Editing Digenea (section)

==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 [[Platyhelminthes|platyhelminths]] comprising the Digenea, [[Aspidogastrea]], [[Monogenea]] and [[Cestoda]].  Digeneans possess a [[vermiform]], unsegmented body-plan and have a solid [[parenchyma]] with no [[body cavity]] ([[coelom]]) as in all platyhelminths.

[[File:Peerj-292-fig-7 Zoogonidae.png|thumb|Anterior sucker of ''[[Overstreetia cribbi]]'' a [[Zoogonidae|zoogonid]] digenean<ref>{{Cite journal  | last1 = Bray | first1 = RA. | last2 = Justine | first2 = J-L. | title = A review of the Zoogonidae (Digenea: Microphalloidea) from fishes of the waters around New Caledonia, with the description of ''Overstreetia cribbi'' n. sp. | journal = PeerJ | volume = 2 | pages = e292 | year = 2014 | doi = 10.7717/peerj.292 | pmid = 24688868|pmc = 3961169 | doi-access = free }}</ref>]]
There are typically two [[Sucker (zoology)|sucker]]s, an [[anterior]] [[Wiktionary:oral|oral]] sucker surrounding the [[mouth]], and a [[ventral]] sucker sometimes termed the [[acetabulum (morphology)|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). [[Trematoda|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]]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]] is internal, with [[sperm]] being transferred via the [[cirrus (biology)|cirrus]] to the [[Laurer's Canal]] or [[genital|genital aperture]]. A key group of digeneans which are [[dioecious]] are the [[schistosoma|schistosomes]].
Asexual reproduction in the first larval stage is ubiquitous.

While the sexual formation of the digenean [[egg (biology)|eggs]] and asexual reproduction in the first [[larva]]l stage (miracidium) is widely reported, the [[developmental biology]] of the asexual stages remains a problem. [[Electron microscope|Electron microscopic]] studies have shown that the [[microscope|light microscopically]] visible germ balls consist of [[mitosis|mitotically]] dividing [[cell (biology)|cell]]s which give rise to [[embryo]]s and to a line of new [[germ cells]] that become included in these embryonic stages. Since the absence of [[meiosis|meiotic processes]] is not proven, the exact definition remains doubtful.

====Male organs====
[[Hermaphrodite|Protandry]] 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]], a [[vas deferens]], [[seminal vesicle]], [[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]] with an [[oviduct]], a [[semen|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 [[caecum|caeca]] may fuse with the body wall posteriorly to make one or more [[anus]]es, 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 [[organ (anatomy)|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]], host blood).  The blood flukes, such as schistosomes, spirorchiids and sanguinicolids, feed exclusively on blood. Asexual stages in [[mollusc]] intermediate hosts feed mostly by direct absorption, although the [[Trematode lifecycle stages|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]]l stages and free-living [[cercaria]]l stages do not feed.

===Nervous system===
Paired [[ganglia]] at the anterior end of the body serve as the [[brain]]. From this [[nerves]] extend anteriorly and posteriorly. [[Sense|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.