Frogs are amphibians that belong to a group called vertebrates. The word amphibian comes from a Greek word that means "two lives." Amphibians are cold - blooded. Their skin absorbs water into their body so they don't have to drink water in order to survive. Frogs have very strong legs for leaping great distances.
<ul><li>Frogs lay their eggs in water or wet places. A floating clump of eggs is called frog spawn. </li></ul><ul><li>The egg begins as a single cell. Several thousand are sometimes laid at once. It becomes surrounded by a jellylike covering, which protects the egg. The female may or may not stay with the eggs to take care of the young after she has laid them. The egg slowly develops. But, only a few develop into adults. Ducks, fish, insects, and other water creatures eat the eggs. </li></ul>
<ul><li>The newly hatched larva head, a small trunk and a short tail. It has no mouth. There is a sucker at the anterior end on the ventral side. The larva attaches it self to the weeds by means of this sucker. It has two pairs of external gills for respiration. After about a week the larva develops into a tadpole larva. </li></ul>
<ul><li>Tadpole larva :- The tadpole larva resembles a small fish. It develops a mouth, a third pair of external gills and a tail. The jaws have teeth. It feeds on plants. The intestine is coiled like a watch spring. The tail increases in size. It helps </li></ul><ul><li>in swimming. </li></ul>
<ul><li>A series of changes take place in tadpole internally and externally to transform it into an adult frog. </li></ul><ul><li>The tadpole larva of frog is fish like. It lives in water and breathes through gills. It grows in size and slowly transforms into a frog. During this transformation, the forelimbs appear. At the same time gills disappear lungs will develop. At this stage, the small frog leaves water as it cannot breathe, with lungs in water. Tail persists for some more time but finally it also disappears. </li></ul>
<ul><li>the marked change in the form of an animal during development is called metamorphosis. </li></ul><ul><li>The metamorphosis in frog is initiated by a harmone called thyroxine. This is secreted by a gland known as thyroid. </li></ul>