Animals are creatures in the kingdom Animalia, one of the five major divisions of organisms (the others are: Monera or bacteria, Fungi, Protists or protozoans, and Plantae or plants). Animals are multicellular, eukaryotic organisms, with cells that do not have walls made of cellulose. Animals are capable of voluntary, spontaneous movements, often in response to sensory perceptions. For their nutrition, animals require sources of biologically fixed energy, that is, the biomass of either plants or other animals. Zoology is the scientific study of animals.
The time of the origin of animals during Earth's evolutionary history is not known, because the first animals were soft-bodied, multicellular life forms that did not preserve well as fossils. By the time multicellular animals became represented in the geological record about 640-670 million years ago, there were already numerous phyla of animals, so the actual origin of the kingdom must have occurred earlier.
Most zoologists recognize the existence of 30-35 phyla of animals, some of which are extinct and only known from their fossil record. The simplest animals with the most ancient lineages are asymmetric or radially symmetric, for example, Porifera (sponges) and Cnidaria (jellyfish and sea anemones). The simplest of the bilaterally symmetric animals include Platyhelminthes (flatworms) and Nematoda (nematodes). Coelomates are a functional group of animals with an enclosed body cavity, the best known of which are Mollusca (snails, clams, octopus, and squid), Annelida (segmented worms and leeches), Arthropoda (insects, spiders, and crustacea), Echinodermata (sea urchins and starfish), and Chordata (fish, amphibians, reptiles, birds, and mammals).
About one million species of animals have been named. However, biologists estimate that a much larger number of animal species has yet to be discovered, and that the actual total could be as large as 30-50 million species. The undiscovered species mostly occur in Earth's richest ecosystems, especially old-growth tropical rain forests, and perhaps the deep oceans. Most of the undiscovered species of terrestrial animals are believed to be insects, especially beetles.