The amphibians were the first vertebrates on land in late Devonian times.
Frogs and salamanders and the wormlike animals called caecilians are the
major members of the class Amphibia that still exist today.
Amphibians are a class of "cold-blooded" vertebrate animals
that have glandular skins lacking any skin coverings such as fur, scales
or feathers.
Metamorphosis - the Two Lives of Amphibians
Amphibian means "two lives," a reference to the
metamorphosis of some frogs and their relatives. The tadpole, the
larval stage of a frog, is usually an aquatic plant-eater with gills
and a long, finned tail. Young tadpoles lack legs and swim using
their tails, like its fishlike ancestors. Salamander larvae are
carnivorous.
During the transformation (metamorphosis) that leads to the "second
life," legs develop and the gills disappear. In most cases,
the young animal crawls onto shore and begins its life as a terrestrial
hunter; some amphibians, such as mudpuppies and clawed-frogs, never
leave the water. In spite of the name amphibian, many do
not go through a "free-living" aquatic larval stage (tadpole),
but develop directly into the adult form within the fluid contained
in the egg.
Reproduction
The amphibian egg has no shell, but is surrounded by jelly-like
material, and it dries up quickly in dry air. Oviparous amphibians
(those that lay eggs which house the developing tadpole for a short
while) generally lay their eggs in ponds or swamps or at least in
moist environments, and the tadpoles spend a long time swimming
in water before transforming to the adult stage (some species overwinter
as tadpoles). There are also live-bearing amphibians, that retain
the eggs in the female reproductive tract, in the skin on the back
of the female, and the females of one frog species in Australia
even carry their eggs in their own stomach. In most cases, eggs
which house the tadpole until it hatches as a fully-formed froglett
or salamander, are laid in moist, sheltered environments, where
embryos can develop without drying out.
Breathing Through the Skin
Even as adults, most frogs and other amphibians must maintain
close ties with water. Although most amphibians have lungs, they
carry out much of their gas exchange with the environment across
their moist skin.
The skin of most amphibians is permeable to water, and the animals
dessicate quickly in a dry environment. Therefore amphibians are
most abundant in damp habitats such as swamps and rain forests.
Even toads (like our Great Basin Spadefoot) and frogs adapted to
drier habitats spend much of their time in burrows where the humidity
is high, and emerge only after a heavy rain.
Endangered Species
Living amphibians are classified in three orders: the Anura (frogs and
toads), the Caudata (salamanders and newts) and the Apoda (Caecilians).
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