After hatching from its egg a larval frog, called a tadpole, has a small body with a large, fin-shaped tail, by which it propels itself through the water. On the tadpole's pharynx are three pairs of external gills for breathing. Later these external gills are replaced by three pairs of internal gills that lie within gill slits. The tadpole lives and swims under water like a fish.
After some weeks or months metamorphosis begins. First hindlegs and then forelegs develop from the tadpole's body. Soon lungs develop, and the tadpole swims to shallow water and begins to inhale air. Then its gills and tail are absorbed into its body. The tadpole gradually assumes the form of an adult frog and begins to hop on land by means of its hindlegs. Adult frogs breathe with lungs, and most of them can live on land. However, most species remain in or near ponds or streams and lay their eggs in water.
About 2,150 different kinds of frogs inhabit the world today. They vary in length from 1 to 10 inches and in color from green to tan and dark brown. They are often marked with black, white, or bronze spots. The long, muscular hindlegs of frogs enable many of them to leap a distance equal to many times their own length. Their smaller, weak forelegs are used to break their fall after a leap and to support the body while they squat. Their hind feet are webbed for swimming. They feed on insects and worms and sometimes on mice and other small animals. Frogs have large mouths, from which they nimbly thrust their long tongues to catch insects.
Most frogs posses huge, bulging eyes that can see sideways and to the rear as well as in front. They are specially constructed to see rapidly flying insects. The eyes can be with-drawn into the head when danger threatens them. Frogs can hear fairly well and have a keen sense of smell. Like many animals that live in water and mud, they have in their skin sensitive organs of touch that enable them to explore their surroundings as they move.
The internal organs of a frog are similar in some respects to those of man. The frog's alimentary canal, or digestive tract, consists serially of a short, wide, elastic gullet; a stomach; a pyloric valve; a long, coiled small intestine; a short, broad large intestine; and a cloaca, through which digestive wastes pass from the body.
The respiratory organs of the adult frog are its skin, the internal lining of its mouth, and its lungs. Even though it has no gills, the adult frog can breathe under water as well as in the air. While lying still under water, it can breathe through its skin. Dissolved oxygen is absorbed into the bloodstream through thin-walled blood vessels that are embedded in the skin. However, while swimming, a frog needs more oxygen than can be obtained through its skin and so must thrust its head above the surface of the water and inhale air into its lungs. While on land the adult frog can breathe through the internal lining of its mouth without using its lungs. Air is drawn into the mouth, and oxygen from the air is absorbed into the bloodstream through thin-walled blood vessels that are embedded in the lining of the mouth. At times while on land the frog inhales air into its lungs, and the absorption of oxygen into the bloodstream occurs in the lungs.
The heart of an adult frog consists of five chambers—two atria, one ventricle, one conus arteriqsus, and one sinus venosus. In the frog's heart deoxygenated blood flows into the right atrium by way of the sinus venosus from veins of various parts of the body. Oxygenated blood from the lungs flows into the left atrium. The atria contract simultaneously and force oxygenated blood and deoxygenated blood into the ventricle, where they are mixed together. Then the ventricle contracts and forces the blood through the arteries of various parts of the body to the capillaries, where food and oxygen are absorbed from the blood by the body cells. Then the deoxygenated blood returns through the veins and sinus venosus into the right atrium. Part of the blood in the ventricle is pumped through the pulmonary arteries to the lungs, where it is oxygenated and returned to the left auricle.
