Amphibians generally start life in water, but later change
so that they can live on land. Most return to the water to mate. The life cycle
involves three stages: egg, larva, and adult. The change from larva to adult is
known as metamorphosis. Amphibians are ectothermic (cold-blooded) animals that
have a bony internal skeleton. They have small lungs and can also breathe
through their smooth skin, which must be kept moist.
Amphibians can lose water through their skin. There are no scales or
hair protecting the skin. Most live in damp places. There are about 5,000
species in three orders.
(caecilians)
Features: limbless, worm-like, poor
sight
(frogs, toads)
Features: wide head, no tail, powerful
back legs
(newts,
salamanders)
Features: long tail, carnivorous larvae,
good sense of smell
A frog looks wet all the time because glands in its skin produce
mucus to keep it moist. The frog can then absorb oxygen through its thin skin
directly into the bloodstream. Frogs are the only amphibians that can hop. Some
have webbed feet and are good swimmers. They live mainly on land and catch
worms and insects with their long, sticky tongue.
A frog lays its eggs in water, because they have no shell to stop
them from drying out. Tadpoles hatch from the eggs and take about 12 weeks to
grow and change into frogs. At first the tadpoles breathe like fish, using
gills, but as they change, lungs replace the gills. The tadpoles grow back
legs, front legs, a big head, and finally their tail disappears.
