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One feature shared by all animals is that they are ‘multicellular’. That is, their bodies are made of many specialized cells. By this criterion, single-celled organisms such as the familiar Amoeba are not considered to be animals, contrary to the views of a century ago. Indeed, many biologists now carefully avoid the term ‘protozoa’ for organisms such as Amoeba, since by definition an organism cannot be both ‘proto’ (meaning ‘first’ and implying one cell) and ‘zoa’ (implying animal).
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