The multicellular eukaryotic species known as animals, animals, fauna, wildlife, or animals comprise the biological kingdom Animalia. Animals, with very few exceptions, are able to move, reproduce sexually, eat organic materials, breathe oxygen, and evolve from a hollow ball of cells called a blastula during the embryonic stage of development. A total of more than 7 million animal species are thought to exist, while more over 1.5 million living animal species—of which roughly 1 million are insects—have been described.