Tadpole in the context of "Odonata"

The beak or bill is an external rostrum structure found mostly in birds. A beak is used for pecking, grasping, and holding (in probing for food, eating, manipulating and carrying objects, killing prey, or fighting), preening, courtship, and feeding young. The terms beak and rostrum are also used to refer to a similar mouth part in some ornithischians, pterosaurs, cetaceans, dicynodonts, rhynchosaurs, anuran tadpoles, monotremes (i.e. echidnas and platypuses, which have a bill-like structure), sirens, pufferfish, billfishes, and cephalopods.

Although beaks vary significantly in size, shape, color and texture, they share a similar underlying structure. Two bony projections–the upper and lower mandibles–are covered with a thin keratinized layer of epidermis known as the rhamphotheca. In most species, two holes called nares lead to the respiratory system.

Several groups of tetrapods have undergone secondary aquatic adaptation, an evolutionary transition from being purely terrestrial to living at least partly aquatic. These animals are called "secondarily aquatic" because although all tetrapods descended from freshwater lobe finned fish (see evolution of tetrapods), their more recent ancestors are terrestrial vertebrates that evolved on land for hundreds of millions of years, and their clades only re-adapted to aquatic environment much later.

Unlike primarily aquatic vertebrates (i.e. fish), secondarily aquatic tetrapods (especially aquatic amniotes), while having appendages such as flippers, dorsal fin and tail fins (flukes) that resemble fish fins due to convergent evolution, still have physiology based on their terrestrial ancestry, most notably their air-breathing respiration via lungs (instead of aquatic respiration via gills) and excretion of nitrogenous waste as urea or uric acid (instead of ammonia like most fish). Nearly all extant aquatic tetrapods are secondarily aquatic, with only larval amphibians (tadpoles) being primarily aquatic with gills, and only some species of paedomorphic mole salamanders (most notably the fully aquatic axolotl) retain the gill-based physiology into adulthood.

Amphibians are ectothermic, anamniotic, four-limbed vertebrate animals that constitute the class Amphibia. In its broadest sense, it is a paraphyletic group encompassing all tetrapods, but excluding the amniotes (tetrapods with an amniotic membrane, such as modern reptiles, birds and mammals). All extant (living) amphibians belong to the monophyletic subclass Lissamphibia, with three living orders: Anura (frogs and toads), Urodela (salamanders), and Gymnophiona (caecilians). Evolved to be mostly semiaquatic, amphibians have adapted to inhabit a wide variety of habitats, with most species living in freshwater, wetland or terrestrial ecosystems (such as riparian woodland, fossorial and even arboreal habitats). Their life cycle typically starts out as aquatic larvae with gills known as tadpoles, but some species have developed behavioural adaptations to bypass this.

Young amphibians generally undergo metamorphosis from an aquatic larval form with gills to an air-breathing adult form with lungs. Amphibians use their skin as a secondary respiratory interface, and some small terrestrial salamanders and frogs even lack lungs and rely entirely on their skin. They are superficially similar to reptiles like lizards, but unlike reptiles and other amniotes, require access to water bodies to breed. With their complex reproductive needs and permeable skins, amphibians are often ecological indicators to habitat conditions; in recent decades there has been a dramatic decline in amphibian populations for many species around the globe.

The evolution of tetrapods began about 400 million years ago in the Devonian Period with the earliest tetrapods evolved from lobe-finned fishes. Tetrapods (under the apomorphy-based definition used on this page) are categorized as animals in the biological superclass Tetrapoda, which includes all living and extinct amphibians, reptiles, birds, and mammals. While most species today are terrestrial, little evidence supports the idea that any of the earliest tetrapods could move about on land, as their limbs could not have held their midsections off the ground and the known trackways do not indicate they dragged their bellies around. Presumably, the tracks were made by animals walking along the bottoms of shallow bodies of water. The specific aquatic ancestors of the tetrapods, and the process by which land colonization occurred, remain unclear. They are areas of active research and debate among palaeontologists at present.

Most amphibians today remain semiaquatic, living the first stage of their lives as fish-like tadpoles. Several groups of tetrapods, such as the snakes and cetaceans, have lost some or all of their limbs. In addition, many tetrapods have returned to partially aquatic or fully aquatic lives throughout the history of the group (modern examples of fully aquatic tetrapods include cetaceans and sirenians). The first returns to an aquatic lifestyle may have occurred as early as the Carboniferous Period whereas other returns occurred as recently as the Cenozoic, as in cetaceans, pinnipeds, and several modern amphibians.

The common toad, European toad, or in Anglophone parts of Europe, simply the toad (Bufo bufo, from Latin bufo "toad"), is a toad found throughout most of Europe (with the exception of Ireland, Iceland, parts of Scandinavia, and some Mediterranean islands), in the western part of North Asia, and in a small portion of Northwest Africa. It is one of a group of closely related animals that are descended from a common ancestral line of toads and which form a species complex. The toad is an inconspicuous animal as it usually lies hidden during the day. It becomes active at dusk and spends the night hunting for the invertebrates on which it feeds. It moves with a slow, ungainly walk or short jumps, and has greyish-brown skin covered with wart-like lumps.

Although toads are usually solitary animals, in the breeding season, large numbers of toads converge on certain breeding ponds, where the males compete to mate with the females. Eggs are laid in gelatinous strings in the water and later hatch out into tadpoles. After several months of growth and development, these sprout limbs and undergo metamorphosis into tiny toads. The juveniles emerge from the water and remain largely terrestrial for the rest of their lives.

Ovoviviparity, ovovivipary, ovivipary, internal oviparity, or aplacental viviparity is a "bridging" form of reproduction between egg-laying oviparous and live-bearing viviparous reproduction. Ovoviviparous animals possess embryos that develop inside eggs that remain in the mother's body until they are ready to hatch.

The young of some ovoviviparous amphibians, such as Limnonectes larvaepartus, are born as larvae, and undergo further metamorphosis outside the body of the mother. Members of genera Nectophrynoides and Eleutherodactylus bear froglets, with not only the hatching, but all the most conspicuous metamorphosis, being completed inside the body of the mother before birth.

Tunicates are marine invertebrates belonging to the subphylum Tunicata (/ˌtjuːnɪˈkeɪtə/ TEW-nih-KAY-tə). This grouping is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (including vertebrates). The subphylum was at one time called Urochordata, and the term urochordates is still sometimes used for these animals.

Despite their simple appearance and very different adult form, their close relationship to the vertebrates is certain. Both groups are chordates, as evidenced by the fact that during their mobile larval stage, tunicates possess a notochord, a hollow dorsal nerve cord, pharyngeal slits, post-anal tail, and an endostyle. They resemble a tadpole.

The axolotl (/ˈæksəlɒtəl/ ; from Classical Nahuatl: āxōlōtl [aːˈʃoːloːtɬ] ) (Ambystoma mexicanum) is a species of paedomorphic mole salamander, meaning they mature without undergoing metamorphosis into the terrestrial adult form; the adults remain fully aquatic with obvious external gills. This trait, although somewhat unusual among the majority of amphibians, is not unique to axolotls. Furthermore, this is apparent as axolotls may be difficult to distinguish from the larval stage of other neotenic adult mole salamanders (Ambystoma spp.), such as the occasionally paedomorphic tiger salamander (A. tigrinum) which are widespread in North America; or with mudpuppies (Necturus spp.), which bear a superficial resemblance to axolotls, but are from a different family of salamanders.

Axolotls originally inhabited a system of interconnected wetlands and lakes in the Mexican highlands; they were known to inhabit the smaller lakes of Xochimilco and Chalco, and are also presumed to have inhabited the larger lakes of Texcoco and Zumpango. These waterways were mostly drained by Spanish settlers after the conquest of the Aztec Empire, leading to the destruction of much of the axolotl's natural habitat, which is now largely occupied by Mexico City. Despite this, they remained abundant enough to form part of the staple in the diet of native Mexica during the colonial era. Today, due to continued urbanization in Mexico City, which causes water pollution in the remaining waterways, as well as the introduction of invasive species such as tilapia and carp, the axolotls are now near extinction. This can be seen as the species has been listed as critically endangered in the wild, with a decreasing population of around 50 to 1,000 adult individuals, by the International Union for Conservation of Nature (IUCN), and is listed under Appendix II of the Convention on International Trade in Endangered Species (CITES).