Etymology
The word "animal" comes from the Latin , meaning 'having breath', 'having soul' or 'living being'. The biological definition includes all members of the kingdom Animalia. In colloquial usage, the term ''animal'' is often used to refer only to nonhuman animals. The term "metazoa" is from Ancient Greek μετα (''meta'', used to mean "later") and ζῷᾰ (''zōia'', plural of ζῷον ''zōion'' "animal").Characteristics
Animals have several characteristics that set them apart from other living things. Animals are eukaryotic and multicellular. Unlike plants and algae, which produce their own nutrients, animals are heterotrophic, feeding on organic material and digesting it internally. With very few exceptions, (example; '' Henneguya zschokkei'') animals respire aerobically. All animals are motile (able to spontaneously move their bodies) during at least part of their life cycle, but some animals, such as sponges, corals, mussels, and barnacles, later become sessile. TheStructure
All animals are composed of cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins. During development, the animal extracellular matrix forms a relatively flexible framework upon which cells can move about and be reorganised, making the formation of complex structures possible. This may be calcified, forming structures such as shells, bones, andReproduction and development
Nearly all animals make use of some form of sexual reproduction. They produce haploid gametes by meiosis; the smaller, motile gametes are spermatozoa and the larger, non-motile gametes are ova. These fuse to form zygotes, which develop via mitosis into a hollow sphere, called a blastula. In sponges, blastula larvae swim to a new location, attach to the seabed, and develop into a new sponge. In most other groups, the blastula undergoes more complicated rearrangement. It first invaginates to form a gastrula with a digestive chamber and two separate germ layers, an external ectoderm and an internal endoderm. In most cases, a third germ layer, the mesoderm, also develops between them. These germ layers then differentiate to form tissues and organs. Repeated instances of mating with a close relative during sexual reproduction generally leads to inbreeding depression within a population due to the increased prevalence of harmful recessive traits. Animals have evolved numerous mechanisms for avoiding close inbreeding. Some animals are capable of asexual reproduction, which often results in a genetic clone of the parent. This may take place through fragmentation; budding, such as in ''Hydra'' and other cnidarians; or parthenogenesis, where fertile eggs are produced without mating, such as in aphids.Ecology
Animals are categorised into ecological groups depending on how they obtain or consume organic material, including carnivores, herbivores, omnivores, detritivores, and parasites. Interactions between animals form complex food webs. In carnivorous or omnivorous species, predation is a consumer–resource interaction where a predator feeds on another organism (called its ''prey''). Selective pressures imposed on one another lead to an evolutionary arms race between predator and prey, resulting in various anti-predator adaptations. Almost all multicellular predators are animals. Some consumers use multiple methods; for example, in parasitoid wasps, the larvae feed on the hosts' living tissues, killing them in the process, but the adults primarily consume nectar from flowers. Other animals may have very specificDiversity
Size
The blue whale (''Balaenoptera musculus'') is the largest animal that has ever lived, weighing up to 190 tonnes and measuring up to long. The largest extant terrestrial animal is the African bush elephant (''Loxodonta africana''), weighing up to 12.25 tonnes and measuring up to long. The largest terrestrial animals that ever lived wereNumbers and habitats
The following table lists estimated numbers of described extant species for the animal groups with the largest numbers of species, along with their principal habitats (terrestrial, fresh water, and marine), and free-living or parasitic ways of life. Species estimates shown here are based on numbers described scientifically; much larger estimates have been calculated based on various means of prediction, and these can vary wildly. For instance, around 25,000–27,000 species of nematodes have been described, while published estimates of the total number of nematode species include 10,000–20,000; 500,000; 10 million; and 100 million. Using patterns within the taxonomic hierarchy, the total number of animal species—including those not yet described—was calculated to be about 7.77 million in 2011.Evolutionary origin
Animals are found as long ago as the Ediacaran biota, towards the end of the Precambrian, and possibly somewhat earlier. It had long been doubted whether these life-forms included animals, but the discovery of the animal lipid cholesterol in fossils of '' Dickinsonia'' establishes their nature. Animals are thought to have originated under low-oxygen conditions, suggesting that they were capable of living entirely byPhylogeny
Animals are monophyletic, meaning they are derived from a common ancestor. Animals are sister to the Choanoflagellata, with which they form theNon-Bilateria
Several animal phyla lack bilateral symmetry. Among these, the sponges (Porifera) probably diverged first, representing the oldest animal phylum. Sponges lack the complex organization found in most other animal phyla; their cells are differentiated, but in most cases not organised into distinct tissues. They typically feed by drawing in water through pores. The Ctenophora (comb jellies) and Cnidaria (which includes jellyfish, sea anemones, and corals) are radially symmetric and have digestive chambers with a single opening, which serves as both mouth and anus. They are sometimes placed together in the group Coelenterata because of common traits, not because of close relationships. Animals in both phyla have distinct tissues, but these are not organised into organs. They areBilateria
The remaining animals, the great majority—comprising some 29 phyla and over a million species—form a clade, the Bilateria, which have a bilaterally symmetric body plan. The Bilateria are triploblastic, with three well-developed germ layers, and their tissues form distinct organs. The digestive chamber has two openings, a mouth and an anus, and there is an internal body cavity, a coelom or pseudocoelom. These animals have a head end (anterior) and a tail end (posterior), a back (dorsal) surface and a belly (ventral) surface, and a left and a right side. Having a front end means that this part of the body encounters stimuli, such as food, favouring cephalisation, the development of a head with sense organs and a mouth. Many bilaterians have a combination of circular muscles that constrict the body, making it longer, and an opposing set of longitudinal muscles, that shorten the body; these enable soft-bodied animals with a hydrostatic skeleton to move by peristalsis. They also have a gut that extends through the basically cylindrical body from mouth to anus. Many bilaterian phyla have primary larvae which swim with cilia and have an apical organ containing sensory cells. However, over evolutionary time, descendant spaces have evolved which have lost one or more of each of these characteristics. For example, adult echinoderms are radially symmetric (unlike their larvae), while some parasitic worms have extremely simplified body structures. Genetic studies have considerably changed zoologists' understanding of the relationships within the Bilateria. Most appear to belong to two major lineages, theProtostomes and deuterostomes
Protostomes and deuterostomes differ in several ways. Early in development, deuterostome embryos undergo radial cleavage during cell division, while many protostomes (the Spiralia) undergo spiral cleavage. Animals from both groups possess a complete digestive tract, but in protostomes the first opening of the embryonic gut develops into the mouth, and the anus forms secondarily. In deuterostomes, the anus forms first while the mouth develops secondarily. Most protostomes have schizocoelous development, where cells simply fill in the interior of the gastrula to form the mesoderm. In deuterostomes, the mesoderm forms by enterocoelic pouching, through invagination of the endoderm. The main deuterostome phyla are the Echinodermata and the Chordata. Echinoderms are exclusively marine and include starfish, sea urchins, and sea cucumbers. The chordates are dominated by the vertebrates (animals with backbones), which consist of fishes,=Ecdysozoa
= The Ecdysozoa are protostomes, named after their shared trait of ecdysis, growth by moulting. They include the largest animal phylum, the Arthropoda, which contains insects, spiders, crabs, and their kin. All of these have a body divided into repeating segments, typically with paired appendages. Two smaller phyla, the Onychophora and Tardigrada, are close relatives of the arthropods and share these traits. The ecdysozoans also include the Nematoda or roundworms, perhaps the second largest animal phylum. Roundworms are typically microscopic, and occur in nearly every environment where there is water; some are important parasites. Smaller phyla related to them are the Nematomorpha or horsehair worms, and the= Spiralia
= The Spiralia are a large group of protostomes that develop by spiral cleavage in the early embryo. The Spiralia's phylogeny has been disputed, but it contains a large clade, the superphylum Lophotrochozoa, and smaller groups of phyla such as the Rouphozoa which includes the gastrotrichs and the flatworms. All of these are grouped as the Platytrochozoa, which has a sister group, the Gnathifera (clade), Gnathifera, which includes the rotifers. The Lophotrochozoa includes the molluscs, annelids, brachiopods, nemerteans, bryozoa and Entoprocta, entoprocts. The molluscs, the second-largest animal phylum by number of described species, includes snails, clams, and squids, while the annelids are the segmented worms, such as earthworms, lugworms, and leeches. These two groups have long been considered close relatives because they share trochophore larvae.History of classification
In the classical era, Aristotle Aristotle's biology, divided animals, based on his own observations, into those with blood (roughly, the vertebrates) and those without. The animals were then Scala naturae, arranged on a scale from man (with blood, 2 legs, rational soul) down through the live-bearing tetrapods (with blood, 4 legs, sensitive soul) and other groups such as crustaceans (no blood, many legs, sensitive soul) down to spontaneously generating creatures like sponges (no blood, no legs, vegetable soul). Aristotle was uncertain whether sponges were animals, which in his system ought to have sensation, appetite, and locomotion, or plants, which did not: he knew that sponges could sense touch, and would contract if about to be pulled off their rocks, but that they were rooted like plants and never moved about. In 1758, Carl Linnaeus created the first hierarchical classification in his '' Systema Naturae''. In his original scheme, the animals were one of three kingdoms, divided into the classes of Vermes in the 10th edition of Systema Naturae, Vermes, Insecta in the 10th edition of Systema Naturae, Insecta, Pisces in the 10th edition of Systema Naturae, Pisces, Amphibia in the 10th edition of Systema Naturae, Amphibia, Aves in the 10th edition of Systema Naturae, Aves, and Mammalia in the 10th edition of Systema Naturae, Mammalia. Since then the last four have all been subsumed into a single phylum, the chordate, Chordata, while his Insecta (which included the crustaceans and arachnids) and Vermes have been renamed or broken up. The process was begun in 1793 by Jean-Baptiste de Lamarck, who called the Vermes ''une espèce de chaos'' (a chaotic mess) and split the group into three new phyla: worms, echinoderms, and polyps (which contained corals and jellyfish). By 1809, in his ''Philosophie Zoologique'', Lamarck had created 9 phyla apart from vertebrates (where he still had 4 phyla: mammals, birds, reptiles, and fish) and molluscs, namely cirripedes, annelids, crustaceans, arachnids, insects, worms, Radiata, radiates, polyps, and infusorians. In his 1817 ''Le Règne Animal'', Georges Cuvier used comparative anatomy to group the animals into four ''embranchements'' ("branches" with different body plans, roughly corresponding to phyla), namely vertebrates, molluscs, articulated animals (arthropods and annelids), and zoophytes, zoophytes (radiata) (echinoderms, cnidaria and other forms). This division into four was followed by the embryologist Karl Ernst von Baer in 1828, the zoologist Louis Agassiz in 1857, and the comparative anatomist Richard Owen in 1860. In 1874, Ernst Haeckel divided the animal kingdom into two subkingdoms: Metazoa (multicellular animals, with five phyla: coelenterates, echinoderms, articulates, molluscs, and vertebrates) and Protozoa (single-celled animals), including a sixth animal phylum, sponges. The protozoa were later moved to the former kingdom Protista, leaving only the Metazoa as a synonym of Animalia.In human culture
Practical uses
The human population exploits a large number of other animal species for food, both of domestication of animals, domesticated livestock species in animal husbandry and, mainly at sea, by hunting wild species. Marine fish of many species are fishing, caught commercially for food. A smaller number of species are fish farming, farmed commercially. Humans and their livestock make up more than 90% of the biomass of all terrestrial vertebrates, and almost as much as all insects combined. Invertebrates including cephalopods, crustaceans, and bivalve or gastropod molluscs are hunted or farmed for food. Chickens, cattle, sheep, pigs, and other animals are raised as livestock for meat across the world. Animal fibres such as wool are used to make textiles, while animal sinews have been used as lashings and bindings, and leather is widely used to make shoes and other items. Animals have been hunted and farmed for their fur to make items such as coats and hats. Dyestuffs including carmine (cochineal), shellac, and Kermes (dye), kermes have been made from the bodies of insects. Working animals including cattle and horses have been used for work and transport from the first days of agriculture. Animals such as the fruit fly ''Drosophila melanogaster'' serve a major role in science as model organism, experimental models. Animals have been used to create vaccines since their discovery in the 18th century. Some medicines such as the cancer drug Yondelis are based on toxins or other molecules of animal origin. People have used hunting dogs to help chase down and retrieve animals, and Bird of prey, birds of prey to catch birds and mammals, while tethered cormorants have been Cormorant fishing, used to catch fish. Poison dart frogs have been used to poison the tips of blowdart, blowpipe darts. A wide variety of animals are kept as pets, from invertebrates such as tarantulas and octopuses, insects including praying mantises, reptiles such as snakes and chameleons, and birds including Domestic canary, canaries, parakeets, and parrots all finding a place. However, the most kept pet species are mammals, namely dogs, cats, and rabbits. There is a tension between the role of animals as companions to humans, and their existence as animal rights, individuals with rights of their own. A wide variety of terrestrial and aquatic animals are hunted Animals in sport, for sport.Symbolic uses
Animals have been the Animal style, subjects of art from the earliest times, both historical, as in Ancient Egypt, and prehistoric, as in the Lascaux, cave paintings at Lascaux. Major animal paintings include Albrecht Dürer's 1515 ''Dürer's Rhinoceros, The Rhinoceros'', and George Stubbs's c. 1762 horse portrait ''Whistlejacket''. Arthropods in film, Insects, birds and mammals play roles in literature and film, such as in Big bug movie, giant bug movies. Animals including Insects in mythology, insects and mammals feature in mythology and religion. In both Japan and Europe, a butterfly was seen as the personification of a person's soul, while the Scarab (artifact), scarab beetle was sacred in ancient Egypt. Among the mammals, Cattle in religion and mythology, cattle, Deer in mythology, deer, Horse worship, horses, Cultural depictions of lions, lions, Bat#Cultural significance, bats, bear worship, bears, and Wolves in folklore, religion and mythology, wolves are the subjects of myths and worship. The signs of the zodiac, signs of the Western and Chinese zodiacs are based on animals.See also
* Animal attacks * Animal coloration * Ethology * Fauna * List of animal names * Lists of organisms by populationNotes
References
External links