Amphibians are members of the class Amphibia. The living ones are frogs (including toads), salamanders (including newts) and caecilians. They are four-legged vertebrates which are cold blooded.

Amphibians lay their eggs in water, usually in a foam nest. After hatching they are tadpoles, which live in the water and have gills. The tadpoles change into adults in a process called metamorphosis.  When they are adult, they have lungs to breathe instead of gills, and legs. Adult amphibians also use their skin to take in oxygen, and some species of salamanders do not have lungs.

The earliest amphibians evolved in the Devonian from lobe-finned fish which had jointed leg-like fins with digits. They could crawl along the sea bottom. Some had developed primitive lungs to help them breathe air when the stagnant pools of the Devonian swamps were low in oxygen. They could also use their strong fins to hoist themselves out of the water and onto dry land if necessary.[1][2]

For tens of millions of years, during the Carboniferous and early Permian, amphibia were top predators on land, especially in the low-lying tropical river systems.  In drier conditions, they were less effective, and the ancestors of mammals and reptiles (the Synapsids and Sauropsids) gradually took over the land. They laid cleidoic eggs, which had hard shells, and could be laid out of water. Most of the early large amphibians went extinct in the Triassic period; a few survived to the Lower Cretaceous.[2]

The only living amphibiana today are the Lissamphibia. These include the Anura (frogs and toads), Caudata (salamanders and newts) and Gymnophiona (caecilians). They are all rather small, compared with mammals or reptiles. The smallest frog and vertebrate in the world is the New Guinea frog (Paedophryne amauensis). The biggest amphibian is the Chinese Giant Salamander (Andrias davidianus).

Amphibia are found everywhere in the world, except Antarctica, and there are about 5,565 different species: 88% of them are in the Anura.[3] In number of species, they are more successful than mammals, though they occupy a smaller range of habitats. However, it is said that amphibian populations have been declining all over the world.[4] Conservation is therefore an important concern.

Amphibians like to live near freshwater in warm weather. There have also been species which live in forests, deserts and arctic conditions. Adult amphibians use lungs, and they also get oxygen through their skin, so long as it is moist.[5]

Amphibians may be camouflaged in brown and green, and if so they are prey for birds and reptiles. Their colour gives them camouflage, which is their main defence.

Alternatively, many other amphibia have toxic skin, which is harmful to predators. These are poisonous to eat. This is an important defence against predation. Connected to this is the use of warning colouration. They may be in vivid colours of red, black, and yellow. Research into the rough-skinned newt and the garter snake shows this is a typical case of co-evolution. Where they live in the same area, the newts get more poisonous, and the snakes develop more resistance to the poison.[6][7][8]

Amphibians have colour vision and depth of focus for clear sight. They also have eyelids, glands and ducts which keep the eyes moist. These are adaptations to life on land: amphibia were the first vertebrates to have these features.

Some amphibians, such as the common coquí, lay eggs out of water (in this case, on palm leaves). The eggs develop directly into adult frogs, by-passing the tadpole stage. Others, like mudpuppies and olms, have a different development. In a process called neoteny, they become sexually developed as tadpoles, and continue to live in the water with gills.

The order Anura includes the frogs and toads. There is no fundamental difference between frogs and toads. Frogs have a short body, webbed digits (fingers or toes), protruding eyes, forked tongue and no tail. They are exceptional jumpers: many of their features, particularly their long, powerful legs, are adaptations to improve jumping performance. They often live in semi-aquatic or inhabit humid areas.[9]

A popular distinction is often made between frogs and toads on the basis of their appearance. Toads' warty skin is an adaptation for making their toxic slime. Apart from these glands, their skin is dry, and that is an adaptation to drier habitats. These features have evolved a number of times independently: convergent evolution. The distinction has no taxonomic basis. The only family exclusively given the common name "toad" is Bufonidae (the "true toads"), but many species from other families are commonly called "toads".[9]

The order Gymnophiona contains caecilians.  They look like worms or snakes, but they are amphibians.  Scientists say it is difficult to study them because most of them live underground.  Their skins make a mucus that makes them slippery enough to slip through the dirt.  They can make a toxic material in their skins, like many frogs and toads do.[10][11]

The order Caudata is the salamanders.

Newts are salamanders which spend their life in the water even though they are adults. They are classified in the subfamily Pleurodelinae of the family Salamandridae.

Respiration differs between species of salamanders. Species that lack lungs respire through gills. In most cases, these are external gills, visible as tufts on either side of the head. Some salamanders that are terrestrial have lungs that are used in respiration, although these are simple and sac-like, unlike the more complex organs found in mammals. Many species, such as the Olm, have both lungs and gills as adults.[12]

Some terrestrial species lack both lungs and gills and perform gas exchange through their skin. Even some species with lungs also respire through the skin in this manner.

The skin of salamanders secretes mucus. This helps to keep the animal moist when on dry land, keeps their salt balance while in water, and lubricates during swimming. Salamanders also secrete poison from glands in their skin, and some additionally have skin glands for secreting courtship pheromones.[12]

Axolotls, from  the genus Ambystoma (or mole salamanders),  are neotenic amphibians. This means they get to sexual maturity and reproduce while still in a larval form.

Most salamanders and newts have some defence against predators, usually a poison which makes them uneatable. Their bright colours are warning colouration. If, instead, they are camouflaged, this means they are probably not protected by a toxin.

The second line of defence is to shed their tail, which can grow again. The tail wriggles a bit, attracts the predator while the business part of the salamander moves off.

There are over 350 lungless salamanders. Most of them are terrestrial and are active in daytime. Lungless salamanders may communicate with their nose.[13]p168 Slender salamanders are found in the Pacific Coast. They are sometimes called "worm salamanders". This is because they have slimmer (skinny) bodies than most salamanders.[13]p182 If touched, slender salamanders will bounce on the ground and then run away.

The order Gymnophiona includes the caecilians. These are long, cylindrical, limbless animals that look like snakes or worms. Their skin has circular folds, increasing their similarity to the segments of earthworms. Some are aquatic but most live underground in burrows they hollow out. Many caecilians give birth to live young, and in the animals that do not do this, the eggs may undergo metamorphosis before they hatch. Caecilians are found in tropical Africa, Asia and Central and South America. There are 171 different species.

They are burrowing amphibians. This means that they dig themselves in wet soil like worms. Their heads are strong and have bones that help them dig.[14]p7 Because caecilians have a lot of vertebrae, they can bend easily.

Amphibians are the only vertebrates to go through metamorphosis. This means that their young look different from their adult.[15]p8 Amphibians usually reproduce in early spring to late summer, though some reproduce in winter and fall.[13]p156 Most frogs and toads, such as the common frog (Rana temporalis), gather in large groups to ponds, rivers, swamps and lakes to breed.[15]p10 Male frogs and toads may croak to attract a female. When a female frog has chosen a mate, the male frog hops on top of her. They swim together as she lay eggs in the water.[14]p7 Sometimes, males fight to mate with a female.[14]p7 Frogs can lay up to 100 to 60,000 eggs in one clutch. This is called "frogspawn".

It is a fundamental feature of amphibia that their reproduction is, one way or another, tied to water. This is because their eggs, although covered by jelly, cannot survive long in dry conditions.

Most female amphibians lays her eggs in water. Males release sperm to fertilize them. The eggs are laid one by one or in batches. Batches of eggs can look like a long chain or a ball of foam. They may wrap their eggs around plants in the water. They do this so their eggs will not drift away.[15]p8

Tree frogs usually lay their eggs on a leaf in a rainwater pool. Bullfrogs, such as the male American bullfrog and the male African bullfrog, stay with their tadpoles and protect them from predators. They also move their tadpoles by using their nose to dig a channel to another place where there is more water.[15]p9 They do this so their tadpoles do not dry up. Most amphibians leave their eggs to look after themselves. Fish and other animals eat most of their eggs. Male midwife toads carry their eggs on their backs. When they are ready to hatch, the toad goes back to the water and release them.[14]p10

Tadpoles do not have lungs when they hatch and instead have gills. Because gills have a large surface area, tadpoles can get more oxygen by using them. Young tadpoles have their gills exposed. When they get older, their gills are covered over by skin.[15]p6  When they hatch, tadpoles eat constantly. The tadpoles eat what is left of their eggs, this is usually their first food.[15]p8

Frog, toad and newt tadpoles eat plants such as algae and pondweed or filter feed. When they get older, they may start to feed on tiny animals in the water. Salamander tadpoles and surinam horned toad tadpoles are carnivorous throughout their tadpole stage.[15]p9 Surinam horned toad tadpoles are very aggressive. They eat other tadpoles if food is nowhere to be found. The eggs of the spadefoot toad hatches in three days. Their tadpoles complete their metamorphosis in six to eight days. This is because spadefoot toads lay their eggs in places where water will dry up soon.[15]p13

Tadpoles of frogs and toads start to grow their back legs first. They then grow front legs a few weeks later. When tadpoles grow their limbs they are called "froglets". This is because they look rather like a smaller version of adult frogs and toads. Tadpoles will also start to grow a backbone after growing their front limbs. After this, their mouths get bigger and their eyes will stick out more. After a tadpole has grown its hands, their tails continue to get shorter until there is nothing left of them.[15]p11

Salamanders and newts can be found living in streams. Salamanders can be found in rotten logs, holes or underground places that are wet such as under leaves.[13]p152 Web-toed salamanders live in habitats where there are a lot of rocks. They like to hide under rocks and stones.[13]p195 The tailed frogs, like to live in cold water habitats.[13]p199 In their habitat, amphibians like to live where there are a lot of places to hide. These include nearby small trees, logs and plants. While underwater they like to hide near aquatic plants and rocks. Tree and dart frogs like to live in forests on trees, plants and on the ground under leaves.

Some amphibia can be found living in the desert or the arctic.[15]p12 The desert froglet lives in the desert. They are only active at night, when temperatures are much cooler. It rarely rains in the desert and because of this, desert frogs will burrow to keep cool. They use their mucus to keep them wet. They will spread it all over their bodies. The mucus will harden to keep the water it produces from escaping. Once the desert frog has done this, it will stay in its cocoon and will not move. They will stay like this for several months to years until a rainstorm. Desert frogs and toads lose water more quickly. The spadefoot toad will spit on the ground. Once they have done this, they will lay on it. Their bodies will take in the water. Their bodies are thin and have a lot of blood vessels, this helps them to be able to take water through their skin. The California newt can survive a fire by spreading its mucous over its body.[15]p12

Arctic frogs such as the wood frog, moor frog and the common frog has to live with freezing temperatures for a long time. They will burrow in places where they can get into a cocoon. Like every living organisms, amphibians must have water to survive. Amphibians however, need freshwater. Some frogs such as burrowing frogs can keep water in their bladders. This allows them to stay underground without drying up. The crab-eating frog lives near water that is somewhat salty. They will eat saltwater crabs. Torrent salamanders lives in cold waters. Because of this they have shorter lungs. Short lungs helps them to float easily.

Amphibia are world-wide, though restricted in distribution by their need for moist or watery habitats to reproduce.

Many amphibia have secretions in their skin which makes them toxic. They do not produce toxins themselves.[16] They get toxins from what they eat. They eat insects in their habitat. These insects get the poison from a plant. The toxin has been discovered in beetles.[16] This means that they are likely the cause of poisons found in amphibians. Amphibians do not produce batrachotoxin in captivity, which means that it is not harmful to touch them. The American Indian tribe comechingóns used the toxins of the arrow dart frogs when hunting.[16]

Newts in the genus Taricha has a poison called tetrodotoxin, a neurotoxin. Scientists believe that toxins in newts are caused by bacteria in the genera Pseudoalteromonas, Pseudomonas and Vibrio. Because of this, the newts do not have a lot of predators. However, some species of snakes have develop a resistance. This means that they can eat newts without the toxin hurting them. It is a case of co-evolution.

Amphibians' eyes have lids, glands and ducts. They have good colour vision[17]  Caecilian eyes are small and dark. Most of them are blind. Most amphibians have a good sense of smell, even underwater.

The skeletal system of amphibians are similar to other four-legged animals. They have a spine, rib cage, long bones such as the humerus and femur. They also have short bones such as the phalanges, metacarpals, and metatarsals. Most amphibians have four limbs, except for caecilians. The bones in amphibians are hollow and do not weigh much.[18]

Amphibians are predatory animals. They mostly eat live invertebrates and animals that do not move too quickly. These include caterpillars, earthworms, crayfish, water beetles, snails and dragon fly larvae.[19]p667 Many amphibians use their sticky tongues to catch their prey. They will swallow the animal whole, but may chew it just a bit for it to go down their throats. The Ranidae family and the Ceratophrys genus will eat almost anything they can fit into their mouths.[19]p668 These include rodents, birds, ducklings, small fish and small mammals.[20] Most frogs are cannibalistic, and will eat each other if food is no where to be found. Some amphibians will even eat their own tadpoles and eggs if there is no food for them.[21]

In captivity, pet frogs will be given crickets, worms, small fish, rodents and fruit flies.[22] Adult amphibians can help decrease the mosquito population by eating most of their larvae.[23]

Caecilians eat earthworms, termites and beetle larvae, and also small lizards.[24]p31 Caecilians rely on their smell to find food. They like to eat earthworms and will find them by picking up their chemical signals. Salamanders and newts are fed a lot of different types of worms. These include blood worms and earthworms. They can eat small fish such as goldfish, fathead minnows and guppies. Salamanders also eat crickets and pinkies, which are baby rats.[19]p771

The amphibian population have been decreasing from all locations in the world.[4] Scientists have said that the declining of amphibians is one of the most critical threats to global biodiversity.[4] A number of causes are believed to be involved. These include habitat destruction, over-exploitation, pollution, introduced species, climate change, destruction of the ozone layer, and diseases like chytridiomycosis. Ultraviolet radiation damages the skin, eyes and eggs of amphibians. However, the declines of amphibian population are still not understood.[25]

The Amphibian Conservation Action Plan (ACAP) have released a global strategy to help the amphibian population. It was developed by over 80 leading experts.[26] The Amphibian Specialist Group of the World Conservation Union (IUCN) are working on another global strategy to help amphibian populations.[26] The Amphibian Ark (AA) is an organization that was created to help the public be aware of the decline in amphibian populations. They have been working with zoos and aquaria around the world. They try to encourage them to create a natural habitat for threatened amphibians.[26] Another project is the Panama Amphibian Rescue and Conservation Project (PARCP) which are trying to spread awareness about chyridiomycosis. The disease is spreading into eastern Panama and threatening all amphibians living there.[27]

On January 21, 2008, Evolutionarily Distinct and Globally Endangered (EDGE) released a statement to the public.[28] It was created by Helen Meredith, who identified nature's most endangered species. Meredith explains that 85% of the top 100 endangered amphibians list are receiving little to no conservation attention.

Bullfrog legs are a source of food for Southern United States and the Midwestern United States.[29] People hunt bullfrogs at night near rivers. The bullfrogs' legs are cooked, while their backs are fried.[30]p9 In China, bullfrogs are sold alive for eating. However, they are later cooked dead with vegetables. In the state of California, people must have a license to catch bullfrogs for food.[31]p256 In schools, bullfrogs are dissected in biology classes. Usually, this is done in grammar school.[32]p85 The dissecting is a method for teaching students the anatomy of a bullfrog.[32]p85 The emperor newt is hunted in China for food. They are also used there for medicine. Burrowing frogs are able to hold water in their bladder, because of this indigenous Australians use them to drink water.[15]p13

Amphibians are also kept as pets.[33]p4 They are kept in aquariums or a terrarium. A terrarium is a tank that is decorated with plants and soil on one side. On the other side, there is water. Most amphibians would need one place for land and another for water.[33]p8 Each type of amphibian should have its special needs taken care of. Semi-aquatic amphibians need both land and water divided in the tank. Tropical frogs would need mist and high humidity in their terrariums.[34]p7 Water for amphibia needs dechlorination. The chlorine in tap water can kill amphibia. Some amphibians popular exotic pets, and are found in pet stores that sell reptiles.[33]p22