Animals have several characteristics that set them apart from other living things. Animals are eukaryotic and multicellular,^[5] which separates them from bacteria and most protists. They are heterotrophic,^[6] generally digesting food in an internal chamber, which separates them from plants and algae.^[7] They are also distinguished from plants, algae, and fungi by lacking rigid cell walls.^[8] All animals are motile,^[9] if only at certain life stages. In most animals, embryos pass through a blastula stage,^[10] which is a characteristic exclusive to animals.

Animals are multicellular, eukaryotic organisms of the kingdom Animalia (also called Metazoa). Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their lives. All animals are motile, meaning they can move spontaneously and independently, at some point in their lives. All animals are heterotrophs: they must ingest other organisms or their products for sustenance.