Amphibia — a life in two places
This is the class of amphibians, able to live in terrestrial and aquatic environments alike.
They were the first onto land
In the course of evolution, amphibians were the first form of life to invade the land. Which is to say every vertebrate on land today is descended from them.
Figure to be added
- Water is essential to completing the life cycle — there is an aquatic stage in it.
- They undergo metamorphosis.
- They bear a thin, slimy, glandular skin. No scales are present in the skin.
- Pentadactyl limbs are present for movement.
- The heart has three chambers — two atria and a single ventricle.
- Respiration is carried out by lungs, by the moist skin, or by the mouth.
- They are poikilothermic.
Moist skin has a price
Being able to breathe through the skin is an advantage — but for it the skin has to stay moist. So no amphibian can live in a desert, and they never became wholly free of water.