Frogs

If frogs were not so common and familiar, they would be regarded as among the strangest of vertebrate animals. The typical frog has a broad head with an enormous mouth and protruding eyes. The body is short and plump, and there is no tail. The forelegs are rather short but normal-looking, and are used mainly for propping up the front part of the body and for stuffing food into the mouth. The hind limbs are much larger and more muscular, and have an extra joint that makes them even longer and provides extra power for jumping, which is their major mode of locomotion. Among aquatic frogs, the hind limbs also provide the propulsion for swimming.

The frog skeleton has been evolutionarily reduced. The skull is a framework of bones that hold the brain-case, eyes, internal ears, and jaws, while giving support to the jaw muscles. The vertebral column has been reduced to only 5-9 body vertebrae, and the caudal (tail) vertebrae have become fused into a single mass, the urostyle. Although the bones of the forelimbs are relatively normal-looking, those of the hindlimbs are highly modified for jumping. The tibia and fibula are fused into a single rod, and an extra joint has developed from the elongation of some of the foot bones, thus providing a jumping apparatus considerably longer than the torso.

Most frogs have a smooth, obviously moist skin. Even toads, with their warty, seemingly dry skin, have a surface cover that is moist and permeable to liquids and gases. This has advantages and disadvantages, but is necessary for frogs to carry on normal respiration. The lungs of amphibians are too small and simple in construction to provide adequate gas exchange, and the skin plays an important role in this regard. A significant amount of oxygen comes into the body via the skin, and as much as half of the carbon dioxide produced is released through this covering.

The internal anatomy is broadly similar to that of other vertebrate animals. There is a heart and associated circulatory system, a brain and nervous system, and a digestive system made up of esophagus, stomach, and small and large intestines, with the associated liver and other organs. The urinary system is relatively simple, having two kidneys as in most vertebrates. The reproductive system consists of paired ovaries or testes, with associated ducts. As in many vertebrates, the digestive, urinary, and reproductive systems empty through a common posterior chamber, the cloaca.