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The [[brain]] consists of a pair of [[ganglion|ganglia]] in the head region, from which two or three pairs of [[Ventral nerve cord|nerve cord]]s run down the length of the body. The nerve cords running along the ventral surface are always the largest, while the dorsal cords are present only in the [[Aspidogastrea]]. Trematodes generally lack any specialized [[sense organs]], although some [[ectoparasite|ectoparasitic]] [[species]] do possess one or two pairs of simple [[ocellus|ocelli]].<ref name=IZ/>
===Reproductive system===
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The ootype is connected to an elongated uterus that opens to the exterior in the genital pore, close to the male opening. Most commonly found in trematodes, the sperm cells travel through the uterus to reach the ootype, where fertilization occurs. The ovary is sometimes also associated with a storage sac for sperm, and a copulatory duct termed ''Laurer's canal''.<ref name=IZ/> Laurer’s canal is described to be the primitive vagina but it is a vestigial organ in some species. In some instances it can serve as a tube outside the worm for potential waste products from the egg-formation system.<ref name="Horák 266–266"/>
▲'''Body wall musculature:''' Formed of three different muscle layers: circular, longitudinal, and diagonal. The outermost layer is formed by the circular muscle fibers, directly behind that are the longitudinal muscle fibers. The inner layer is formed by the diagonal muscle fibers. Together these muscle fibers form the segmented body wall of trematodes.<ref name=":0">{{Cite journal |last=D’ávila |first=Sthefane |date=18 March 2010 |title=Gross anatomy of the musculature and a new description of the reproductive system of Tanaisia bragai and Tanaisia inopina (Trematoda: Eucotylidae) analysed by confocal laser scanning microscopy |url=https://onlinelibrary.wiley.com/doi/full/10.1111/j.1463-6395.2008.00393.x |journal= Acta Zoologica|volume=91| issue = 2|pages=139–149 |doi=10.1111/j.1463-6395.2008.00393.x }}</ref>
▲'''Oral sucker and acetabulum:''' In some species of Trematoda, such as T. bragai, there is an [[Acetabulum (morphology)|acetabulum]]. This saucer-shaped organ is attached to the oral sucker in some Trematodes and other parasitic worms. This allows for parasitic worms to attach to their host by penetrating the host’s tissue with spines lining the acetabulum organ. In trematodes, the oral sucker is linked to the [[pharynx]] via a canal composed of meridional, equatorial, and radial muscle fibers.<ref name=":0" /> Together, the mouth, pharynx, and [[esophagus]] form the foregut in Trematodes.<ref name=":2">{{Cite book |last1=Bogitsh |first1=Burton J. |url=https://books.google.com/books?id=UexdDwAAQBAJ&dq=Human+parasitology++Burton+Bogitsh+2005&pg=PP1 |title=Human Parasitology |last2=Carter |first2=Clint E. |last3=Oeltmann |first3=Thomas N. |date=2018-05-28 |publisher=Academic Press |isbn=978-0-12-813713-0 |language=en}}</ref>
▲'''Male reproductive system:''' The male reproductive system generally includes two [[Testicle|testes]], while some species may have more. Along with this, the size and position of the testes in the body may vary depending on the species. Due to this species-level variation, the male reproductive system can be very useful in species identification. Sperm production, or spermatogenesis, occurs in the testes. [[Spermatogenesis]] in Trematodes produces biflagellate sperm or sperm with two tails. This phenomenon occurs across very few invertebrate and vertebrate species. Sperm is stored in the seminal vesicle, this organ is connected to the testes by the [[vas deferens]].<ref name=":2" />
==Life cycles==
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