Animal

Animal

For other uses, see Animal (disambiguation).

"Animalia" redirects here. For other uses, see Animalia (disambiguation).

Animals
Temporal range: Cryogenian – present, 665–0 Ma

Pha.

Proterozoic

Archean

Had‘n

Scientific classification

Domain:Eukaryota

(unranked):Unikonta

(unranked):Obazoa

(unranked):Opisthokonta

(unranked):Holozoa

(unranked):Filozoa

Kingdom:Animalia
Linnaeus, 1758

Major divisions

show

Major animal taxa

Synonyms

Metazoa

Choanoblastaea

Animals (also called Metazoa) are multicellular eukaryotic organisms that form the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and grow from a hollow sphere of cells, the blastula, during embryonic development. Over 1.5 million living animal species have been described—of which around 1 million are insects—but it has been estimated there are over 7 million animal species in total. Animals range in length from 8.5 micrometres (0.00033 in) to 33.6 metres (110 ft). They have complex interactions with each other and their environments, forming intricate food webs. 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.

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 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. Life forms interpreted as early animals were present in the Ediacaran biota of the late Precambrian. Many modern animal phyla became clearly established in the fossil record as marine species during the Cambrian explosion, which began around 542 million years ago. 6,331 groups of genes common to all living animals have been identified; these may have arisen from a single common ancestor that lived 650 million years ago.

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 phyla 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 animal taxa.

Humans make use of many other animal species, such as for food (including meat, milk, and eggs), for materials (such as leather and wool), and also as pets, and for transports, as working animals. 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.

Contents

1Etymology

2Characteristics

2.1Structure

2.2Reproduction and development

3Ecology

4Diversity

4.1Largest and smallest

4.2Numbers and habitats

5Evolutionary origin

6Phylogeny

6.1Non-bilaterian animals

6.2Bilaterian animals6.2.1Protostomes and deuterostomes

6.2.1.1Ecdysozoa

6.2.1.2Spiralia

7History of classification

8In human culture

9See also

10Notes

11References

12External links

Etymology

The word "animal" comes from the Latin animalis, meaning having breath, having soul or living being.[1] The biological definition includes all members of the kingdom Animalia.[2] In colloquial usage, as a consequence of anthropocentrism, the term animal is sometimes used nonscientifically to refer only to non-human animals.[3][4][5][6]

Characteristics

Animals are unique in having the ball of cells of the early embryo (1) develop into a hollow ball or blastula (2).

Animals have several characteristics that set them apart from other living things. Animals are eukaryotic and multicellular,[7][8] unlike bacteria, which are prokaryotic, and unlike protists, which are eukaryotic but unicellular. Unlike plants and algae, which produce their own nutrients[9] animals are heterotrophic,[8][10] feeding on organic material and digesting it internally.[11] With very few exceptions, animals respire aerobically.[12] All animals are motile[13] (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. The blastula is a stage in embryonic development that is unique to most animals,[14] allowing cells to be differentiated into specialised tissues and organs.

Structure

All animals are composed of cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins.[15] 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, and spicules.[16] 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.[17] Animal cells uniquely possess the cell junctions called tight junctions, gap junctions, and desmosomes.[18]

With few exceptions—in particular, the sponges and placozoans—animal bodies are differentiated into tissues.[19] These include muscles, which enable locomotion, and nerve tissues, which transmit signals and coordinate the body. Typically, there is also an internal digestive chamber with either one opening (in Ctenophora, Cnidaria, and flatworms) or two openings (in most bilaterians).[20]

Reproduction and development

Sexual reproduction is nearly universal in animals, such as these dragonflies.

See also: Sexual reproduction § Animals, and Asexual reproduction § Examples in animals

Nearly all animals make use of some form of sexual reproduction.[21] They produce haploid gametes by meiosis; the smaller, motile gametes are spermatozoa and the larger, non-motile gametes are ova.[22] These fuse to form zygotes,[23] 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.[24] In most other groups, the blastula undergoes more complicated rearrangement.[25] It first invaginates to form a gastrula with a digestive chamber and two separate germ layers, an external ectoderm and an internal endoderm.[26] In most cases, a third germ layer, the mesoderm, also develops between them.[27] These germ layers then differentiate to form tissues and organs.[28]

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.[29][30] Animals have evolved numerous mechanisms for avoiding close inbreeding.[31]

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.[32][33]

Ecology

Predators, such as this ultramarine flycatcher (Ficedula superciliaris), feed on other animals.

Animals are categorised into ecological groups depending on how they obtain or consume organic material, including carnivores, herbivores, omnivores, detritivores,[34] and parasites.[35] 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).[36] Selective pressures imposed on one another lead to an evolutionary arms race between predator and prey, resulting in various anti-predator adaptations.[37][38] Almost all multicellular predators are animals.[39] Some consumers use multiple methods; for example, in parasitoid wasps, the larvae feed on the hosts’ living tissues, killing them in the process,[40] but the adults primarily consume nectar from flowers.[41] Other animals may have very specific feeding behaviours, such as hawksbill sea turtles primarily eating sponges.[42]

Hydrothermal vent mussels and shrimps

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,[43] which allows the animal to grow and to sustain biological processes such as locomotion.[44][45][46] 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).[47]