Frogs and toads are members of the order Anura, sometimes called Salientia, which is divided into approximately 30 families and more than 4,500 species. Anurans are a diverse group of amphibians, ranging in size from a few millimeters to a couple feet in length, and are found in nearly every niche on Earth. Externally, anurans differ from caudates and caecilians by the presence of four limbs, of which the hind limbs are typically larger, and modified for leaping or climbing. Anurans are also unique in that they are capable of vocalizing, and produce an array of sounds from squeaks to barking noises. Unlike the majority of salamanders and caecilians, most anurans are external fertilizers. 

Members of the order Caudata, often called Urodela, include salamanders, newts, waterdogs, mudpuppies, sirens, and amphiuma. There are 10 recognized salamander families, with a just more than 500 species. Caudates are known as the "tailed amphibians" because all species possess a tail. Typical caudates have four limbs of similar size that are used for terrestrial walking. Notable exceptions to this rule are the Amphiuma, which possess reduced limbs, and the Sirens, which lack hind limbs and possess reduced forelimbs. In general, caudates are incapable of vocalization, with the exception of Dicamptodon ensatus, who can make squeaking noises when disturbed. Although found mostly in temperate zones of the world, caudates have evolved some amazing anatomical characteristics, including the elongate bodies of amphiuma and sirens, the mostly aquatic lifestyles of some newts and salamanders, the arboreal habits of the neotropical species, obligate and facultative neoteny, and more. Caudates also vary in size a great deal, and include the largest amphibian in the world, Andrias davidanius, which can reach nearly 6 feet long. 

Caecilians, order Gymnophiona, or Apoda, are perhaps the least studied amphibians, and definitely the most alien form. Caecilians are long, segmented, almost worm-like amphibians that live underground and in aquatic habitats. Most species are found in tropical and subtropical environments in South America, Asia, and Africa. Caecilians lack appendages, and have powerful heads and highly ossified sculls for burrowing, highly acute olfactory systems, and are the only amphibians to possess dermal scales.

Amphibians first colonized land in the mid Devonian, some 350 million years ago. These ancient creatures were typically larger, possessing massive teeth, and some with scaled skin like modern reptiles. Modern amphibians are much smaller in statute, and have evolved a remarkable number of specializations that suit their habitat. Because amphibians are so diverse in nature, there are few defining characteristics that pertain to all species. In general, amphibians are regarded as requiring both land and water for survival. This is true for many species, however, others may be fully terrestrial, even being born on land (viviparity). Others may only need a moist habitat, not necessarily a pond or pool, while others may be completely aquatic. These variations are the results of a wide scope of evolution, however, all amphibians evolved from a common ancestor, and share a general life cycle, despite discrepancies among specific species or genera. 

Although there are several exceptions, most amphibians are biphasic, meaning that they go through an aquatic stage, and a terrestrial stage at some point in life. Most species produce shell-less (amamniotic) eggs that are deposited in the water. Amamniotic eggs rely on moisture from their surrounding water source, as opposed to amniotic eggs, which are enclosed in their own "internal pond" of amniotic fluids. This enables amniotic eggs to be deposited on land, without the risk of desiccation. Amamniotic eggs are enclosed in a protective, gelatinous envelope, as is the case with biphasic amphibians. Tiny larvae  hatch from these translucent eggs, and are nourished for a short period by a yolk sac. Salamander larvae possess bushy, external gills that are usually lost upon metamorphosis, except in the case of neotenic species or populations. Amphiuma are the only caudates that possess internal gills. Frogs and toad larvae, referred to as tadpoles, possess internal gills, making the two types of larvae easily distinguishable. Many species begin growing limbs and other adult features very quickly, whereas some larvae are forced to remain aquatic for several months, even years, until climatic conditions are suitable for metamorphosis. Neoteny, sometimes called paedomorphism, is common in many caudate species. Neoteny can be briefly described as the retention of larval characteristics in sexually mature animals.

As aquatic larvae, biphasic species begin to metamorphose into terrestrial juveniles or adults at varying times, depending on the species. The transition from aquatic larvae to terrestrial juvenile entails many physical changes, including the absorption of the gills (salamanders) and tail (frogs), modified eyesight to see on land, thicker skin to cope with atmospheric environments, stronger limbs to support the weight of the body, and a more cylindrical tail. New morphs must also learn to catch food on land, which may prove more difficult than in the water. A typical biphasic species will return to the water a few years later, when sexual maturity has been reached, where it will begin the lifecycle all over again with its own offspring. Some caudate species  will remain in the water at this point, a common characteristic of newts. Others may only return to the water for the breeding season, and resume a hidden, terrestrial life until the next season. Many caudates that spend time in the water during the breeding season go through a second, lesser metamorphosis when returning to the water, where physical modifications manifest in adaptation to an aquatic lifestyle, including modified vision to see underwater, thinner skin to absorb more oxygen, a laterally compressed tail for propulsion, etc. Semi-aquatic species, particularly newts, are capable of going back and forth between terrestrial and aquatic physiology at will, or when environmental conditions create the necessity to do so. There are also viviparous, and external fertilizing salamanders. 

Anurans typically remain terrestrial as adults, only depositing their eggs in water, with the exception of the fully aquatic species. Anurans have also developed several ways of depositing their eggs and larvae into water sources, including carrying them on their backs, and foam nests placed on leaves above water sources, where the larvae drop out of their egg cases directly into the water. Some species may burry their eggs close to a water source, where the larvae will scurry upon hatching. There are also viviparous, marsupial, and mouth brooding anurans.  

So, what defines an amphibian? Although highly diverse, all amphibians are quadrupedal (four-legged) vertebrates, or were at some point in the evolutionary process, possessing two occipital condyles on the scull, and no more than one sacral vertebrate. Amphibian skin is naked, lacking hair, feathers, or "true" scales, and is highly glandular with mucous and glandular glands (in many species). Internally, amphibians possess a three-chambered heart, and usually two lungs (except the lungless species, and some reduced-lung species). Amphibians also possess pedicellate teeth and specialized papillae in the inner ear. It is important to note that there are some amphibians groups considered highly advanced, and some that are rather primitive, and although the present physiology may be drastically different, the ancestral forms were more similar. For instance, lungless salamanders did at one time possess lungs, but lost them through evolution. In other words, in many cases recent evolutionary characteristics must be disregarded, in a sense, in order to validate specific taxonomic classification.

Amphibian Biology, by Taxon

 

	Anuran Biology - Frogs and Toads are amphibians of the order Anura. Found all over the world, in nearly every habitat, anurans are a highly diverse group of amphibians. Frogs are likely the most recognizable group of amphibians.
	Caudate Biology - Salamanders, Newts, Sirens, Amphiuma, Waterdogs, and Mudpuppies are amphibians of the order Caudata (Urodela). Caudates are commonly referred to as the "tailed amphibians", a feature lacking in frogs and often overlooked in caecilians.
	Caecilian Biology - Caecilians are amphibians of the order Gymnophiona (Apoda). Caecilians are long, segmented, worm-like amphibians, making them undoubtedly distinguishable from any other amphibian order. Some species are aquatic, and some are terrestrial, or more accurately, fossorial. Caecilians are found in South America, Africa, and Asia.

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