Vertebrates are the only group to have an adaptive immune system, which may be partly responsible for their size and success. Vertebrates have a backbone, or spine invertebrates do not. Several million species may exist, but biologists have only identified about one million.Īnimals can mainly be divided into two main groups: the invertebrates and the vertebrates. They use this to group the animals in a biological classification. When biologists look at animals, they find things that certain animals have in common. The common animals most people know are only about 3% of the animal kingdom. There are many different types of animals. Plants are also multicellular eukaryotic organisms, but live by using light, water and basic elements to make their tissues. Also, the cells of animals have different cell membranes to other eukaryotes like plants and fungi. In both these ways they are different from plants.

This cellular respiration is part of their metabolism (chemical working). Animals take in oxygen, and give out carbon dioxide. Most animals are mobile, meaning they can move around. The study of ancient life is called palaeontology. Animals, plants, fungi, and some other living things have complex cells, so they are grouped together as eukaryotes. Animals get their energy from other living things. Animals (or Metazoa) are living creatures with many cells.

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