Frogs

Frogs are tail-less amphibians (class Amphibia, order Anura). With some 3,500 living species, frogs are the most numerous and best known of amphibians. They are found on all continents except Antarctica and are common on many oceanic islands. The terms "frog" and "toad" are derived from early usage in England and northern Europe, where two families of the order Anura occur. One includes slender, long-legged, smooth-skinned animals that live near water: frogs; the other includes short-legged warty animals that live in fields and gardens: toads. When other kinds of animals of this group were discovered elsewhere, such as tree-frogs, fire-bellied toads, and others, it was realized that these various forms actually represented one major group. This group, the anurans, is now commonly referred to as frogs.

Frogs are amphibians, a term derived from two Greek words: amphi meaning double and bios meaning life. The double life of frogs involves living in water and also on land. Because of this amphibious habit, they must have adaptations for each environment. As in other animals that have a separate larval stage and a complex life cycle, frogs have two extremely different morphologies.

Tadpoles, the larval stage of frogs, are adapted to a purely aquatic life. They are seemingly reduced to the essentials, which in this case includes a globular body with a muscular, finned tail. Typically, tadpoles have no bones but rather a simple cartilaginous skull and skeleton. They also have no true teeth, instead having rows of denticles and a beak of keratin (a fingernail-like substance). The globular body is mainly filled with a long, highly coiled intestine.