Amphibians

Amphibians are a group of tetrapod vertebrates that include modern-day frogs and toads, caecilians, and newts and salamanders. The first amphibians evolved from lobe-finned fishes approximately 370 million years ago during the Devonian Period. The world of those early amphibians was quite different than it is today.
There were no birds, no mammals, and no reptiles on land (or in the water for that matter). There were only invertebrates and an assortment of prehistoric plants. It was a silent place, void of birdsong and lacking the growl of predators. The land lay wide open to amphibians and those with the evolutionary tenacity to set forth from the shallow shores began a new and important stage in the history of life on our planet. Several types of fishes had developed lungs. Among those lung-bearing fishes were the lobe-finned fish and the lungfishes.
The Crossopterygians, a group of primative lobe-finned fish are believed to be the ancestors of amphibians. They evolved several key features that enabled them to colonize land:

* a more rigid skeleton that would support the animal's body weight on land
* nostrils
* leg bones

Early amphibians included creatures such as Diplocaulus, Ophiderpeton, Adelospondylus, Diplocaulus, and Pelodosotis.
Most lineages of amphibians never fully severed their ties with aquatic habitats. A majority of amphibian species return to the water to breed and some species remain entirely aquatic throughout their entire life cycle. Most amphibians go through a complex metamorphosis process as they grow to adulthood.
The life cycles of amphibians reflects their evolutionary history of bridging land and water. Most amphibians lay their eggs in freshwater. A few species tolerate brackish water and some species lay their eggs on land. Extraordinarily, some species even carry their eggs inside their body. Although life cycles of amphibians vary from species to species, they all share the following three basic stages of development: egg, larva, adult.
Amphibian eggs do not have a hardened shells which means amphibians are not amniotes like reptiles, birds, and mammals. Instead, amphibian eggs consist of a gelatinous envelope that must remain moist to survive. Eggs hatch to release tiny larvae which later undergo a metamorphosis into the adult form.
Many amphibians can absorb oxygen directly into their bloodstream through their skin and are also able to expell carbon dioxide waste back into the air. The skin of amphibians lacks scales and hair. It is smooth and sometimes moist, making it quite permeable to gases and water. This permeability is thought to make amphibians particularly vulnerable to toxins in air and water such as herbicides, pesticides, and pollutants. Amphibians have been sharp decline throughout many areas worldwide. This is thought to be an early warning sign of a troubled environment.
Classification:

* Kingdom: Animalia
* Phylum Chordata
* Class Amphibia
* Subclass Lissamphibia
The Subclass Lissamphibia contains the following subgroups:
o Order: Caudata (newts and salamanders)
o Order: Gymnophiona (caecilians)
o Order: Anura (frogs and toads)