Frogs, toad, newts, and
salamanders are amphibians. They are mostly land animals,
but their lives are tied to the water. Amphibians lay their
eggs in water and they develop into tadpoles, which swim and have
gills to breathe. Later the tadpoles develop legs, lose
their gills, and develop lungs. Now as tiny frogs or
newts, they move to the land. But even then, they mostly
stay in damp places, because their smooth, slimy skins are not
waterproof. The advantage of these skins is that they can be
used for breathing and help out the simple lungs.
Over 350 million years ago
some lobe-finned fish lived where pools sometimes dried up.
They had to wriggle to the next pool to survive. Eventually
some became better adapted for making these trips. The fins
turned into legs. The breathing of air became
important. Amphibians had arrived. The head of
amphibians differs from that of fish. Amphibians have eyes
with tear glands and eyelids. They blink to clean the
surface of the eye. Fish have internal ears and hear
well. On land something more is needed to pick up sounds in
air. Amphibians have eardrums at the back of the head.
Frogs depend upon sound to attract a mate.
Newts probably look most
like the early amphibians. Toads and frogs are relatively
new, but they are very successful, especially in the
tropics. There are 3,500 species of toads and frogs compared
to about 500 other amphibians.