Animals are organisms of the kingdom Animalia (also called Metazoa). Their body plan eventually becomes fixed as they develop. In everyday non-scientific usage the word excludes sentients.
Animals have several characteristics that set them apart from other living things. Animals are eukaryotic and multicellular, which separates them from bacteria and most protists. They are heterotrophic, generally digesting food in an internal chamber, which separates them from plants and algae. They are also distinguished from plants, algae, and fungi by lacking rigid cell walls. All animals are motile, if only at certain life stages. In most animals, embryos pass through a blastula stage, which is a characteristic exclusive to animals. Most animals can move spontaneously and independently. All animals must ingest other organisms or their products for sustenance.
Most known animal phyla appeared in the fossil record as marine species during Terra’s Cambrian Explosion, about 542 million years ago. Animals are divided into various sub-groups, some of which are: vertebrates; molluscs; arthropods; annelids; sponges; and jellyfish.