Why some frogs are called tree frogs?
As you can guess, tree frogs are called by this name because they make their home in the branches of trees. Some inhabit smaller plants, and a few species are ground-dwellers. Tree frogs are found in most parts of the world. They vary in length from less than two and a half centimetres to more than ten centimetres.Tree frogs are very agile, and can climb quickly. How do they do this? They have pads at the ends of their fingers and toes. These pads help in climbing, because they are rough and are covered with a sticky secretion. Some tree frogs have claw-shaped fingers and toes, and most kinds have webbed hands, webbed feet, or both. Many tree frogs can change colour, usually to match their surroundings. As winter approaches, tree frogs burrow into the humus on the forest floor. Their tissues and body fluids are protected from freezing by glycerol, made in the frog’s cells.
Tree frogs are noted for the loudness and variety of their calls, made only by the male. Most tree frogs lay their eggs in water. There are three main families of tree frogs that include the so-called flying frogs. Actually, flying frogs don't fly- they just glide from tree to tree.
Frogs lay their eggs in water or wet places. The egg begins as a single cell. Several thousand are sometimes laid at once. Gradually, a jellylike covering develops, which protects the egg. The single cell in the egg eventually splits into two. These two split making four cells, and so on. Eventually, there are many cells in the egg. The mass of cells in the egg come to form an embryo. Organs and gills begin to form, and in the mean-time, the embryo lives off its internal yolk.
The main reason for the frog being chosen for the laboratory is that it has a similar orientation to the human body.
Frogs have very special skin- it not only protects them but they drink and breathe through it. Frogs don't usually swallow water like we do. Instead they absorb most of the moisture they need through their skin. They also get extra oxygen from the water absorbed through their skin. So it is important for frogs to keep their skin moist or they might suffocate. Some times you'll find frogs that are slimy. This is because the frog skin secretes mucus that helps keep it moist. Even with the slimy skin, these frogs need to stay near water.
Frogs and toads are both amphibians, and they are very similar. There are some differences however. Frogs generally have bulging eyes, strong long webbed hind legs for leaping and jumping, and smooth or slimy skin. Frogs also usually lay their eggs in clusters. Toads, on the other hand, have stubby bodies with short hind legs, and warty and dry skin. Their eyes are less bulging and more football shaped, with poison glands behind them. Toads move in short hops and jumps, and they lay eggs in long chains. However, there is really no hard and fast rule. Toads belong to the frog family, but they are not true frogs. To make things more complicated, it is not uncommon, for example, to find a warty skinned frog that isn't a toad, or even a slimy toad!
The first true frogs evolved about 200 million years ago during the time of the dinosaurs. Frogs are amphibians – in fact, about 90% of amphibian are frogs. They spend their lives near water, because they must return to the water to lay their eggs. Frog eggs are laid in the water. When they hatch into tadpoles, they breathe with gills, and swim using a tail. As they mature, they lose their tail, develop lungs for breathing air, and live on land. Though adult frogs have no tail, they have well developed hind legs, and are experts at leaping and jumping. They are found on every continent except Antarctica. There are thousands of species of frogs and they come in all shapes, colours and sizes.
Amphibians hear very well- they can even hear sounds which we can't hear. If you look closely at a frog, you will see small circles covered with a membrane behind its eyes, on the side of its head. This membrane is the frog's eardrum. Sound waves in the air make the membrane vibrate. The vibrations are transmitted to the inner ear, and from there to the brain, which recognizes it as a sound.
Since amphibians have been around a long time, and have adapted to a 
variety of environments, their eye structures vary slightly for different environments. The frog has large, bulging eyes, which sit at the top of its head. Its eyes bulge out so far that it can see in nearly all different directions, which is helpful for an animal that can't turn its head. A transparent inner eyelid called the nictitating membrane protects the frog's eyes when the animal is underwater. A frog can also shut its eyes completely. To do this, the frog pulls its eyeballs deep into their sockets. This closes the upper and lower eyelids together.
Amphibians can be red, blue, yellow, brown, or green and they come with spots, stripes, and many different types of markings. Some are darker on top with a completely different colour and pattern underneath. An amphibian's colour may vary with humidity too, becoming pale when warm and dry, and dark when cold and damp.
Amphibians live on both land and water, and so, their respiratory system must allow them to take in oxygen not only from the air, but also from water. Thus, they have a very complex respiratory system.
For amphibians, water is essential for survival. To begin with, water keeps their skin moist, and they get the oxygen they need from the water that they absorb through their skin. Water is also vital for their reproduction. This is because the eggs of amphibians do not have a hard shell so they need water to keep them from drying out, and to protect them from the rays of the sun. Some species of amphibians are aquatic, and spend all their lives in water. Others spend the beginning of their lives in water as larvae, and later undergo a change called metamorphosis, and move on to land.
Amphibians are found all over the world, except in Antarctica and Greenland. They are found both on land, and in fresh water. Frogs are the common amphibians. You can find frogs in water, or near places that have water, like ponds and streams. However, some frogs will never enter the water. They live mainly on land. Then, there are some kinds that live in trees. Some frogs are burrowers. Frogs that live in cold places, hibernate during winter time.
Amphibian’s skeletons are made of bones. They have fewer bones than their fishy ancestors, and over the years, the evolutionary changes in the amphibian family have gone in two directions. The frog family has a broad head, large eye sockets, short spine, no tail, and long hind legs. The caecilian group has skeletons with small tubular skulls, tiny eye sockets, long spines, and no legs at all.
Sirens are probably the most ancient line of salamanders now alive on planet Earth. They are large eel like creatures that can reach 40 centimetres in length and are 2.5 centimetres in diameter. They have a pair of small front legs that they use to pull themselves along through the muddy murky world of the swamps that they live in. They have no hind legs.
Amphibians evolved from fish about 400 million years ago, when the amount of dry land on Earth increased greatly. Certain fish adapted to these changing conditions by developing limbs to crawl with, and lungs to breathe. They came to be called ‘amphibians’, and were the first animals with backbones to live on land. The largest variety of amphibians lived around 360 million to 230 million years ago, when the climate was constantly alternating between wet and dry.