Amphibians

When I was young, I learnt that amphibians were animals that could live on land and water. This definition was not totally wrong but then it would include crocodiles which could live on land and water. The more scientific readers would promptly add that amphibians need to lay their eggs in water. This is almost correct except that it would exclude some tree frogs whose nest is on trees. It also excludes a toad that keeps its eggs under its skin.

As a rule of thumb, an amphibian needs to keep its eggs and skin moist (There are still some exceptions). The advantage of laying eggs in water is that the embryo can absorb water from its surrounding. For example, some amphibians have adapted some unique tricks to survive in arid conditions. Some frogs drink/soak up a large amount of water and then burrow themselves underground. These frogs then hibernate and wait for the next rain storm. There is also the sand rain frog which lays its eggs underground. In this case, the frog does not have an aquatic tadpole stage in its life. And yes, there are tadpoles that are born and "swim" in sand.

Amphibians are cold-blooded vertebrates. Before any reader suggests the character of having legs in its adulthood, Caecilians which are often limbless are also amphibians. Most Caecilians also give birth to their young live. This means the egg laying characteristic of amphibians is a heuristic. Amphibians also have the ability to breathe through their skin.