Animals have developed muscles which give them mobility, also a characteristic that has stimulated further development of other type of tissues and organ systems. Animals differ also in that they obtain energy by eating. Without a doubt, animals are the most diverse forms of life inhabiting the Earth, currently we know of more than 2 million species, and many more are awaiting identification. From 100ft whales to microscopic organisms, animals inhabit a vast range of ecosystems from deserts to sub-zero Arctic tundra to the ocean depths. One of the most amazing characteristics of animals is their ability to adapt to their surroundings and shape patterns of behavior, learning from past experience.
Animals also hold the distinction of having developed sense organs and nervous systems.
Like all the inhabitants of Earth, animals are classified into groups according to their similarities and differences. Reptile have scales, birds have feathers, mammals are the only ones that have fur and so on. Animals are classified according to a wide range of characteristics, including genetic makeup, internal anatomy and patterns of development. Not only does this provide a reliable guide to where different animals belong in the world, but shows how certain species are related through evolution. The animal kingdom is divided into approximately 30 groups, each called a phylum.