Module: Accordion – demo

Etymology

The word “animal” comes from the Latin animalis, meaning ‘having breath’, ‘having soul’ or ‘living being’.[4] The biological definition includes all members of the kingdom Animalia.[5] In colloquial usage, the term animal is often used to refer only to nonhuman animals.[6][7][8][9] The term “metazoa” is derived from the Ancient Greek μετα (meta, meaning “later”) and ζῷᾰ (zōia, plural of ζῷον zōion, meaning animal).[10][11]

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.[12][13] Unlike plants and algae, which produce their own nutrients,[14] animals are heterotrophic,[13][15] feeding on organic material and digesting it internally.[16] With very few exceptions, animals respire aerobically.[a][18] All animals are motile[19] (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 animals, allowing cells to be differentiated into specialised tissues and organs.[20]

Structure

All animals are composed of cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins.[21] 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.[22] 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.[23] Animal cells uniquely possess the cell junctions called tight junctions, gap junctions, and desmosomes.[24]

With few exceptions—in particular, the sponges and placozoans—animal bodies are differentiated into tissues.[25] 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).[26]

Reproduction and development

Sexual reproduction is nearly universal in animals, such as these dragonflies.
Nearly all animals make use of some form of sexual reproduction.[27] They produce haploid gametes by meiosis; the smaller, motile gametes are spermatozoa and the larger, non-motile gametes are ova.[28] These fuse to form zygotes,[29] 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.[30] In most other groups, the blastula undergoes more complicated rearrangement.[31] It first invaginates to form a gastrula with a digestive chamber and two separate germ layers, an external ectoderm and an internal endoderm.[32] In most cases, a third germ layer, the mesoderm, also develops between them.[33] These germ layers then differentiate to form tissues and organs.[34]

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.[35][36] Animals have evolved numerous mechanisms for avoiding close inbreeding.[37]

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.[38][39]

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,[40] and parasites.[41] 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).[42] Selective pressures imposed on one another lead to an evolutionary arms race between predator and prey, resulting in various anti-predator adaptations.[43][44] Almost all multicellular predators are animals.[45] Some consumers use multiple methods; for example, in parasitoid wasps, the larvae feed on the hosts’ living tissues, killing them in the process,[46] but the adults primarily consume nectar from flowers.[47] Other animals may have very specific feeding behaviours, such as hawksbill sea turtles primarily eating sponges.[48]

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, which allows the animal to grow and to sustain biological processes such as locomotion.[49][50][51] 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).[52]

Animals originally evolved in the sea. Lineages of arthropods colonised land around the same time as land plants, probably between 510 and 471 million years ago during the Late Cambrian or Early Ordovician.[53] Vertebrates such as the lobe-finned fish Tiktaalik started to move on to land in the late Devonian, about 375 million years ago.[54][55] 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 other animals, plants, fungi, and rocks.[56] Animals are however not particularly heat tolerant; very few of them can survive at constant temperatures above 50 °C (122 °F).[57] Only very few species of animals (mostly nematodes) inhabit the most extreme cold deserts of continental Antarctica.[58]

Diversity

The blue whale is the largest animal that has ever lived.
The blue whale (Balaenoptera musculus) is the largest animal that has ever lived, weighing up to 190 tonnes and measuring up to 33.6 metres (110 ft) long.[59][60][61] The largest extant terrestrial animal is the African bush elephant (Loxodonta africana), weighing up to 12.25 tonnes[59] and measuring up to 10.67 metres (35.0 ft) long.[59] The largest terrestrial animals that ever lived were titanosaur sauropod dinosaurs such as Argentinosaurus, which may have weighed as much as 73 tonnes, and Supersaurus which may have reached 39 meters.[62][63] Several animals are microscopic; some Myxozoa (obligate parasites within the Cnidaria) never grow larger than 20 µm,[64] and one of the smallest species (Myxobolus shekel) is no more than 8.5 µm when fully grown.[65]

Numbers and habitats of major phyla
The following table lists estimated numbers of described extant species for the major animal phyla,[66] along with their principal habitats (terrestrial, fresh water,[67] and marine),[68] and free-living or parasitic ways of life.[69] 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.[70] 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.[71][72][b]