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AmphibiansThe amphibians evolved around 370 million years ago (mya) and there are now around 4,000 species, the group includes frogs, toads, newts, salamanders and caecilians. They are the first stage of vertebrate evolution (animals with back bones) and adaptation to living on land. Amphibians have limbs instead of fins like fish; these are much stronger to support the body out of water. They have primitive lungs that allow them to breathe air; they can also absorb oxygen through their skin, which is kept moist to help this process. Many amphibians can keep their skin moist by excreting mucus or by licking saliva over their body. This allows them to survive when water is not easily available and to live in unusual habitats such as trees. These features have been found in primitive forms in fish such as the coelacanth and the lungfish supporting theories of their evolution. The biggest difficulty for amphibians is that the young are aquatic. The eggs do not have a hard shell to prevent desiccation (drying out) in air so they must be laid in water. The young start out life as tadpoles and metamorphose into the adult form before emerging onto land. As the tadpole grows it develops four limbs and either retains the tail (e.g. newts and salamanders) or loses it (e.g. frogs and toads). Orders (types) of amphibians alive today:
Anura: frogs and toads |
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