Amphibians form a Class of Vertebrates. Vertebrates are animals that have backbones that surround their nerve cords. The nerve cord is found along the back of an animal. The bodies of most vertebrates have a head and a trunk. Many vertebrates have two pairs of limbs attached to the trunk (e.g. arms and legs).
There are five main classes or groups of vertebrates: amphibians, birds, fish, mammals and reptiles with over 4000 species worldwide. They are called ectotherms or cold-blooded. That means that amphibians do not hold their body temperature constant like mammals, but their body temperature depends upon that of the surrounding environment.
Amphibians have soft, generally moist skin without scales.
Amphibians go through a two-stage life cycle. The are laid in water or a damp environment to keep from drying out. They experience a metamorphosis or change.
When an amphibian hatches, it is in a gilled larval form. In frogs and toads this larva is called a tadpole. After a few weeks or even months, the larvae transform into the adult form of a frog.
Amphibians are divided into three major groups or Orders. Two of these, the Frogs (including toads) and the Salamanders are found throughout the world. The third group, known as Caecilians, are limbless amphibians found only in the tropics. Frogs belong to the Class Amphibia, and rely on moisture to breathe, reproduce and survive. Amphibians are nocturnal because this is one way of minimizing the risk of drying out; the evening is the coolest part of the day. Most frogs are active at night.
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