Editing Gill (section)

===Fish===
{{main|Fish gill}}
The gills of fish form a number of slits connecting the pharynx to the outside of the animal  on either side of the fish behind the head. Originally there were many slits, but during evolution, the number reduced, and modern fish mostly have five pairs, and never more than eight.<ref name=Hughes>{{cite book|author=Hughes, George Morgan|title=Comparative Physiology of Vertebrate Respiration |url=https://archive.org/details/ost-biology-comparativephysi00hugh |year=1963 |publisher=Harvard University Press |isbn=978-0-674-15250-2 |pages=[https://archive.org/details/ost-biology-comparativephysi00hugh/page/n25 8]–9}}</ref>

====Cartilaginous fish====
[[Shark]]s and [[ray (fish)|rays]] typically have five pairs of gill slits that open directly to the outside of the body, though some more primitive sharks have six pairs with the [[Broadnose sevengill shark]] being the only cartilaginous fish exceeding this number. Adjacent slits are separated by a [[cartilage|cartilaginous]] gill arch from which projects a cartilaginous '''gill ray'''. This gill ray is the support for the sheet-like interbranchial [[septum]], which the individual lamellae of the gills lie on either side of. The base of the arch may also support [[gill raker]]s, projections into the pharyngeal cavity that help to prevent large pieces of debris from damaging the delicate gills.<ref name=VB>{{cite book |author=Romer, Alfred Sherwood |author-link=Alfred Romer |author2=Parsons, Thomas S.|year=1977|title=The Vertebrate Body |publisher=Holt-Saunders International|location= Philadelphia, PA|pages= 316–327|isbn= 0-03-910284-X}}</ref>

A smaller opening, the [[Spiracle (vertebrates)|spiracle]], lies in the back of the first [[gill slit]]. This bears a small '''pseudobranch''' that resembles a gill in structure, but only receives blood already oxygenated by the true gills.<ref name=VB/> The spiracle is thought to be [[Homology (biology)|homologous]] to the ear opening in [[higher vertebrates]].<ref>Laurin M. (1998): The importance of global parsimony and historical bias in understanding tetrapod evolution. Part I-systematics, middle ear evolution, and jaw suspension. ''Annales des Sciences Naturelles, Zoologie, Paris'', 13e Série 19: pp 1–42.</ref>

Most sharks rely on ram ventilation, forcing water into the mouth and over the gills by rapidly swimming forward. In slow-moving or bottom-dwelling species, especially among skates and rays, the spiracle may be enlarged, and the fish breathes by sucking water through this opening, instead of through the mouth.<ref name=VB/>

[[Chimaera]]s differ from other cartilagenous fish, having lost both the spiracle and the fifth gill slit. The remaining slits are covered by an [[operculum (fish)|operculum]], developed from the septum of the gill arch in front of the first gill.<ref name=VB/>

====Bony fish====
[[File:Tuna Gills in Situ cut.jpg|thumb|175px|left|The red gills inside a detached [[tuna]] head (viewed from behind)]]
In [[Osteichthyes|bony fish]], the gills lie in a branchial chamber covered by a bony operculum. The great majority of bony fish species have five pairs of gills, although a few have lost some over the course of evolution. The operculum can be important in adjusting the pressure of water inside of the pharynx to allow proper ventilation of the gills, so bony fish do not have to rely on ram ventilation (and hence near constant motion) to breathe. Valves inside the mouth keep the water from escaping.<ref name=VB/>

The gill arches of bony fish typically have no septum, so the gills alone project from the arch, supported by individual gill rays. Some species retain gill rakers. Though all but the most primitive bony fish lack spiracles, the pseudobranch associated with them often remains, being located at the base of the operculum. This is, however, often greatly reduced, consisting of a small mass of cells without any remaining gill-like structure.<ref name=VB/>

Marine [[teleost]]s also use their gills to excrete osmolytes (e.g. Na⁺, Cl<sup>−</sup>). The gills' large surface area tends to create a problem for fish that seek to regulate the [[osmolarity]] of their internal fluids. Seawater contains more osmolytes than the fish's internal fluids, so marine fishes naturally lose water through their gills via osmosis. To regain the water, marine fishes drink large amounts of [[sea water]] while simultaneously expending energy to excrete [[salt]] through the [[Na+/K+-ATPase|Na<sup>+</sup>/K<sup>+</sup>-ATPase]] ionocytes (formerly known as mitochondrion-rich cells and [[chloride cell]]s).<ref name=":0">{{Cite journal|last1=Evans|first1=David H.|last2=Piermarini|first2=Peter M.|last3=Choe|first3=Keith P.|date=January 2005|title=The Multifunctional Fish Gill: Dominant Site of Gas Exchange, Osmoregulation, Acid-Base Regulation, and Excretion of Nitrogenous Waste|url=https://www.physiology.org/doi/10.1152/physrev.00050.2003|journal=Physiological Reviews|language=en|volume=85|issue=1|pages=97–177|doi=10.1152/physrev.00050.2003|pmid=15618479|issn=0031-9333}}</ref> Conversely, fresh water contains less osmolytes than the fish's internal fluids. Therefore, freshwater fishes must utilize their gill ionocytes to attain ions from their environment to maintain optimal blood osmolarity.<ref name=VB/><ref name=":0" />

[[Lamprey]]s and [[hagfish]] do not have gill slits as such. Instead, the gills are contained in spherical pouches, with a circular opening to the outside. Like the gill slits of higher fish, each pouch contains two gills. In some cases, the openings may be fused together, effectively forming an operculum. Lampreys have seven pairs of pouches, while hagfishes may have six to fourteen, depending on the species. In the hagfish, the pouches connect with the pharynx internally and a separate tube which has no respiratory tissue (the pharyngocutaneous duct) develops beneath the pharynx proper, expelling ingested debris by closing a valve at its anterior end.<ref name=VB/> [[Lungfish]] larvae also have [[external gills]], as does the primitive [[Actinopterygii|ray-finned fish]] ''[[Polypterus]]'', though the latter has a structure different from amphibians.<ref name=VB/>

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