Editing Hagfish (section)

===Respiration===
A hagfish generally respires by taking in water through its [[pharynx]], past the velar chamber, and bringing the water through the internal [[gill]] pouches, which can vary in number from five to 16 pairs, depending on species.<ref>{{cite book|author1=Springer, Joseph|author2=Holley, Dennis|title=An Introduction to Zoology|url=https://books.google.com/books?id=BzgNZca_L5AC&pg=PA376|date=2012|publisher=Jones & Bartlett Publishers|isbn=978-1-4496-9544-6|pages=376–|access-date=2016-03-13|archive-date=2024-08-29|archive-url=https://web.archive.org/web/20240829210757/https://books.google.com/books?id=BzgNZca_L5AC&pg=PA376|url-status=live}}</ref> The gill pouches open individually, but in'' Myxine'', the openings have coalesced, with canals running backwards from each opening under the skin, uniting to form a common aperture on the [[ventral]] side known as the branchial opening. The [[esophagus]] is also connected to the left branchial opening, which is therefore larger than the right one, through a pharyngocutaneous duct (esophageocutaneous duct), which has no respiratory tissue. This pharyngocutaneous duct is used to clear large particles from the pharynx, a function also partly taking place through the nasopharyngeal canal. In other species, the coalescence of the gill openings is less complete, and in ''Bdellostoma'', each pouch opens separately to the outside, as in lampreys.<ref>{{cite book|last=Hughes |first=George Morgan |title=Comparative Physiology of Vertebrate Respiration|url=https://archive.org/details/ost-biology-comparativephysi00hugh|date=1963|publisher=Harvard University Press|isbn=978-0-674-15250-2|pages=[https://archive.org/details/ost-biology-comparativephysi00hugh/page/n26 9]–}}</ref><ref>{{cite book|author=Wake, Marvalee H.|title=Hyman's Comparative Vertebrate Anatomy|url=https://books.google.com/books?id=VKlWjdOkiMwC&pg=PA81|date=1992|publisher=University of Chicago Press|isbn=978-0-226-87013-7|pages=81–|access-date=2016-03-13|archive-date=2024-08-29|archive-url=https://web.archive.org/web/20240829210815/https://books.google.com/books?id=VKlWjdOkiMwC&pg=PA81#v=onepage&q&f=false|url-status=live}}</ref> The unidirectional water flow passing the gills is produced by rolling and unrolling velar folds located inside a chamber developed from the nasohypophyseal tract, and is operated by a complex set of muscles inserting into cartilages of the neurocranium, assisted by peristaltic contractions of the gill pouches and their ducts.<ref>{{cite book|author1=Bone, Quentin|author2=Moore, Richard|title=Biology of Fishes|url=https://books.google.com/books?id=e2N4AgAAQBAJ&pg=PA128|date=2008|publisher=Taylor & Francis|isbn=978-1-134-18631-0|pages=128–|access-date=2016-03-13|archive-date=2024-08-29|archive-url=https://web.archive.org/web/20240829210751/https://books.google.com/books?id=e2N4AgAAQBAJ&pg=PA128#v=onepage&q&f=false|url-status=live}}</ref> Hagfish also have a well-developed dermal capillary network that supplies the skin with oxygen when the animal is buried in anoxic mud, as well as a high tolerance for both hypoxia and anoxia, with a well-developed anaerobic metabolism.<ref name="Jørgensen">{{cite book|author=Jørgensen, Jørgen Mørup |title=The Biology of Hagfishes|url=https://books.google.com/books?id=vu1uzL0p7xsC&pg=PA231|year=1998|publisher=Springer Science & Business Media|isbn=978-0-412-78530-6|pages=231–}}</ref> Members of the group have spent 36 hours in water completely devoid of dissolved oxygen, and made a complete recovery.<ref>[https://www.sciencedirect.com/science/article/abs/pii/S1095643321001501 Hypoxia modifies calcium handling in the Pacific hagfish, Eptatretus stoutii]</ref> The skin has also been suggested to be capable of [[cutaneous respiration]].<ref>{{cite book|author1=Helfman, Gene|author2=Collette, Bruce B.|author3=Facey, Douglas E.|author4=Bowen, Brian W.|title=The Diversity of Fishes: Biology, Evolution, and Ecology|url=https://books.google.com/books?id=FyehAR6hsUUC&pg=PA235|date=2009|publisher=John Wiley & Sons|isbn=978-1-4443-1190-7|pages=235–|access-date=2016-03-13|archive-date=2024-08-29|archive-url=https://web.archive.org/web/20240829210753/https://books.google.com/books?id=FyehAR6hsUUC&pg=PA235#v=onepage&q&f=false|url-status=live}}</ref>