For other uses, see Animal (disambiguation).
"Animalia" redirects here.
For other uses, see Animalia (disambiguation).
Animals range in length from 8.5 micrometres (0.00033 in) to 33.6 metres (110 ft).
The kingdom Animalia includes humans but in colloquial use the term animal often refers only to non-human animals.
The scientific study of animals is known as zoology.
The Bilateria include the protostomes—in which many groups of invertebrates are found, such as nematodes, arthropods, and molluscs—and the deuterostomes, containing both the echinoderms as well as the chordates, the latter containing the vertebrates.
Historically, Aristotle divided animals into those with blood and those without.
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 taxa.
Dogs have been used in hunting, 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.
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, as a consequence of anthropocentrism, the term animal is sometimes used nonscientifically to refer only to non-human animals.
Animals have several characteristics that set them apart from other living things.
With very few exceptions, animals respire aerobically.
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.
In contrast, the cells of other multicellular organisms (primarily algae, plants, and fungi) are held in place by cell walls, and so develop by progressive growth.
Typically, there is also an internal digestive chamber with either one opening (in Ctenophora, Cnidaria, and flatworms) or two openings (in most bilaterians).
Reproduction and development
Nearly all animals make use of some form of sexual reproduction.
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.
In most cases, a third germ layer, the mesoderm, also develops between them.
These germ layers then differentiate to form tissues and organs.
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.
Interactions between animals form complex food webs.
Almost all multicellular predators are animals.
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 trophic levels typically acquire it indirectly by eating other animals.
Animals oxidize carbohydrates, lipids, proteins, 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 cold seeps on the dark sea floor consume organic matter of archaea and bacteria produced in these locations through chemosynthesis (by oxidizing inorganic compounds, such as hydrogen sulfide).
Animals originally evolved in the sea.
Animals occupy virtually all of earth's habitats 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 heat tolerant; very few of them can survive at constant temperatures above 50 °C (122 °F).
Largest and smallest
The largest extant terrestrial animal is the African bush elephant (Loxodonta africana), weighing up to 12.25 tonnes and measuring up to 10.67 metres (35.0 ft) long.
Several animals are microscopic; some Myxozoa (obligate parasites within the Cnidaria) never grow larger than 20 µm, and one of the smallest species (Myxobolus shekel) is no more than 8.5 µm when fully grown.
Numbers 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.
|Total number of described extant species as of 2013: 1,525,728|
Further information: Urmetazoan
These fossils are interpreted as most probably being early sponges.
The oldest animals are found in the Ediacaran biota, towards the end of the Precambrian, around 610 million years ago.
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.
Extant phyla in these rocks include molluscs, brachiopods, onychophorans, tardigrades, arthropods, echinoderms and hemichordates, along with numerous now-extinct forms such as the predatory 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.
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.
Further information: Lists of animals
Animals are monophyletic, meaning they are derived from a common ancestor.
These genes are found in the Placozoa and the higher animals, the Bilateria.
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 (mya) the lineages split.
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.
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 placozoans are similar, but they do not have a permanent digestive chamber.
The remaining animals, the great majority—comprising some 29 phyla and over a million species—form a clade, the Bilateria.
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.
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.
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.
Genetic studies have considerably changed zoologists' understanding of the relationships within the Bilateria.
The basalmost bilaterians are the Xenacoelomorpha.
Protostomes and deuterostomes
Further information: Embryological origins of the mouth and anus
Protostomes and deuterostomes differ in several ways.
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.
The deuterostomes also include the Hemichordata (acorn worms).
Main article: Ecdysozoa
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.
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.
These groups have a reduced coelom, called a pseudocoelom.
Main article: 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.
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
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.
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) 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.
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).
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
Main article: Animals in culture
Marine fish of many species are caught commercially for food.
A smaller number of species are farmed commercially.
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.
Working animals including cattle and horses have been used for work and transport from the first days of agriculture.
Animals have been used to create vaccines since their discovery in the 18th century.
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 canaries, parakeets, and parrots all finding a place.
There is a tension between the role of animals as companions to humans, and their existence as individuals with rights of their own.
A wide variety of terrestrial and aquatic animals are hunted for sport.
Animals including insects and mammals feature in mythology and religion.
- Animal attacks
- Animal coloration
- List of animal names
- Lists of organisms by population
Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Animal.