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The first animals to evolve, as far back as a billion years ago, invertebrates are characterized by their lack of backbones and internal skeletons, as well as their relatively simple anatomy and behavior, at least as compared to most vertebrates. Today, invertebrates account for a whopping 97 percent of all animal species; this widely varied group includes insects, worms, arthropods, sponges, mollusks, octopuses, and countless other families.
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