Editing Kidney (section)

==Other animals==
{{Main|Kidney (vertebrates)}}
In the majority of vertebrates, the [[mesonephros]] persists into the adult, albeit usually fused with the more advanced [[metanephros]]; only in [[amniote]]s is the mesonephros restricted to the embryo. The kidneys of [[fish]] and [[amphibian]]s are typically narrow, elongated organs, occupying a significant portion of the trunk. The collecting ducts from each cluster of nephrons usually drain into an ''archinephric duct'', which is [[homology (biology)|homologous]] with the [[vas deferens]] of amniotes. However, the situation is not always so simple; in [[cartilaginous fish]] and some amphibians, there is also a shorter duct, similar to the amniote ureter, which drains the posterior (metanephric) parts of the kidney, and joins with the archinephric duct at the [[bladder]] or [[cloaca]]. Indeed, in many cartilaginous fish, the anterior portion of the kidney may degenerate or cease to function altogether in the adult.<ref name=VB/>

In the most primitive vertebrates, the [[hagfish]] and [[lamprey]]s, the kidney is unusually simple: it consists of a row of nephrons, each emptying directly into the archinephric duct. Invertebrates may possess excretory organs that are sometimes referred to as "kidneys", but, even in ''[[Amphioxus]]'', these are never homologous with the kidneys of vertebrates, and are more accurately referred to by other names, such as [[nephridia]].<ref name=VB/> In [[amphibian]]s, kidneys and the [[urinary bladder]] harbour specialized [[parasite]]s, [[monogenea]]ns of the family Polystomatidae.<ref name=Theunissen>{{cite journal | vauthors = Theunissen M, Tiedt L, Du Preez LH | title = The morphology and attachment of Protopolystoma xenopodis (Monogenea: Polystomatidae) infecting the African clawed frog Xenopus laevis | journal = Parasite | volume = 21 | pages = 20 | year = 2014 | pmid = 24823278 | pmc = 4018937 | doi = 10.1051/parasite/2014020 }}</ref>

The kidneys of [[reptile]]s consist of a number of lobules arranged in a broadly linear pattern. Each lobule contains a single branch of the ureter in its centre, into which the collecting ducts empty. Reptiles have relatively few nephrons compared with other amniotes of a similar size, possibly because of their lower [[metabolic rate]].<ref name=VB>{{cite book | vauthors = Romer AS, Parsons TS |year=1977 |title=The Vertebrate Body |publisher=Holt-Saunders International |location= Philadelphia, PA|pages= 367–376|isbn= 978-0-03-910284-5}}</ref>

[[Bird]]s have relatively large, elongated kidneys, each of which is divided into three or more distinct lobes. The lobes consists of several small, irregularly arranged, lobules, each centred on a branch of the ureter. Birds have small glomeruli, but about twice as many nephrons as similarly sized mammals.<ref name=VB/>

The human kidney is fairly typical of that of [[mammal]]s. Distinctive features of the mammalian kidney, in comparison with that of other vertebrates, include the presence of the renal pelvis and renal pyramids and a clearly distinguishable cortex and medulla. The latter feature is due to the presence of elongated [[Loop of Henle|loops of Henle]]; these are much shorter in birds, and not truly present in other vertebrates (although the nephron often has a short ''intermediate segment'' between the convoluted tubules). It is only in mammals that the kidney takes on its classical "kidney" shape, although there are some exceptions, such as the multilobed [[reniculate kidney]]s of [[pinniped]]s and [[cetacea]]ns.<ref name=VB/>

===Evolutionary adaptation===
Kidneys of various animals show evidence of evolutionary [[adaptation]] and have long been studied in [[ecophysiology]] and [[comparative physiology]]. Kidney morphology, often indexed as the relative medullary thickness, is associated with habitat [[aridity]] among species of mammals<ref name='Al-kahtani2004'>{{cite journal | vauthors = al-Kahtani MA, Zuleta C, Caviedes-Vidal E, Garland T | title = Kidney mass and relative medullary thickness of rodents in relation to habitat, body size, and phylogeny | journal = Physiological and Biochemical Zoology | volume = 77 | issue = 3 | pages = 346–365 | year = 2004 | pmid = 15286910 | doi = 10.1086/420941 | url = http://www.biology.ucr.edu/people/faculty/Garland/Al-kahtaniEA2004.pdf | access-date = 2009-03-28 | url-status = dead | s2cid = 12420368 | citeseerx = 10.1.1.407.8690 | archive-url = https://web.archive.org/web/20100617221052/http://www.biology.ucr.edu/people/faculty/Garland/Al-kahtaniEA2004.pdf | archive-date = 2010-06-17 }}</ref> and diet (e.g., carnivores have only long loops of Henle).<ref name=SRT/>