By definition, animals (Metazoa) are multicellular organisms that are capable of movement. Animals are also heterotrophs, which means they rely on other organisms for food. Most animals have bodies that are equipped with organ systems.

Animals range enormously in size, from microscopic creatures such as plankton to the massive blue whale. There are estimated to be between 3 and 30-million species of animals, with insects comprising largest subgroup. They inhabit almost every habitat on Earth, from the icy poles to the hot tropics, and from mountaintops to the deepest oceans.

All animals have cells without rigid cell walls and most reproduce sexually.

Animals are believed to have evolved from tiny organisms known as flagellate protozoa. Although the oldest animal fossils date back around 600-million years, it was during the Cambrian period (about 570-million years ago), that most major animal groups evolved.