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Animals, beasts, fauna, wildlife, or animals are multicellular eukaryotic organisms that make up the biological kingdom Animalia. With few exceptions, animals consume organic matter, breathe oxygen, are mobile, reproduce sexually, and grow from a hollow ball of cells, the blastula, during the embryonic phase of development. More than 1.5 million living animal species have been described—about 1 million of which are insects—but it is estimated that there are more than 7 million animal species in all. Animals range in length from 8.5 micrometers to 33.6 meters and have complex interactions with each other and with their environment, and form complex food webs. The study of animals is called zoology. Most living animal species are classified in the Bilateria, a clade whose members are bilaterally symmetrical. Bilateria include protostomes and deuterostomes. Within the protostomes there are many groups of invertebrates, such as nematodes, arthropods, and mollusks, while deuterostomes include echinoderms and chordates (including vertebrates). Life forms that are interpreted as ancient animals are classified in the Ediacaran biota that lived in the late Precambrian eon. The modern animal phyla formed clearly in the fossil record as marine species during the Cambrian explosion about 542 million years ago. 6,331 gene pools have been identified that are shared by all living animals; these genes probably arose from a single common ancestor who lived 650 million years ago.