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550 mya

Non-bilaterian animals

Non-bilaterians include sponges (centre) and corals (background).

Several animal ph

Several animal phyla lack bilateral symmetry. Among these, the sponges (Porifera) probably diverged first, representing the oldest animal phylum.[105] Sponges lack the complex organization found in most other animal phyla;[106] their cells are differentiated, but in most cases not organised into distinct tissues.[107] They typically feed by drawing in water through pores.[108]

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.[109] Animals in both phyla have distinct tissues, but these are not organised into organs.[110] They are diploblastic, having only two main germ layers, ectoderm and endoderm.[111] The tiny 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.[109] Animals in both phyla have distinct tissues, but these are not organised into organs.[110] They are diploblastic, having only two main germ layers, ectoderm and endoderm.[111] The tiny placozoans are similar, but they do not have a permanent digestive chamber.[112][113]

The remaining animals, the great majority—comprising some 29 phyla and over a million species—form a clade, the Bilateria. The body is 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. Animals with this bilaterally symmetric body plan and a tendency to move in one direction have a head end (anterior) and a tail end (posterior) as well as a back (dorsal) and a belly (ventral); therefore they also have a left side and a right side.[114][115]

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;[115] these enable soft-bodied animals with a hydrostatic skeleton to move by peristalsis.[116] 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, there are exceptions to each of these characteristics; for example, adult echinoderms are radially symmetric (unlike their larvae), while some parasitic worms have extremely simplified body structures.[114]<

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;[115] these enable soft-bodied animals with a hydrostatic skeleton to move by peristalsis.[116] 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, there are exceptions to each of these characteristics; for example, adult echinoderms are radially symmetric (unlike their larvae), while some parasitic worms have extremely simplified body structures.[114][115]

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.[117] The basalmost bilaterians are the Xenacoelomorpha.[118][119][120]

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.[121] 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.[122][123] 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.[124]

The main deuterostome phyla are the Echinodermata and the Chordata.[125] Echinoderms are exclusively marine and include starfish, sea urchins, and sea cucumbers.[126] The chordates are dominated by the vertebrates (animals with backbones),[127] which consist of fishes, amphibians, reptiles, birds, and mammals.[128] The deuterostomes also include the Hemichordata (acorn worms).[129][130]

Ecdysozoa
The main deuterostome phyla are the Echinodermata and the Chordata.[125] Echinoderms are exclusively marine and include starfish, sea urchins, and sea cucumbers.[126] The chordates are dominated by the vertebrates (animals with backbones),[127] which consist of fishes, amphibians, reptiles, birds, and mammals.[128] The deuterostomes also include the Hemichordata (acorn worms).[129][130]

The Ecdysozoa are protostomes, named after their shared trait of ecdysis, growth by moulting.[131] 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;[132] some are important parasites.[133] 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.[134]

Spiralia
Spiral cleavage in a sea snail embryo

The Spiralia are a large group of protostomes that develop by spiral cleavage in the early embryo.[135] The Spiralia's phylogeny has been disputed, but it contains a large clade, the s

The Spiralia are a large group of protostomes that develop by spiral cleavage in the early embryo.[135] 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, which includes the rotifers.[136][137]

The Lophotrochozoa includes the molluscs, annelids, brachiopods, nemerteans, bryozoa and entoprocts.[136][138][139] 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 molluscs, annelids, brachiopods, nemerteans, bryozoa and entoprocts.[136][138][139] 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.[140][141]

In the classical era, Aristotle divided animals,[d] based on his own observations, into those with blood (roughly, the vertebrates) and those without. The animals were then 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.[143]

In 1758, Carl Linnaeus created the first hierarchical classification in his Systema Naturae.[144] 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 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

In 1758, Carl Linnaeus created the first hierarchical classification in his Systema Naturae.[144] 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 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)[e] 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, radiates, polyps, and infusorians.[142]

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 (radiata) (echinoderms, cnidaria and other forms).[146] 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.[147]

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.[148][147] The protozoa were later moved to the former kingdom Protista, leaving only the Metazoa as a synonym of Animalia.[149]

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.[150][151] Marine fish of many species are caught commercially for food. A smaller number of species are farmed commercially.[150][152][153] Invertebrates including cephalopods, crustaceans, and bivalve or gastropod molluscs are hunted or farmed for food.[154] Chickens, cattle, sheep, pigs and other animals are raised as livestock for meat across the world.[151][155][156] 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.[157] Dyestuffs including carmine (cochineal),[158][159] shellac,[160][161] and kermes[162][163] 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.[164]

Animals such as the fruit fly Drosophila melanogaster serve a major role in science as experimental models.[165][166][167][168] Animals have been used to create vaccines since their discovery in the 18th century.[169] Some medicines such as the cancer drug Yondelis are based on toxins or other molecules of animal origin.[170]

Animals such as the fruit fly Drosophila melanogaster serve a major role in science as experimental models.[165][166][167][168] Animals have been used to create vaccines since their discovery in the 18th century.[169] Some medicines such as the cancer drug Yondelis are based on toxins or other molecules of animal origin.[170]

People have used hunting dogs to help chase down and retrieve animals,[171] and birds of prey to catch birds and mammals,[172] while tethered cormorants have been used to catch fish.[173] Poison dart frogs have been used to poison the tips of blowpipe darts.[174][175] A wide variety of animals are kept as pets, from invertebrates such as tarantulas and octopuses, insects including praying mantises,[176] reptiles such as snakes and chameleons,[177] and birds including canaries, parakeets, and parrots[178] all finding a place. However, the most kept pet species are mammals, namely dogs, cats, and rabbits.[179][180][181] There is a tension between the role of animals as companions to humans, and their existence as individuals with rights of their own.[182] A wide variety of terrestrial and aquatic animals are hunted for sport.[183]

subjects of art from the earliest times, both historical, as in Ancient Egypt, and prehistoric, as in the cave paintings at Lascaux. Major animal paintings include Albrecht Dürer's 1515 The Rhinoceros, and George Stubbs's c. 1762 horse portrait Whistlejacket.[184] Insects, birds and mammals play roles in literature and film,[185] such as in giant bug movies.[186][187][188] Animals including insects[189] and mammals[190] feature in mythology and religion. In both Japan and Europe, a butterfly was seen as the personification of a person's soul,[189][191][192] while the scarab beetle was sacred in ancient Egypt.[193] Among the mammals, cattle,[194] deer,[190] horses,[195] lions,[196] bats,[197] bears,[198] and wolves[199] are the subjects of myths and worship. The signs of the Western and Chinese zodiacs are based on animals.[200][201]

See also