Editing Salamander (section)

===Respiration===
[[Respiration (physiology)|Respiration]] differs among the different species of salamanders, and can involve gills, lungs, skin, and the membranes of mouth and throat. Larval salamanders breathe primarily by means of [[gill]]s, which are usually external and feathery in appearance. Water is drawn in through the mouth and flows out through the gill slits. Some [[neotenic]] species such as the [[mudpuppy]] (''Necturus maculosus'') retain their gills throughout their lives, but most species lose them at [[metamorphosis]]. The [[embryo]]s of some terrestrial lungless salamanders, such as ''Ensatina'', that undergo direct development, have large gills that lie close to the egg's surface.<ref name=stebbins25>Stebbins & Cohen (1995) pp. 17–25</ref>

When present in adult salamanders, lungs vary greatly among different species in size and structure. In aquatic, cold-water species like the [[torrent salamander]]s (''Rhyacotriton''), the lungs are very small with smooth walls, while species living in warm water with little dissolved oxygen, such as the [[lesser siren]] (''Siren intermedia''), have large lungs with convoluted surfaces. In the lungless salamanders ([[Family (biology)|family]] [[Plethodontidae]] and the [[clawed salamander]]s in the family of [[Asiatic salamander]]s), no lungs or gills are present, and [[gas exchange]] mostly takes place through the skin, known as [[cutaneous respiration]], supplemented by the tissues lining the mouth. To facilitate this, these salamanders have a dense network of blood vessels just under the skin and in the mouth.<ref name="stebbins25" /><ref>Cogger & Zweifel (1998), pp. 74–75.</ref><ref>{{Cite journal |title=Patterns of Natural Selection on Mitochondrial Protein-Coding Genes in Lungless Salamanders: Relaxed Purifying Selection and Presence of Positively Selected Codon Sites in the Family Plethodontidae |date=2021 |doi=10.1155/2021/6671300 |doi-access=free |last1=Kakehashi |first1=Ryosuke |last2=Kurabayashi |first2=Atsushi |journal=International Journal of Genomics |volume=2021 |pages=1–12 |pmid=33928143 |pmc=8053045 }}</ref>

In the [[amphiuma]]s, metamorphosis is incomplete, and they retain one pair of [[gill slit]]s as adults, with fully functioning internal lungs.<ref>{{Cite journal|last=Toews|first=Daniel P.|date=1974|title=Respiratory Mechanisms in the Aquatic Salamander, Amphiuma tridactylum|journal=Copeia|volume=1974|issue=4|pages=917–920|doi=10.2307/1442591|jstor=1442591}}</ref> Some species that lack lungs respire through gills. In most cases, these are external gills, visible as tufts on either side of the head. Some terrestrial salamanders have lungs used in respiration, although these are simple and sac-like, unlike the more complex organs found in [[mammal]]s. Many species, such as the [[olm]], have both lungs and gills as adults.<ref name="EoR pp.60–68"/>

[[File:Levatores arcuum of Necturus maculosus.png|thumb|upright|A dissected view of the levatores arcuum muscles in a ''Necturus maculosus'' specimen. These (shown in the purple circles) move the external gills, as a means of respiration.]]

In the ''Necturus'', external gills begin to form as a means of combating hypoxia in the egg as egg yolk is converted into metabolically active tissue.<ref>{{Cite journal|last1=Rogge|first1=Jessica R.|last2=Warkentin|first2=Karen M.|date=2008-11-15|title=External gills and adaptive embryo behavior facilitate synchronous development and hatching plasticity under respiratory constraint|journal=Journal of Experimental Biology|volume=211|issue=22|pages=3627–3635|doi=10.1242/jeb.020958|issn=0022-0949|pmid=18978228|doi-access=free}}</ref> Molecular changes in the mudpuppy during post-embryonic development primarily due to the [[thyroid gland]] prevent the internalization of the external gills as seen in most salamanders that undergo metamorphosis.<ref name=":12">{{Cite book|title=Biology of Amphibians|last=Duellman|first=William Edward|publisher=The Johns Hopkins University Press |year=1994 }}</ref> The external gills seen in salamanders differs greatly from that of amphibians with internalized gills. Unlike amphibians with internalized gills which typically rely on the changing of pressures within the buccal and pharyngeal cavities to ensure diffusion of oxygen onto the gill curtain, neotenic salamanders such as Necturus use specified musculature, such as the levatores arcuum, to move external gills to keep the respiratory surfaces constantly in contact with new oxygenated water.<ref>{{Cite book|title=Vertebrates : comparative anatomy, function, evolution|last=V.|first=Kardong, Kenneth|date=2012|publisher=McGraw-Hill|isbn=9780073524238|oclc=939087630}}</ref><ref>{{Cite book|title=Comparative vertebrate anatomy a laboratory dissection guide|author=Zalisko, Edward J. |date=2015|publisher=McGraw-Hill Education|isbn=9780077657055|oclc=935173274}}</ref>