Animals

 Animals—complex, multicellular organisms equipped with nervous systems and the ability to pursue or capture their food—can be divided into six broad categories. Here are the six main animal groups, ranging from the simplest (the spineless invertebrates) to the most complex (mammals, which can adapt to a wide range of habitats).

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Invertebrates 

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 compared with those of most vertebrates. Today, invertebrates account for a whopping 97 percent of all animal species, a widely varied group that includes insects, worms, arthropods, sponges, mollusks, octopuses, and countless other families.