Invertebrate is a term coined by Chevalier de Lamarck to describe any animal without a backbone or vertebra, such as insects, squids and worms.
He divided them into two groups, the Insecta and the Vermes. However, the invertebrates are not a coherent group of animals, as many are much more closely related to vertebrates than to one another. This is one of the places where formal taxonomy and common usage diverge, as the invertebrates comprise 97% of animal species.
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