With few exceptions, animals consume organic material , breathe oxygen , have myocytes and are able to move , can reproduce sexually , and grow from a hollow sphere of cells, the blastula , during embryonic development.
Animals form a clade , meaning that they arose from a single common ancestor. Over 1. It has been estimated there are as many as 7. Animal body lengths range from 8. They have complex ecologies and interactions with each other and their environments, forming intricate food webs.
The scientific study of animals is known as zoology , and the study of animal behaviour is known as ethology. Most living animal species belong to the infrakingdom Bilateria, a highly proliferative clade whose members have a bilaterally symmetric and significantly cephalised body plan , and the vast majority of bilaterians belong to two large superphyla : the protostomes , which includes organisms such as arthropods , molluscs , flatworms , annelids and nematodes ; and the deuterostomes , which include echinoderms , hemichordates and chordates , the latter of which contains the vertebrates.
The much smaller basal phylum Xenacoelomorpha have an uncertain position within Bilateria. Animals first appear in the fossil record in the late Cryogenian period and diversified in the subsequent Ediacaran period in what is known as the Avalon explosion.
Earlier evidence of animals is still controversial; the sponge -like organism Otavia has been dated back to the Tonian period at the start of the Neoproterozoic , but its identity as an animal is heavily contested. Historically, Aristotle divided animals into those with blood and those without.
