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Animals (also called Metazoa) are multicellular eukaryotic organisms that form the biological kingdom Animalia. With few exceptions, animals Heterotroph, breathe oxygen, are Motility, can reproduce sexually, and grow from a hollow sphere of cells, the blastula, during Embryogenesis. Over 1.5 million Extant taxon animal species have been described—of which around 1 million are Insecta—but it has been estimated there are over 7 million animal species in total. Animals range in length from to . They have Ecology with each other and their environments, forming intricate . The kingdom Animalia includes but in colloquial use the term animal often refers only to non-human animals. The scientific study of animals is known as zoology.
Most living animal species are in Bilateria, a clade whose members have a bilaterally symmetric body plan. The Bilateria include the protostomes—in which many groups of are found, such as , , and —and the deuterostomes, containing both the as well as the , the latter containing the . Life forms interpreted as early animals were present in the Ediacaran biota of the late Precambrian. Many modern animal Phylum became clearly established in the fossil record as Marine life during the Cambrian explosion, which began around 542 million years ago. 6,331 groups of common to all living animals have been identified; these may have arisen from a single common ancestor that lived Cryogenian.
Historically, Aristotle divided animals into those with blood and those without. Carl Linnaeus created the first hierarchical biological classification for animals in 1758 with his Systema Naturae, which Jean-Baptiste Lamarck expanded into 14 Phylum by 1809. In 1874, Ernst Haeckel divided the animal kingdom into the multicellular Metazoa (now synonymous for Animalia) and the Protozoa, single-celled organisms no longer considered animals. In modern times, the biological classification of animals relies on advanced techniques, such as molecular phylogenetics, which are effective at demonstrating the evolutionary relationships between Taxon.
Humans make use of many other animal species, such as for food (including meat, milk, and ), for materials (such as leather and wool), as , and as including for transport. Dogs have been Hunting dog, while many terrestrial and aquatic animals were hunted for sports. Non-human animals have appeared in art from the earliest times and are featured in mythology and religion.
With few exceptions—in particular, the sponges and —animal bodies are differentiated into tissues. These include , which enable locomotion, and , which transmit signals and coordinate the body. Typically, there is also an internal Digestion chamber with either one opening (in Ctenophora, Cnidaria, and flatworms) or two openings (in most bilaterians).
Repeated instances of inbreeding 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 ; or parthenogenesis, where fertile eggs are produced without mating, such as in . (2002). 9780471489689, Wiley. ISBN 9780471489689
Most animals rely on the biomass and energy produced by plants through photosynthesis. Herbivores eat plant material directly, while carnivores, and other animals on higher typically acquire it indirectly by eating other animals. Animals oxidize , , , and other biomolecules to unlock the chemical energy of molecular oxygen, which allows the animal to grow and to sustain biological processes such as locomotion. Animals living close to hydrothermal vents and on the dark Seabed consume organic matter of archaea and bacteria produced in these locations through chemosynthesis (by oxidizing inorganic compounds, such as hydrogen sulfide). (2020). 9780077221249, McGraw-Hill. ISBN 9780077221249
Animals originally evolved in the sea. Lineages of arthropods colonised land around the same time as , probably between 510 and 471 million years ago during the Late Cambrian or Early Ordovician. such as the Sarcopterygii Tiktaalik started to move on to land in the late Devonian, about 375 million years ago. Animals occupy virtually all of earth's and microhabitats, including salt water, hydrothermal vents, fresh water, hot springs, swamps, forests, pastures, deserts, air, and the interiors of animals, plants, fungi and rocks. Animals are however not particularly Thermophile; very few of them can survive at constant temperatures above . Only very few species of animals (mostly nematodes) inhabit the most extreme cold deserts of continental Antarctica.
| 17,000 | Yes (soil) | Yes | 1,750 | Yes | 400 | ||
| Arthropods | 1,257,000 | 1,000,000 (insects) Stork notes that 1m insects have been named, making much larger predicted estimates. | >40,000 (Malac- ostraca) | 94,000 | Yes | >45,000 | |
| Bryozoa | 6,000 | Yes | 60–80 | Yes | |||
| Chordates | 65,000 45,000 | 23,000 | 13,000 | 18,000 9,000 | Yes | 40 (catfish) | |
| Cnidaria | 16,000 | Yes | Yes (few) | Yes | >1,350 (Myxozoa) | ||
| Echinoderms | 7,500 | 7,500 | Yes | ||||
| Molluscs | 85,000 107,000 | 35,000 | 60,000 | 5,000 12,000 | Yes | >5,600 | |
| Nematodes | 25,000 | Yes (soil) | 4,000 | 2,000 | 11,000 | 14,000 | |
| Platyhelminthes | 29,500 | Yes | Yes | 1,300 | Yes 3,000–6,500 | >40,000 4,000–25,000 | |
| 2,000 | >400 | 2,000 | Yes | ||||
| Sponges | 10,800 | Yes | 200-300 | Yes | Yes | ||
The oldest animals are found in the Ediacaran biota, towards the end of the Precambrian, around 610 million years ago. It had long been doubtful whether these included animals, but the discovery of the animal lipid cholesterol in fossils of Dickinsonia establishes that these were indeed animals. Animals are thought to have originated under low-oxygen conditions, suggesting that they were capable of living entirely by anaerobic respiration, but as they became specialized for aerobic metabolism they became fully dependent on oxygen in their environments.
Many animal phyla first appear in the fossil record during the Cambrian explosion, starting about 542 million years ago, in beds such as the Burgess shale. Extant phyla in these rocks include , , , , , and , along with numerous now-extinct forms such as the Anomalocaris. The apparent suddenness of the event may however be an artefact of the fossil record, rather than showing that all these animals appeared simultaneously.
Some palaeontologists have suggested that animals appeared much earlier than the Cambrian explosion, possibly as early as 1 billion years ago. (2020). 9780805371710, Pearson, Benjamin Cummings. ISBN 9780805371710 such as tracks and burrows found in the Tonian period may indicate the presence of triploblastic worm-like animals, roughly as large (about 5 mm wide) and complex as earthworms. However, similar tracks are produced today by the giant single-celled protist Gromia sphaerica, so the Tonian trace fossils may not indicate early animal evolution. Around the same time, the layered mats of called decreased in diversity, perhaps due to grazing by newly evolved animals.
These genes are found in the Placozoa and the higher animals, the Bilateria. 6,331 groups of common to all living animals have been identified; these may have arisen from a single common ancestor that lived Cryogenian in the Precambrian. 25 of these are novel core gene groups, found only in animals; of those, 8 are for essential components of the Wnt and TGF-beta signalling pathways which may have enabled animals to become multicellular by providing a pattern for the body's system of axes (in three dimensions), and another 7 are for transcription factors including homeodomain proteins involved in the control of development.
The phylogenetic tree (of major lineages only) indicates approximately how many millions of years ago () the lineages split.
The Ctenophora (comb jellies) and Cnidaria (which includes jellyfish, , and corals) are radially symmetric and have digestive chambers with a single opening, which serves as both mouth and anus. Animals in both phyla have distinct tissues, but these are not organised into organs. They are diploblastic, having only two main germ layers, ectoderm and endoderm. The tiny are similar, but they do not have a permanent digestive chamber.
Having a front end means that this part of the body encounters stimuli, such as food, favouring cephalisation, the development of a head with and a mouth. Many bilaterians have a combination of circular 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 which swim with cilia and have an apical organ containing sensory cells. However, there are exceptions to each of these characteristics; for example, adult echinoderms are radially symmetric (unlike their larvae), while some Helminths 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, the protostomes and the deuterostomes. The basalmost bilaterians are the Xenacoelomorpha.
The main deuterostome phyla are the Echinodermata and the Chordata. Echinoderms are exclusively marine and include starfish, , and . The chordates are dominated by the vertebrates (animals with Vertebral column), which consist of , , , , and mammals. The deuterostomes also include the Hemichordata (acorn worms).
The Lophotrochozoa includes the , , , , bryozoa and Entoprocta. The molluscs, the second-largest animal phylum by number of described species, includes , , and , while the annelids are the segmented worms, such as , , and . These two groups have long been considered close relatives because they share trochophore larvae.
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, Insecta, Pisces, Amphibia, Aves, and Mammalia. Since then the last four have all been subsumed into a single phylum, the chordate, 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 , annelids, crustaceans, arachnids, insects, worms, Radiata, polyps, and .
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 (echinoderms, cnidaria and other forms).
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.
Animals such as the fruit fly Drosophila melanogaster serve a major role in science as model organism. Animals have been used to create since their discovery in the 18th century. Some medicines such as the cancer drug Yondelis are based on or other molecules of animal origin.
People have used to help chase down and retrieve animals, and birds of prey to catch birds and mammals, while tethered have been used to catch fish. Poison dart frogs have been used to poison the tips of blowdart.
A wide variety of animals are kept as pets, from invertebrates such as tarantulas and octopuses, insects including , reptiles such as and , and birds including canaries, , and all finding a place. However, the most kept pet species are mammals, namely , , and . There is a tension between the role of animals as companions to humans, and their existence as animal rights of their own.
A wide variety of terrestrial and aquatic animals are hunted for sport.
Animals have been the Animal style from the earliest times, both historical, as in Ancient Egypt, and prehistoric, as in the Lascaux. Major animal paintings include Albrecht Dürer's 1515 The Rhinoceros, and George Stubbs's c. 1762 horse portrait Whistlejacket.
Insects, birds and mammals play roles in literature and film, such as in giant bug movies. Animals including 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,Hutchins, M., Arthur V. Evans, Rosser W. Garrison and Neil Schlager (Eds) (2003) Grzimek's Animal Life Encyclopedia, 2nd edition. Volume 3, Insects. Gale, 2003. while the scarab beetle was sacred in ancient Egypt. Among the mammals, cattle, deer, Horse worship, lions, bats, bear worship, and wolves are the subjects of myths and worship. The signs of the Western and are based on animals.
Protostomes 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 archenteron develops into the mouth, and the anus forms secondarily. In deuterostomes, the anus forms first while the mouth develops secondarily. (2020). 9780521837620, Cambridge University Press. ISBN 9780521837620 Most protostomes have Schizocoely, where cells simply fill in the interior of the gastrula to form the mesoderm. In deuterostomes, the mesoderm forms by Enterocoely, through invagination of the endoderm.
(2020). 9780534406530, Cengage Learning. ISBN 9780534406530
Ecdysozoa
The Ecdysozoa are protostomes, named after their shared Phenotypic 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 Kinorhyncha, Priapulida, and Loricifera. These groups have a reduced coelom, called a pseudocoelom.
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 and the . All of these are grouped as the Platytrochozoa, which has a sister group, the Gnathifera, which includes the .
History of classification
In the classical era, Aristotle divided animals, based on his own observations, into those with blood (roughly, the vertebrates) and those without. The animals were then Scala naturae 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.
Hendrik C. D. De Wit (1994). 9782880742645, Presses Polytechniques et Universitaires Romandes. ISBN 9782880742645 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 human culture
The human population exploits a large number of other animal species for food, both of domesticated livestock species in animal husbandry and, mainly at sea, by hunting wild species. Marine fish of many species are fishing for food. A smaller number of species are fish farming.
Invertebrates including , , 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 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 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.
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