There are almost 1 million different species of animals living in the world today, and new ones are constantly being discovered. They range in size and complexity from microscopic creatures made up of only a few cells to the giant blue whale, weighing up to 180 tons. Faced with the task of bringing order to the living world, zoologists consider two key factors: definition (is it an animal?) and classification (what place does it occupy in the animal kingdom?).
What is an animal?
Apart from inanimate objects, humans share the world with living organisms, including plants, animals, and single-celled organisms called monerans and protists. The difference between inanimate objects and living things is not always apparent. For example, one of the characteristics of living things is that they grow. Yet crystals, which are inanimate, also grow. But only living things share in common the fundamental life process called metabolism. Metabolism is the sum of the chemical processes by which cells produce the substances and energy that an organism needs to live.
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