All living organisms, with the exception of basic single-celled organisms, are made up of various different parts which enable them to function. The majority of an animal's body is made up of water, cells and a protective or supportive structure such as a skeleton made out of bone or a hard outer shell to protect it. There are numerous processes that occur in an animal's body consistently and together such as circulation and digestion.
Animals have a number of different body systems, each of which does a specific job in order to keep the animal functioning. The heaviest body systems are the skeletal system (although it is not present in all animals), and the muscular system which connects the muscles to the brain so that they know what to do. Other body systems include the circulatory system, where blood is pumped around the body of the animal by it's heart; the digestive system which breaks down food and extracts the nutrients from it so they can be turned into energy; and the respiration system which controls an animal's breathing. All body systems are vital in the working of a healthy animal.
Many animals have some kind of hard structure either on the inside of their body (known as an endoskeleton) for support or on the outside of their body (known as an exoskeleton) for both support and protection. Other animals may not have a hard support system at all, like jellyfish for example, which are held together by a thin layer of cells known as a membrane. Skeletons help to keep the animal stable by supporting it's body shape, and some land animals are so large that their bodies even contain extra bony plates as the skeleton on it's own is not strong enough to support the animal's weight.
Similar cells within the body of an animal form together in groups which are organised into tissues, and various tissues make up different organs in an animal's body. There are a number of different organs found in the body of an animal, each of which has a specific task. Organs are linked to form organ systems, which carry out the vital processes in an animal's body that are key to it's survival, like the heart pumping the blood around the body or the skin of an animal, protecting it's delicate insides from the outside world.
Animals need oxygen in order to survive and many animals get their oxygen by breathing the surrounding air into their lungs, which is passed through the body and exhaled as carbon dioxide (so animals breathe in oxygen and breathe out carbon dioxide). Respiratory systems differ between animal species, so birds and mammals for example process the oxygen in different ways. Some animals however, do not have lungs and instead use other methods in which they can get oxygen into their bodies. Fish live in the water and so use gills in order to extract oxygen from the water surrounding them. Gills are made up of thin, feathery flaps which draw the blood close to the surface of the skin allowing the oxygen particles to pass into it. Other animals, like some frog species also breathe through pores in their skin.
Typically, most animal species have five senses which are sight, smell, taste, touch and balance. Certain species of animal have more heightened senses than others, like a mole that lives under-ground so can barely see, to an eagle that can spot prey on the ground from miles up in the air. Some animal species have also developed a "sixth sense" such as echolocation in dolphins, whales and bats.
Being able to move independently is one of the factors that makes an organism an animal. Movement is caused by the contracting of muscles within an animal's body, and is displayed in countless ways throughout the animal kingdom. Humans walk, cheetahs can run as fast as a car, ducks and geese waddle, fish swim, birds fly and some insects get around by jumping. Some adult animals however, such as barnacles and sponges become sessile by permanently fixing themselves to another animal or object, they are still animals however as they still respire and are able to move in their immature form.