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Invertebrate, animal without a backbone. Invertebrates are a miscellaneous collection of groups from single-celled protozoa to highly specialized insects and spiders, although the term usually refers to multicellular organisms. Apart from the universal lack of an internal backbone of vertebrae, many of these groups have little in common.

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21st Century Webster's Family Encyclopedia21st Century Webster's Family Encyclopedia - Inert gas to Jaruzelski, Wojciech