Animals are multicellular, eukaryotic organisms of the kingdom Animalia (also called Metazoa). All animals are motile, meaning they can move spontaneously and independently at some point in their lives. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their lives. All animals are heterotrophs: they must ingest other organisms or their products for sustenance.Most known animal phyla appeared in the fossil record as marine species during the Cambrian explosion, about 542 million years ago. Animals can be divided broadly into vertebrates and invertebrates. Vertebrates have a backbone or spine (vertebral column), and amount to less than five percent of all described animal species. They include fish, amphibians, reptiles, birds and mammals. The remaining animals are the invertebrates, which lack a backbone. These include molluscs (clams, oysters, octopuses, squid, snails); arthropods (millipedes, centipedes, insects, spiders, scorpions, crabs, lobsters, shrimp); annelids (earthworms, leeches), cnidarians (jellyfish, sea anemones, corals), and sponges. Aristotle divided the living world between animals and plants, and this was followed by Carl Linnaeus, in the first hierarchical classification. [4] In Linnaeus's original scheme, the animals were one of three kingdoms, divided into the classes of Vermes, Insecta, Pisces , Amphibia, Aves, and Mammalia. Since then the last four have all been subsumed into a single phylum, the Chordata, whereas the various other forms have been separated out.In 1874, Ernst Haeckel divided the animal kingdom into two subkingdoms: Metazoa (multicellular animals) and Protozoa (single- celled animals). [5] the protozoa were later moved to the kingdom Protista, leaving only the metazoa. Thus Metazoa is now considered a synonym of Animalia.Animals have several characteristics that set them apart from other living things. Animals are eukaryotic and multicellular, [7] which separates them from bacteria and most protists. They are heterotrophic, [8] generally digesting food in an internal chamber, which separates them from plants and algae. [9] They are also distinguished from plants, algae, and fungi by lacking rigid cell walls. [10] All animals are motile , [11] if only at certain life stages. In most animals, embryos pass through a blastula stage, [12] which is a characteristic exclusive to animals.With a few exceptions, most notably the sponges (Phylum Porifera) and Placozoa, animals have bodies differentiat