An animal is a Animals (also called Metazoa) are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume
organic material, breathe
oxygen, are able to
move, can reproduce sexually, and go through an ontogenetic stage in which their body consists of a hollow
sphere of cells, the blastula, during embryonic development. Over 1.5 million living animal species have been described—of which around 1 million are insects—but it has been estimated there are over 7 million animal species in total. Animals range in length from 8.5 micrometres (0.00033 in) to
33.6 metres (110
ft). They have complex interactions with each other and their environments, forming intricate
food webs. The scientific
study of animals is known as zoology.
Most living animal species are in Bilateria, a clade whose members have a bilaterally symmetric body plan. The Bilateria include the protostomes, containing invertebrates such as nematodes, arthropods, and molluscs, and the deuterostomes, containing the echinoderms and the chordates, the latter including the vertebrates. Life forms interpreted as early animals were
present in the Ediacaran biota of the
late Precambrian. Many modern animal phyla became clearly established in the fossil record as marine species during the Cambrian explosion, which began around 542 million years ago.
