplostomiasis cerebralis del Pejerrey. Una grave epizootia que afecta a la economia nacional prod ...

Poirier, 1886 ****

Proterodiplostomidae Proterodiplostomidae is a monotypic family of trematodes in the order Diplostomida. It consists of one subfamily, Polycotylinae Monticelli, 1888,Monticelli, F. S. (1888). ''Saggio di una morfologia del trematodi. Tesi per ottenere la privata doce ...

Dubois, 1936 ****

Strigeidae Strigeidae is a family of trematodes in the order Diplostomida. Genera *''Apatemon'' Szidat, 1928Szidat, L. (1928). Zur Revision der Trematodengattung ''Strigea'' Abildgaard. ''Centralblatt für Bakteriologie, Parasitenkunde und Infektionskrankh ...

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

Aporocotylidae Aporocotylidae is a family of trematodes within the order Diplostomida, which contains species commonly known as fish blood flukes. It contains more than 40 genera, the largest being '' Cardicola''. Species in this family parasite fish in both ...

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 genus ...

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 Stunka ...

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, organ ...

**Apocreadiata ***Apocreadioidea Skrjabin, 1942 **** Apocreadiidae Skrjabin, 1942 **Bivesiculata ***Bivesiculoidea ****

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 f ...

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

Botulisaccidae Botulisaccidae 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 ...

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 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, 1929 ****

Cyclocoelidae Cyclocoelidae 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 signifi ...

Stossich, 1902 ****

Echinochasmidae Echinochasmidae 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 signif ...

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 few sig ...

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 signific ...

Odhner, 1910 ****

Philophthalmidae Philophthalmidae is a family of trematodes 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 reproductio ...

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 fe ...

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 ...

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 f ...

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 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 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 Lepidapedidae 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 ...

Yamaguti, 1958 **** Lepocreadiidae Odhner, 1905 **** Liliatrematidae Gubanov, 1953 **Monorchiata ***Monorchioidea Odhner, 1911 ****

Lissorchiidae Lissorchiidae is a family of flatworms 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 si ...

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 cyrcumoral spines. Pharynx presented. Genital synus presented. Ventral and genital ...

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 f ...

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 Notocotylidae 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 signifi ...

Lühe, 1909 **** Nudacotylidae Barker, 1916 **** Opisthotrematidae Poche, 1926 **** Pronocephalidae Looss, 1899 ****

Rhabdiopoeidae Rhabdiopoeidae 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 ...

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 Ozaki, 1925 ***Gorgoderoidea Looss, 1901 **** Callodistomidae Odhner, 1910 ****

Dicrocoeliidae Dicrocoeliidae is a family of flatworms belonging to the order Plagiorchiida. Genera Genera: * '' Allocorrigia'' Turner & Corkum, 1977 * '' Athesmia'' Looss, 1899 * '' Brachydistomum '' , 1944 * '' Brachylecithum'' Shtrom, 1940 * '' Brodenia'' Ge ...

Looss, 1899 ****

Gorgoderidae Gorgoderidae is a Family (biology), family of Trematoda, trematodes in the Order (biology), order Plagiorchiida. Genera Family Gorgoderidae *Subfamily Anporrhutinae Looss, 1901 **''Anaporrhutum'' Brandes in Ofenheim, 1900 **''Bicornuata'' Pearse ...

Looss, 1899 ***Haploporoidea Nicoll, 1914 ****

Atractotrematidae Atractotrematidae 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 ...

Yamaguti, 1939 **** Haploporidae Nicoll, 1914 ***Microphalloidea Ward, 1901 **** Diplangidae Yamaguti, 1971 ****

Exotidendriidae Exotidendriidae 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 ...

Mehra, 1935 **** Faustulidae Poche, 1926 ****

Microphallidae ''Microphallidae'' 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 ...

Ward, 1901 **** Pachypsolidae Yamaguti, 1958 **** Phaneropsolidae 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 even ...

Lühe, 1909 **** Renicolidae Dollfus, 1939 ****

Zoogonidae Zoogonidae are a family of trematodes. They are the namesake family of the suborder Zoogonata in the order Plagiorchiida. Selected genera are: * ''Brevicreadium'' * ''Deretrema'' Linton, 1910 * ''Diphtherostomum'' Stossich, 1904 * ''Glauciv ...

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 relatively few ...

Jue Sue & Platt, 1998 **** Echinoporidae Krasnolobova & Timofeeva, 1965 **** Encyclometridae Mehra, 1931 ****

Leptophallidae Leptophallidae 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 ...

Dayal, 1938 **** Macroderoididae McMullen, 1937 **** Meristocotylidae Fischthal & Kuntz, 1981 ****

Ocadiatrematidae Ocadiatrematidae 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 ...

Fischthal & Kuntz, 1981 ****

Orientocreadiidae Orientocreadiidae is a family of trematodes belonging to the order Plagiorchiida. Genera: * ''Macrotrema ''Macrotrema caligans'' is a species In biology, a species is the basic unit of Taxonomy (biology), classification and a taxonomic r ...

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 re ...

Lühe, 1901 **** Styphlotrematidae Baer, 1924 ****

Telorchiidae Telorchiidae 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 ...

Looss, 1899 **** Thrinascotrematidae Jue Sue & Platt, 1999 **** Urotrematidae Poche, 1926

Human digenean infections

Only about 12 of the 6,000 known species are

infectious An 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 dis ...

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 The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, tha ...

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 nom ...

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 Org ...

''.

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 ...

. 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