Editing Gill (section)

====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/>