Frogs are amphibians that spend their lives both in water and on land. As a result, their bodies have evolved inner organs and outer structures adapted to these unique living conditions.
Outer Body Parts
Head and Mouth
Frogs possess two nares, or nostrils, two tympani, or eardrums, and two eyes. The eyes each have three eyelids: one of which is invisible. This third eyelid, also called a nictitating membrane, protects the frog's eyes when it is swimming underwater and helps keep the eyes moist when the frog is on land. Inside the frog's mouth are four distinct teeth that help the frog catch and swallow prey.
Legs and Feet
Frogs have five toes on each webbed hind leg, and four toes on the front legs, which lack webs. During the metamorphosis from tadpole to adult frog, the back legs develop prior to the front legs. This is partly because the frog begins its life in water, and webbed hind feet are more immediately beneficial.
Frogs range in size from the tiny Paedophryne at 9 millimeters (around 1/3 of an inch) to the enormous Goliath frog that can grow to a whopping 30 centimeters (roughly one foot) in length!
Frog skin is permeable, which means that
The frog's skin comes in many different colors and patterns. Most frogs use their skin pattern as a form of camouflage, while other frogs, such as the poison dart frog, have brightly colored skin that warns predators away. Most frogs shed their skin as they grow, eating it to consume additional nutrients.
Inner Body Parts
Skeleton and Muscles
The skeletal structure of the frog consists of a framework of bones and joints to which the frog's voluntary muscles are attached. These muscles are divided into flexors and extensors: When a flexor muscle contracts, that body part bends. When an extensor muscle contracts, the attached body part fully straightens. The combination of flexors and extensors gives the frog its ability to kick, jump, and swim.
Frogs possess a liver, heart, lungs, stomach, gall bladder and intestines. These organs perform the same functions for the frog as they do in human bodies: The heart pumps blood throughout the body, and the lungs aid in breathing. The stomach and intestines, like ours, digest food--from insects to fish, or even other frogs.
While the life span of most frogs in the wild is only a few years, the green tree frog can live up to 30 years in captivity.