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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. These include muscles, which are able to contract and control locomotion, and nerve tissues, which send and process signals. Typically, there is also an internal digestive chamber, with one or two openings.[13] Animals with this sort of organization are called metazoans, or eumetazoans when the former is used for animals in general.All animals have eukaryotic cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins.[15] This may be calcified to form structures like shells, bones, and spicules.[16] During development, it forms a relatively flexible framework[17] upon which cells can move about and be reorganized, making complex structures possible. In contrast, other multicellular organisms, like plants and fungi, have cells held in place by cell walls, and so develop by progressive growth.[13] Also, unique to animal cells are the following intercellular junctions: tight junctions, gap junctions, and desmosomes.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 differentiated into separate tissues.
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