Branchial arches orgill arches are a series of pairedbony/cartilaginous "loops" behind thethroat (pharyngeal cavity) offish, which support thefish gills. Aschordates, allvertebrateembryos developpharyngeal arches, though the eventual fate of these arches varies betweentaxa. In alljawed fish (gnathostomes), the first arch pair (mandibular arches) develops into thejaw, the second gill arches (the hyoid arches) develop into thehyomandibular complex (which supports the back of the jaw and the front of the gill series), and the remaining posterior arches (simply called branchial arches) support the gills. Intetrapods, a mostlyterrestrialclade evolved fromlobe-finned fish, many pharyngeal arch elements are lost, including the gill arches. Inamphibians andreptiles, only the oral jaws and ahyoid apparatus remains, and inmammals andbirds the hyoid is simplified further to support thetongue andfloor of the mouth. In mammals, the first and second pharyngeal arches also give rise to theauditory ossicles.

Most vertebrates areaquatic and breathe withgills, wherewater comes in contact forexchangingdissolved oxygen before flowing out through a series of openings (gill slits) to the outside. Each gill is supported by a cartilaginous or bony gill arch,^[1] which helps to maintain the gill'ssurface area.Bony fish (osteichthyans, mostlyteleostray-finned fish) have four pairs of arches,cartilaginous fish (chondrichthyans) have five to seven pairs, and the morebasaljawless fish ("agnathans") have up to seven. TheCambrian ancestors of vertebrates no doubt had more gill arches, as some of theirchordate relatives have more than 50 pairs of gills.^[2]

In amphibians and someprimitive bony fish, thelarvae bearexternal gills branching out from the gill arches.^[3] These regress uponadulthood, their function taken over by the gillsproper in fish, or bylungs (which arehomologous toswim bladders) andcutaneous respiration in most amphibians. Someneotenic amphibians (such as theaxolotl) retain the external larval gills in adulthood, the complex internal gill system as seen in fish apparently being irrevocably lost very early in the evolution oftetrapods.^[4]

The branchial system is typically used for respiration and/or feeding. Many fish have modified posterior gill arches intopharyngeal jaws, often equipped with specializedpharyngeal teeth for handling particular prey items (long, sharp teeth in carnivorous moray eels compared to broad, crushing teeth in durophagous black carp). In amphibians and reptiles, the hyoid arch is modified for similar reasons. It is often used inbuccal pumping and often plays a role in tongue protrusion for prey capture. In species with highly specializedballistic tongue movements such aschameleons or someplethodontidsalamanders, the hyoid system is highly modified for this purpose, while it is often hypertrophied in species which usesuction feeding. Species such as snakes and monitor lizards, whose tongue has evolved into a purely sensory organ, often have very reduced hyoid systems.

The primitive arrangement is 7 (possibly 8) arches, each consisting of the same series of paired (left and right) elements. order from dorsal-most (highest) to ventral-most (lowest), these elements are the pharyngobranchial, epibranchial, ceratobranchial, hypobranchial, and basibranchial. The pharyngobranchials may articulate with theneurocranium, while the left and right basibranchials connect to each other (often fusing into a single bone). When part of the hyoid arch, the names of the bones are altered by replacing "-branchial" with "-hyal", thus "ceratobranchial" becomes "ceratohyal".^[5]

• TheBasihyals andBasibranchials lie at the midline of the lower edge of the throat. Almost all modernchondrichthyans have a single midline basihyal, as do manyteleosts,lungfish, andtetrapodomorphs. Intetrapods, the basihyal is modified into a structure known as thehyoid bone, which provides muscle attachment for thetongue,pharynx, andlarynx. Basibranchials, which are most common inosteichthyans, have the form of one or more rod-like bones projecting backwards along the midline of the throat.
• TheCeratohyals andCeratobranchials lie above their respective basi- components, slanting backwards and upwards. They are often the largest bony components of the gill system, as well as the most essential and abundant components. Small connecting bones known asHypophyals orHypobranchials may link the basi- and cerato- components, and hypobranchials in particular are common among all types of fish. Paired hypophyals are characteristic of living osteichthyans. Living chondrichthyans lack hypohyals, though several extinct forms are known to have had them.
• TheEpihyals andEpibranchials lie above their respective cerato- components, slanting forwards, upwards, and often inwards. Along with the ceratohyals and ceratobranchials, they are also essential components of the gill system, found in every fish. In filter-feeding fish, the epibranchials often hostgill rakers, specialized spines projecting backwards to trap plankton. The epihyal is more commonly known as thehyomandibula, which is homologous to the sound-sensitivestapes (sometimes known as thecolumnella) of tetrapods.
• ThePharhyngobranchials are the most dorsal bony elements of the gill system, connecting to the upper extent of the epibranchials. Living chondrichthyans have large pharyngobranchials which lean backwards and upwards. Osteichthyans, on the other hand, have two different types of pharyngobranchials:Suprapharyngobranchials are toothless structures similar to those of chondrichthyans, whileInfrapharyngobranchials often possess teeth and lean inwards and forwards, forming the roof of the throat. A hyoid equivalent of the pharyngobranchial, thePharyngohyal, is only found in livingholocephalans, also known aschimaeras.

Amniotes do not havegills. The gill arches form aspharyngeal arches duringembryogenesis, and lay the basis of essential structures such asjaws, thethyroid gland, thelarynx, thecolumella (corresponding to thestapes inmammals) and in mammals, themalleus and incus.^[2] Studies onplacoderms also show that theshoulder girdle also originated from gill arches.^[6]

• The Gill Arches: Overview,Palaeos. Retrieved 30 November 2014.

• Fish anatomy

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