Editing Amphibian (section)

== Anatomy and physiology ==

=== Skin ===
[[File:Tree frog congo.jpg|thumb|left|upright|alt=Common reed frog|The [[Aposematism|bright colours]] of the [[common reed frog]] (''Hyperolius viridiflavus'') are typical of a toxic species]]
The [[Wikt:integument|integumentary]] structure contains some typical characteristics common to terrestrial vertebrates, such as the presence of highly [[Keratin#Cornification|cornified]] outer layers, renewed periodically through a moulting process controlled by the [[pituitary gland|pituitary]] and [[thyroid]] glands. Local thickenings (often called warts) are common, such as those found on toads. The outside of the skin is shed periodically mostly in one piece, in contrast to mammals and birds where it is shed in flakes. Amphibians often eat the sloughed skin.<ref name="Collins" /> Caecilians are unique among amphibians in having mineralized dermal scales embedded in the [[dermis]] between the furrows in the skin. The similarity of these to the scales of bony fish is largely superficial. [[Squamata|Lizards]] and some frogs have somewhat similar [[osteoderm]]s forming bony deposits in the dermis, but this is an example of [[convergent evolution]] with similar structures having arisen independently in diverse vertebrate lineages.<ref>{{cite journal |author1=Zylberberg, Louise |author2=Wake, Marvalee H. |year=1990 |title=Structure of the scales of ''Dermophis'' and ''Microcaecilia'' (Amphibia: Gymnophiona), and a comparison to dermal ossifications of other vertebrates |journal=Journal of Morphology |volume=206 |issue=1 |pages=25–43 |doi=10.1002/jmor.1052060104 |pmid=29865751 |s2cid=46929507 |url=https://www.researchgate.net/publication/227806326 |access-date=November 10, 2016 |archive-date=December 16, 2017 |archive-url=https://web.archive.org/web/20171216161753/https://www.researchgate.net/publication/227806326 |url-status=live }}</ref>
[[File:FrogSkin.png|thumb|upright=1.35|Cross section of frog skin:
{{image key
|list type=ordered
|Mucous gland
|Chromatophore
|Granular poison gland
|[[Connective tissue]]
|[[Stratum corneum]]
|Transition zone
|[[Epidermis]]
|[[Dermis]]}}]]
Amphibian skin is permeable to water. Gas exchange can take place through the skin ([[cutaneous respiration]]) and this allows adult amphibians to respire without rising to the surface of water and to hibernate at the bottom of ponds.<ref name="Collins" /> To compensate for their thin and delicate skin, amphibians have evolved mucous glands, principally on their heads, backs and tails. The secretions produced by these help keep the skin moist. In addition, most species of amphibian have granular glands that secrete distasteful or poisonous substances. Some amphibian toxins can be lethal to humans while others have little effect.<ref name="amphibfacts">{{cite encyclopedia |url=http://www.eoearth.org/article/Amphibian_morphology_and_reproduction |title=Amphibian morphology and reproduction |date=October 12, 2008 |author1=Biodiversity Institute of Ontario |author2=Hebert, Paul D. N. |encyclopedia=Encyclopedia of Earth |access-date=August 15, 2012 |archive-date=February 1, 2008 |archive-url=https://web.archive.org/web/20080201074532/http://www.eoearth.org/article/Amphibian_morphology_and_reproduction |url-status=live }}</ref> The main poison-producing glands, the [[Parotoid gland|parotoids]], produce the neurotoxin [[bufotoxin]] and are located behind the ears of toads, along the backs of frogs, behind the eyes of salamanders and on the upper surface of caecilians.{{sfn | Stebbins | Cohen | 1995 | pp=10–11 }}

The skin colour of amphibians is produced by three layers of pigment cells called [[chromatophore]]s. These three cell layers consist of the melanophores (occupying the deepest layer), the guanophores (forming an intermediate layer and containing many granules, producing a blue-green colour) and the lipophores (yellow, the most superficial layer). The colour change displayed by many species is initiated by [[hormone]]s secreted by the pituitary gland. Unlike bony fish, there is no direct control of the pigment cells by the nervous system, and this results in the colour change taking place more slowly than happens in fish. A vividly coloured skin usually indicates that the species is toxic and is a warning sign to predators.<ref>{{cite book |title=The Integument: A Textbook of Skin Biology |last=Spearman |first=R. I. C. |year=1973 |publisher=Cambridge University Press |isbn=978-0-521-20048-6 |page=[https://archive.org/details/integumenttextbo00spea/page/81 81] |url=https://archive.org/details/integumenttextbo00spea |url-access=registration |quote=Amphibian skin colour. }}</ref>

=== Skeletal system and locomotion ===
[[File:Xenotosuchus skull roof.svg|thumb|Skull diagram of ''[[Xenotosuchus]],'' a temnospondyl]]
Amphibians have a skeletal system that is structurally [[Homology (biology)|homologous]] to other tetrapods, though with a number of variations. They all have four limbs except for the legless caecilians and a few species of salamander with reduced or no limbs. The bones are hollow and lightweight. The musculoskeletal system is strong to enable it to support the head and body. The bones are fully [[ossification|ossified]] and the vertebrae interlock with each other by means of overlapping processes. The [[pectoral girdle]] is supported by muscle, and the well-developed [[pelvic girdle]] is attached to the backbone by a pair of sacral ribs. The [[ilium (bone)|ilium]] slopes forward and the body is held closer to the ground than is the case in mammals.{{sfn | Dorit | Walker | Barnes | 1991 | p=846 }}

[[File:Ceratophrys cornuta skeleton front.jpg|thumb|alt=Frog skeleton|Skeleton of the [[Surinam horned frog]]<br/>(''Ceratophrys cornuta'')]]
In most amphibians, there are four digits on the fore foot and five on the hind foot, but no claws on either. Some salamanders have fewer digits and the [[amphiuma]]s are eel-like in appearance with tiny, stubby legs. The [[Siren (genus)|sirens]] are aquatic salamanders with stumpy forelimbs and no hind limbs. The caecilians are limbless. They burrow in the manner of earthworms with zones of muscle contractions moving along the body. On the surface of the ground or in water they move by undulating their body from side to side.{{sfn | Stebbins | Cohen | 1995 | pp=26–36 }}

In frogs, the hind legs are larger than the fore legs, especially so in those species that principally move by jumping or swimming. In the walkers and runners the hind limbs are not so large, and the burrowers mostly have short limbs and broad bodies. The feet have adaptations for the way of life, with webbing between the toes for swimming, broad adhesive toe pads for climbing, and keratinised tubercles on the hind feet for digging (frogs usually dig backwards into the soil). In most salamanders, the limbs are short and more or less the same length and project at right angles from the body. Locomotion on land is by walking and the tail often swings from side to side or is used as a prop, particularly when climbing. In their normal gait, only one leg is advanced at a time in the manner adopted by their ancestors, the lobe-finned fish.{{sfn | Dorit | Walker | Barnes | 1991 | p=846 }} Some salamanders in the genus ''[[Aneides]]'' and certain [[lungless salamander|plethodontids]] climb trees and have long limbs, large toepads and prehensile tails.<ref name="BritCaudata" /> In aquatic salamanders and in frog tadpoles, the tail has [[dorsal fin|dorsal]] and [[ventral fin|ventral]] fins and is moved from side to side as a means of propulsion. Adult frogs do not have tails and caecilians have only very short ones.{{sfn | Stebbins | Cohen | 1995 | pp=26–36 }}
[[File:Didactic model of an amphibian heart-FMVZ USP-14.jpg|thumb|left|upright=0.65|[[Educational toy|Didactic model]] of an amphibian heart.]]
Salamanders use their tails in defence and some are prepared to jettison them to save their lives in a process known as [[autotomy]]. Certain species in the Plethodontidae have a weak zone at the base of the tail and use this strategy readily. The tail often continues to twitch after separation which may distract the attacker and allow the salamander to escape. Both tails and limbs can be regenerated.<ref name="Beneski">{{cite journal | author=Beneski, John T. Jr. | date=September 1989 | title=Adaptive significance of tail autotomy in the salamander, ''Ensatina'' | journal=Journal of Herpetology | volume=23 | issue=3 | pages=322–324 | doi=10.2307/1564465 | jstor=1564465 }}</ref> Adult frogs are unable to regrow limbs but tadpoles can do so.{{sfn | Stebbins | Cohen | 1995 | pp=26–36 }}

=== Circulatory system ===
[[File:Juvenile Amphibian Circulatory System.svg|thumb|Juvenile amphibian circulatory systems are single loop systems which resemble fish. <!--
-->{{olist
    |Internal gills where the blood is reoxygenated
    |Point where the blood is depleted of oxygen and returns to the heart via veins
    |Two chambered heart
}} Red indicates oxygenated blood, and blue represents oxygen depleted blood.]]
Amphibians have a juvenile stage and an adult stage, and the circulatory systems of the two are distinct. In the juvenile (or tadpole) stage, the circulation is similar to that of a fish; the two-chambered heart pumps the blood through the gills where it is oxygenated, and is spread around the body and back to the heart in a single loop. In the adult stage, amphibians (especially frogs) lose their gills and develop lungs. They have a heart that consists of a single ventricle and two atria. When the ventricle starts contracting, deoxygenated blood is pumped through the [[pulmonary artery]] to the lungs. Continued contraction then pumps oxygenated blood around the rest of the body. Mixing of the two bloodstreams is minimized by the anatomy of the chambers.{{sfn | Dorit | Walker | Barnes | 1991 | p=306 }}

=== Nervous and sensory systems ===
{{see also|Pain in amphibians}}
The [[nervous system]] is basically the same as in other vertebrates, with a central brain, a spinal cord, and nerves throughout the body. The amphibian brain is relatively simple but broadly the same structurally as in reptiles, birds and mammals. Their brains are elongated, except in caecilians, and contain the usual motor and sensory areas of tetrapods.<ref>{{cite book|author1=González, A.|author2=López, J. M.|author3=Morona, R.|author4=Morona, N.|year=2020|chapter=The Organization of the Central Nervous System of Amphibians|title=Evolutionary Neuroscience|editor=Hass, J. H.|page=127|publisher=Elsevier Science |isbn= 978-0-12-820584-6}}</ref> The [[pineal body]], known to regulate sleep patterns in humans, is thought to produce the hormones involved in [[hibernation]] and [[aestivation]] in amphibians.{{sfn | Stebbins | Cohen | 1995 | p=100 }}

Tadpoles retain the lateral line system of their ancestral fishes, but this is lost in terrestrial adult amphibians. Many aquatic salamanders and some caecilians possess [[Electroreception|electroreceptors]] called ampullary organs (completely absent in anurans), that allow them to locate objects around them when submerged in water.<ref>[https://books.google.com/books?id=PpkvEAAAQBAJ&dq=salamanders+caecilians+ampullary+organs&pg=PA73 A Natural History of Amphibians]</ref> The ears are well developed in frogs. There is no external ear, but the large circular [[Tympanum (anatomy)|eardrum]] lies on the surface of the head just behind the eye. This vibrates and sound is transmitted through a single bone, the [[stapes]], to the inner ear. Only high-frequency sounds like mating calls are heard in this way, but low-frequency noises can be detected through another mechanism.{{sfn | Dorit | Walker | Barnes | 1991 | p=846 }} There is a patch of specialized haircells, called ''papilla amphibiorum'', in the inner ear capable of detecting deeper sounds. Another feature, unique to frogs and salamanders, is the columella-operculum complex adjoining the auditory capsule which is involved in the transmission of both airborne and seismic signals.{{sfn | Stebbins | Cohen | 1995 | p=69 }} The ears of salamanders and caecilians are less highly developed than those of frogs as they do not normally communicate with each other through the medium of sound.<ref name="EBAmphibia" />

The eyes of tadpoles lack lids, but at metamorphosis, the [[cornea]] becomes more dome-shaped, the [[Lens (anatomy)|lens]] becomes flatter, and [[eyelid]]s and associated glands and ducts develop.{{sfn | Dorit | Walker | Barnes | 1991 | p=846 }} The adult eyes are an improvement on invertebrate eyes and were a first step in the development of more advanced vertebrate eyes. They allow colour vision and depth of focus. In the retinas are green rods, which are receptive to a wide range of wavelengths.<ref name="EBAmphibia">{{cite encyclopedia |url=https://www.britannica.com/EBchecked/topic/21445/amphibian/ |title=Amphibian |author1=Duellman, William E. |author2=Zug, George R. |year=2012 |encyclopedia=Encyclopædia Britannica |access-date=March 27, 2012 |archive-date=March 8, 2012 |archive-url=https://web.archive.org/web/20120308183351/https://www.britannica.com/EBchecked/topic/21445/amphibian |url-status=live }}</ref>

=== Digestive and excretory systems ===
[[File:Frog anatomy tags.PNG|thumb|alt=Dissected frog|Dissected frog:
{{image key
|list type=ordered
|Right atrium
|Liver
|Aorta
|Egg mass
|Colon
|Left atrium
|Ventricle
|Stomach
|Left lung
|Spleen
|Small intestine
|Cloaca
}}]]
Many amphibians catch their prey by flicking out an elongated tongue with a sticky tip and drawing it back into the mouth before seizing the item with their jaws. Some use inertial feeding to help them swallow the prey, repeatedly thrusting their head forward sharply causing the food to move backwards in their mouth by [[inertia]]. Most amphibians swallow their prey whole without much chewing so they possess voluminous stomachs. The short [[Esophagus|oesophagus]] is lined with [[Cilium|cilia]] that help to move the food to the stomach and [[mucus]] produced by glands in the mouth and [[pharynx]] eases its passage. The enzyme [[chitinase]] produced in the stomach helps digest the [[chitin]]ous cuticle of arthropod prey.{{sfn | Dorit | Walker | Barnes | 1991 | p=847 }}

Amphibians possess a [[pancreas]], [[liver]] and [[gall bladder]]. The liver is usually large with two lobes. Its size is determined by its function as a [[glycogen]] and fat storage unit, and may change with the seasons as these reserves are built or used up. [[Adipose tissue]] is another important means of storing energy and this occurs in the abdomen (in internal structures called fat bodies), under the skin and, in some salamanders, in the tail.{{sfn | Stebbins | Cohen | 1995 | p=66 }}
<!--In aquatic amphibians, the liver plays only a small role in processing nitrogen for excretion, and [[ammonia]] is diffused mainly through the skin. The liver of terrestrial amphibians converts ammonia to urea, a less toxic, water-soluble nitrogenous compound, as a means of water conservation. In some species, urea is further converted into [[uric acid]]. [[Bile]] secretions from the liver collect in the gall bladder and flow into the small intestine. In the small intestine, enzymes digest carbohydrates, fats, and proteins. Salamanders lack a valve separating the small intestine from the large intestine. Salt and water absorption occur in the large intestine, as well as mucous secretion to aid in the transport of faecal matter, which is passed out through the [[cloaca]].<ref name="Anatomy" />---Omitting this until a more reliable source can be found.--->

There are two [[kidney]]s located dorsally, near the roof of the body cavity. Their job is to filter the blood of metabolic waste and transport the urine via ureters to the urinary bladder where it is stored before being passed out periodically through the cloacal vent. Larvae and most aquatic adult amphibians excrete the nitrogen as ammonia in large quantities of dilute urine, while terrestrial species, with a greater need to conserve water, excrete the less toxic product urea. Some tree frogs with limited access to water excrete most of their metabolic waste as uric acid.{{sfn | Dorit | Walker | Barnes | 1991 | p=849 }}
==== Urinary bladder ====
Most aquatic and semi-aquatic amphibians have a membranous skin which allows them to absorb water directly through it. Some semi-aquatic animals also have similarly permeable bladder membrane.<ref>{{Cite journal |last1=Urakabe |first1=Shigeharu |last2=Shirai |first2=Dairoku |last3=Yuasa |first3=Shigekazu |last4=Kimura |first4=Genjiro |last5=Orita |first5=Yoshimasa |last6=Abe |first6=Hiroshi |year=1976 |title=Comparative study of the effects of different diuretics on the permeability properties of the toad bladder |journal=Comparative Biochemistry and Physiology Part C: Comparative Pharmacology |language=en |volume=53 |issue=2 |pages=115–119 |doi=10.1016/0306-4492(76)90063-0 |pmid=5237}}</ref> As a result, they tend to have high rates of urine production to offset this high water intake, and have urine which is low in dissolved salts. The urinary bladder assists such animals to retain salts. Some aquatic amphibian such as ''[[Xenopus]]'' do not reabsorb water, to prevent excessive water influx.<ref>{{Cite journal |last1=Shibata |first1=Yuki |last2=Katayama |first2=Izumi |last3=Nakakura |first3=Takashi |last4=Ogushi |first4=Yuji |last5=Okada |first5=Reiko |last6=Tanaka |first6=Shigeyasu |last7=Suzuki |first7=Masakazu |year=2015 |title=Molecular and cellular characterization of urinary bladder-type aquaporin in Xenopus laevis |journal=General and Comparative Endocrinology |volume=222 |pages=11–19 |doi=10.1016/j.ygcen.2014.09.001 |pmid=25220852}}</ref> For land-dwelling amphibians, dehydration results in reduced urine output.<ref name="VittCaldwell2013">{{cite book |author1=Laurie J. Vitt |url=https://books.google.com/books?id=Gay9N_ry79kC&pg=PA184 |title=Herpetology: An Introductory Biology of Amphibians and Reptiles |author2=Janalee P. Caldwell |date=25 March 2013 |publisher=Academic |isbn=978-0-12-386920-3 |page=184}}</ref>

The amphibian bladder is usually highly distensible and among some land-dwelling species of frogs and salamanders may account for between 20% and 50% of their total body weight.<ref name="VittCaldwell2013" /> Urine flows from the kidneys through the ureters into the bladder and is periodically released from the bladder to the cloaca.<ref>{{Cite book |last1=Feder |first1=Martin E. |url=https://books.google.com/books?id=oaS-OpEjPtUC&pg=PA108 |title=Environmental Physiology of the Amphibians |last2=Burggren |first2=Warren W. |date=1992-10-15 |publisher=University of Chicago Press |isbn=978-0-226-23944-6 |language=en}}</ref>

=== Respiratory system ===
[[File:Axolotl ganz.jpg|thumb|left|alt=Axolotl|The [[axolotl]] (''Ambystoma mexicanum'') retains its larval form with gills into adulthood]]
The lungs in amphibians are primitive compared to those of amniotes, possessing few internal [[alveolar septum|septa]] and large [[Pulmonary alveolus|alveoli]], and consequently having a comparatively slow diffusion rate for oxygen entering the blood. Ventilation is accomplished by [[buccal pumping]].<ref>{{cite journal|last1=Brainerd|first1=E. L. |title=New perspectives on the evolution of lung ventilation mechanisms in vertebrates |journal=Experimental Biology Online |year=1999 |volume=4 |issue=2 |pages=1–28 |doi=10.1007/s00898-999-0002-1 |bibcode=1999EvBO....4b...1B |s2cid=35368264 }}</ref> Most amphibians, however, are able to exchange gases with the water or air via their skin. To enable sufficient [[cutaneous respiration]], the surface of their highly vascularised skin must remain moist to allow the oxygen to diffuse at a sufficiently high rate.{{sfn | Dorit | Walker | Barnes | 1991 | p=847 }} Because oxygen concentration in the water increases at both low temperatures and high flow rates, aquatic amphibians in these situations can rely primarily on cutaneous respiration, as in the [[Telmatobius culeus|Titicaca water frog]] and the [[Cryptobranchus|hellbender salamander]]. In air, where oxygen is more concentrated, some small species can rely solely on cutaneous gas exchange, most famously the [[Plethodontidae|plethodontid salamanders]], which have neither lungs nor gills. Many aquatic salamanders and all tadpoles have gills in their larval stage, with some (such as the [[axolotl]]) retaining gills as aquatic adults.{{sfn | Dorit | Walker | Barnes | 1991 | p=847 }}