Animal

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Animals
Temporal range: Cryogenian – present, 665–0 Ma
Animal diversity b.png
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Clade: Amorphea
Clade: Obazoa
(unranked): Opisthokonta
(unranked): Holozoa
(unranked): Filozoa
Kingdom: Animalia
Linnaeus, 1758
Subdivisions
Synonyms
  • Metazoa Haeckel 1874 [1]
  • Choanoblastaea Nielsen 2008 [2]
  • Gastrobionta Rothm. 1948 [3]
  • Zooaea Barkley 1939 [3]
  • Euanimalia Barkley 1939 [3]

Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, have myocytes and are able to move, can reproduce sexually, and grow from a hollow sphere of cells, the blastula, during embryonic development. Animals form a clade, meaning that they arose from a single common ancestor.

Contents

Over 1.5 million living animal species have been described—of which around 1.05 million are insects, over 85,000 are molluscs, and around 65,000 are vertebrates. It has been estimated there are as many as 7.77 million animal species on Earth. Animal body lengths range from 8.5 μm (0.00033 in) to 33.6 m (110 ft). They have complex ecologies and interactions with each other and their environments, forming intricate food webs. The scientific study of animals is known as zoology, and the study of animal behaviors is known as ethology.

Most living animal species belong to the infrakingdom Bilateria, a highly proliferative clade whose members have a bilaterally symmetric body plan. Extant bilaterians include the basal group Xenacoelomorpha, but the vast majority belong to two large superphyla: the protostomes, which includes organisms such as the arthropods, molluscs, flatworms, annelids and nematodes; and the deuterostomes, which include the echinoderms, hemichordates and chordates, the latter of which contains the vertebrates.

Animals first appear in the fossil record in the late Cryogenian period, and diversified in the subsequent Ediacaran. Earlier evidence of animals is still controversial; the sponge-like organism Otavia has been dated all the way back to the Tonian period, but its identity as an animal is heavily contested. [4] Nearly all modern animal phyla became clearly established in the fossil record as marine species during the Cambrian explosion, which began around 539  million years ago (Mya), and most classes during the Ordovician radiation 485.4 Mya. 6,331 groups of genes common to all living animals have been identified; these may have arisen from a single common ancestor that lived about 650 Mya during the Cryogenian period.

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 with 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 taxa.

Humans make use of many other animal species for food (including meat, eggs and dairies), for materials (such as leather, fur and wool), as pets and as working animals for transportation, and services. Dogs, the first domesticated animal, have been used in hunting, in security and in warfare, as have horses, pigeons and birds of prey, while other terrestrial and aquatic animals are hunted for sports, trophies or profits. Non-human animals are also an important cultural element of human evolution, having appeared in cave arts and totems since the earliest times, and are frequently featured in mythology, religion, arts, literature, heraldry, politics and sports.

Etymology

The word animal comes from the Latin noun animal of the same meaning, which is itself derived from Latin animalis 'having breath or soul'. [5] The biological definition includes all members of the kingdom Animalia. [6] In colloquial usage, the term animal is often used to refer only to nonhuman animals. [7] [8] [9] [10] The term metazoa is derived from Ancient Greek μετα (meta) 'after' (in biology, the prefix meta- stands for 'later') and ζῷᾰ (zōia) 'animals', plural of ζῷον zōion 'animal'. [11] [12]

Characteristics

Animals are unique in having the ball of cells of the early embryo (1) develop into a hollow ball or blastula (2). Blastulation.png
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. [13] Unlike plants and algae, which produce their own nutrients, [14] animals are heterotrophic, [15] [16] feeding on organic material and digesting it internally. [17] With very few exceptions, animals respire aerobically. [lower-alpha 1] [19] All animals are motile [20] (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. [21]

Structure

All animals are composed of cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins. [22] 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. [23] 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. [24] Animal cells uniquely possess the cell junctions called tight junctions, gap junctions, and desmosomes. [25]

With few exceptions—in particular, the sponges and placozoans—animal bodies are differentiated into tissues. [26] 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). [27]

Reproduction and development

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

Nearly all animals make use of some form of sexual reproduction. [28] They produce haploid gametes by meiosis; the smaller, motile gametes are spermatozoa and the larger, non-motile gametes are ova. [29] These fuse to form zygotes, [30] 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. [31] In most other groups, the blastula undergoes more complicated rearrangement. [32] It first invaginates to form a gastrula with a digestive chamber and two separate germ layers, an external ectoderm and an internal endoderm. [33] In most cases, a third germ layer, the mesoderm, also develops between them. [34] These germ layers then differentiate to form tissues and organs. [35]

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. [36] [37] Animals have evolved numerous mechanisms for avoiding close inbreeding. [38]

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. [39] [40]

Ecology

Predators, such as this ultramarine flycatcher (Ficedula superciliaris), feed on other animals. Ultramarine Flycatcher (Ficedula superciliaris) Naggar, Himachal Pradesh, 2013 (cropped).JPG
Predators, such as this ultramarine flycatcher (Ficedula superciliaris), feed on other animals.

Animals are categorised into ecological groups depending on their trophic levels and how they consume organic material. Such groupings include carnivores (further divided into subcategories such as piscivores, insectivores, ovivores, etc.), herbivores (subcategorized into folivores, graminivores, frugivores, granivores, nectarivores, algivores, etc.), omnivores, fungivores, scavengers/detritivores, [41] and parasites. [42] Interactions between animals of each biome form complex food webs within that ecosystem. In carnivorous or omnivorous species, predation is a consumer–resource interaction where the predator feeds on another organism, its prey, [43] who often evolves anti-predator adaptations to avoid being fed upon. Selective pressures imposed on one another lead to an evolutionary arms race between predator and prey, resulting in various antagonistic/competitive coevolutions. [44] [45] Almost all multicellular predators are animals. [46] Some consumers use multiple methods; for example, in parasitoid wasps, the larvae feed on the hosts' living tissues, killing them in the process, [47] but the adults primarily consume nectar from flowers. [48] Other animals may have very specific feeding behaviours, such as hawksbill sea turtles which mainly eat sponges. [49]

Hydrothermal vent mussels and shrimps Expl0072 - Flickr - NOAA Photo Library.jpg
Hydrothermal vent mussels and shrimps

Most animals rely on biomass and bioenergy produced by plants and phytoplanktons (collectively called producers) through photosynthesis. Herbivores, as primary consumers, eat the plant material directly to digest and absorb the nutrients, while carnivores and other animals on higher trophic levels indirectly acquire the nutrients by eating the herbivores or other animals that have eaten the herbivores. Animals oxidize carbohydrates, lipids, proteins and other biomolecules, which allows the animal to grow and to sustain basal metabolism and fuel other biological processes such as locomotion. [50] [51] [52] Some benthic animals living close to hydrothermal vents and cold seeps on the dark sea floor consume organic matter produced through chemosynthesis (via oxidizing inorganic compounds such as hydrogen sulfide) by archaea and bacteria. [53]

Animals 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. [54] Vertebrates such as the lobe-finned fish Tiktaalik started to move on to land in the late Devonian, about 375 million years ago. [55] [56] Animals occupy virtually all of earth's habitats and microhabitats, with faunas adapted to salt water, hydrothermal vents, fresh water, hot springs, swamps, forests, pastures, deserts, air, and the interiors of other organisms. [57] Animals are however not particularly heat tolerant; very few of them can survive at constant temperatures above 50 °C (122 °F) [58] or in the most extreme cold deserts of continental Antarctica. [59]

Diversity

Size

The blue whale is the largest animal that has ever lived. Anim1754 - Flickr - NOAA Photo Library (1).jpg
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. [60] [61] [62] The largest extant terrestrial animal is the African bush elephant (Loxodonta africana), weighing up to 12.25 tonnes [60] and measuring up to 10.67 metres (35.0 ft) long. [60] 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. [63] [64] Several animals are microscopic; some Myxozoa (obligate parasites within the Cnidaria) never grow larger than 20  µm, [65] and one of the smallest species (Myxobolus shekel) is no more than 8.5 µm when fully grown. [66]

Numbers and habitats of major phyla

The following table lists estimated numbers of described extant species for the major animal phyla, [67] along with their principal habitats (terrestrial, fresh water, [68] and marine), [69] and free-living or parasitic ways of life. [70] 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. [71] 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. [72] [73] [lower-alpha 2]

Phylum ExampleDescribed species Land Sea Freshwater Free-living Parasitic
Arthropoda European wasp white bg02.jpg 1,257,000 [67] Yes 1,000,000
(insects) [75] 		Yes >40,000
(Malac-
ostraca) [76] 		Yes 94,000 [68] Yes [69] Yes >45,000 [lower-alpha 3] [70]
Mollusca Grapevinesnail 01.jpg 85,000 [67]
107,000 [77] 		Yes 35,000 [77] Yes 60,000 [77] Yes 5,000 [68]
12,000 [77] 		Yes [69] Yes >5,600 [70]
Chordata Lithobates pipiens.jpg >70,000 [67] [78] Yes 23,000 [79] Yes 13,000 [79] Yes 18,000 [68]
9,000 [79] 		YesYes 40
(catfish) [80] [70]
Platyhelminthes Pseudoceros dimidiatus.jpg 29,500 [67] Yes [81] Yes [69] Yes 1,300 [68] Yes [69]

3,000–6,500 [82]

Yes >40,000 [70]

4,000–25,000 [82]

Nematoda CelegansGoldsteinLabUNC.jpg 25,000 [67] Yes (soil) [69] Yes 4,000 [71] Yes 2,000 [68] Yes
11,000 [71] 		Yes 14,000 [71]
Annelida Nerr0328.jpg 17,000 [67] Yes (soil) [69] Yes [69] Yes 1,750 [68] YesYes 400 [70]
Cnidaria FFS Table bottom.jpg 16,000 [67] Yes [69] Yes (few) [69] Yes [69] Yes >1,350
(Myxozoa) [70]
Porifera A colourful Sponge on the Fathom.jpg 10,800 [67] Yes [69] 200–300 [68] YesYes [83]
Echinodermata Starfish, Caswell Bay - geograph.org.uk - 409413.jpg 7,500 [67] Yes 7,500 [67] Yes [69]
Bryozoa Bryozoan at Ponta do Ouro, Mozambique (6654415783).jpg 6,000 [67] Yes [69] Yes 60–80 [68] Yes
Rotifera 20090730 020239 Rotifer.jpg 2,000 [67] Yes >400 [84] Yes 2,000 [68] Yes
Nemertea Nemerte.jpg 1,350 [85] [86] YesYesYes
Tardigrada Tardigrade (50594282802).jpg 1,335 [67] Yes [87]
(moist plants)		YesYesYes
Total number of described extant species as of 2013: 1,525,728 [67]

Evolutionary origin

Evidence of animals is found as long ago as the Cryogenian period. 24-Isopropylcholestane (24-ipc) has been found in rocks from roughly 650 million years ago; it is only produced by sponges and pelagophyte algae. Its likely origin is from sponges based on molecular clock estimates for the origin of 24-ipc production in both groups. Analyses of pelagophyte algae consistently recover a Phanerozoic origin, while analyses of sponges recover a Neoproterozoic origin, consistent with the appearance of 24-ipc in the fossil record. [88] [89]

The first body fossils of animals appear in the Ediacaran, represented by forms such as Charnia and Spriggina . It had long been doubted whether these fossils truly represented animals, [90] [91] [92] but the discovery of the animal lipid cholesterol in fossils of Dickinsonia establishes their nature. [93] 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. [94]

Many animal phyla first appear in the fossil record during the Cambrian explosion, starting about 539 million years ago, in beds such as the Burgess shale. [95] 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 artifact of the fossil record, rather than showing that all these animals appeared simultaneously. [96] [97] [98] [99] That view is supported by the discovery of Auroralumina attenboroughii , the earliest known Ediacaran crown-group cnidarian (557–562 mya, some 20 million years before the Cambrian explosion) from Charnwood Forest, England. It is thought to be one of the earliest predators, catching small prey with its nematocysts as modern cnidarians do. [100]

Some palaeontologists have suggested that animals appeared much earlier than the Cambrian explosion, possibly as early as 1 billion years ago. [101] Early fossils that might represent animals appear for example in the 665-million-year-old rocks of the Trezona Formation of South Australia. These fossils are interpreted as most probably being early sponges. [102] Trace fossils such as tracks and burrows found in the Tonian period (from 1 gya) may indicate the presence of triploblastic worm-like animals, roughly as large (about 5 mm wide) and complex as earthworms. [103] However, similar tracks are produced by the giant single-celled protist Gromia sphaerica , so the Tonian trace fossils may not indicate early animal evolution. [104] [105] Around the same time, the layered mats of microorganisms called stromatolites decreased in diversity, perhaps due to grazing by newly evolved animals. [106] Objects such as sediment-filled tubes that resemble trace fossils of the burrows of wormlike animals have been found in 1.2 gya rocks in North America, in 1.5 gya rocks in Australia and North America, and in 1.7 gya rocks in Australia. Their interpretation as having an animal origin is disputed, as they might be water-escape or other structures. [107] [108]

Phylogeny

External phylogeny

Animals are monophyletic, meaning they are derived from a common ancestor. Animals are the sister group to the choanoflagellates, with which they form the Choanozoa. [109] The dates on the phylogenetic tree indicate approximately how many millions of years ago (mya) the lineages split. [110] [111] [112] [113] [114]

Ros-Rocher and colleagues (2021) trace the origins of animals to unicellular ancestors, providing the external phylogeny shown in the cladogram. Uncertainty of relationships is indicated with dashed lines. [115]

Opisthokonta

Holomycota (inc. fungi) Asco1013.jpg

Holozoa
1100 mya
1300 mya

Internal phylogeny

The most basal animals, the Porifera, Ctenophora, Cnidaria, and Placozoa, have body plans that lack bilateral symmetry. Their relationships are still disputed; the sister group to all other animals could be the Porifera or the Ctenophora, [116] both of which lack hox genes, which are important for body plan development. [117]

Hox genes are found in the Placozoa [118] [119] , Cnidaria [120] , and Bilateria. [121] [122] 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 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. [123] [124]

Giribet and Edgecombe (2020) provide what they consider to be a consensus internal phylogeny of the animals, embodying uncertainty about the structure at the base of the tree (dashed lines). [125]

An alternative phylogeny, from Kapli and colleagues (2021), proposes a clade Xenambulacraria for the Xenacoelamorpha + Ambulacraria; this is either within Deuterostomia, as sister to Chordata, or the Deuterostomia are recovered as paraphyletic, and Xenambulacraria is sister to the proposed clade Centroneuralia, consisting of Chordata + Protostomia. [126]

Non-bilateria

Non-bilaterians include sponges (centre) and corals (background). Elephant-ear-sponge.jpg
Non-bilaterians include sponges (centre) and corals (background).

Several animal phyla lack bilateral symmetry. These are the Porifera (sea sponges), Placozoa, Cnidaria (which includes jellyfish, sea anemones, and corals), and Ctenophora (comb jellies).

Sponges are physically very distinct from other animals, and were long thought to have diverged first, representing the oldest animal phylum and forming a sister clade to all other animals. [127] Despite their morphological dissimilarity with all other animals, genetic evidence suggests sponges may be more closely related to other animals than the comb jellies are. [128] [129] Sponges lack the complex organization found in most other animal phyla; [130] their cells are differentiated, but in most cases not organised into distinct tissues, unlike all other animals. [131] They typically feed by drawing in water through pores, filtering out food and nutrients. [132]

The comb jellies and Cnidaria are radially symmetric and have digestive chambers with a single opening, which serves as both mouth and anus. [133] Animals in both phyla have distinct tissues, but these are not organised into discrete organs. [134] They are diploblastic, having only two main germ layers, ectoderm and endoderm. [135]

The tiny placozoans have no permanent digestive chamber and no symmetry; they superficially resemble amoebae. [136] [137] Their phylogeny is poorly defined, and under active research. [128] [138]

Bilateria

Idealised bilaterian body plan. With an elongated body and a direction of movement the animal has head and tail ends. Sense organs and mouth form the basis of the head. Opposed circular and longitudinal muscles enable peristaltic motion. Bilaterian body plan.svg
Idealised bilaterian body plan. With an elongated body and a direction of movement the animal has head and tail ends. Sense organs and mouth form the basis of the head. Opposed circular and longitudinal muscles enable peristaltic motion.

The remaining animals, the great majority—comprising some 29 phyla and over a million species—form a clade, the Bilateria, which have a bilaterally symmetric body plan. The Bilateria are 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. These animals have a head end (anterior) and a tail end (posterior), a back (dorsal) surface and a belly (ventral) surface, and a left and a right side. [139] [140]

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; [140] these enable soft-bodied animals with a hydrostatic skeleton to move by peristalsis. [141] 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, over evolutionary time, descendant spaces have evolved which have lost one or more of each of these characteristics. For example, adult echinoderms are radially symmetric (unlike their larvae), while some parasitic worms have extremely simplified body structures. [139] [140]

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. [142] It is often suggested that the basalmost bilaterians are the Xenacoelomorpha, with all other bilaterians belonging to the subclade Nephrozoa [143] [144] [145] However, this suggestion has been contested, with other studies finding that xenacoelomorphs are more closely related to Ambulacraria than to other bilaterians. [126]

Protostomes and deuterostomes

The bilaterian gut develops in two ways. In many protostomes, the blastopore develops into the mouth, while in deuterostomes it becomes the anus. Protovsdeuterostomes.svg
The bilaterian gut develops in two ways. In many protostomes, the blastopore develops into the mouth, while in deuterostomes it becomes the anus.

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. [146] 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. [147] [148] 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. [149]

The main deuterostome phyla are the Echinodermata and the Chordata. [150] Echinoderms are exclusively marine and include starfish, sea urchins, and sea cucumbers. [151] The chordates are dominated by the vertebrates (animals with backbones), [152] which consist of fishes, amphibians, reptiles, birds, and mammals. [153] The deuterostomes also include the Hemichordata (acorn worms). [154] [155]

Ecdysozoa
Ecdysis: a dragonfly has emerged from its dry exuviae and is expanding its wings. Like other arthropods, its body is divided into segments. Anax Imperator 2(loz).JPG
Ecdysis: a dragonfly has emerged from its dry exuviae and is expanding its wings. Like other arthropods, its body is divided into segments.

The Ecdysozoa are protostomes, named after their shared trait of ecdysis, growth by moulting. [156] 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; [157] some are important parasites. [158] 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. [159]

Spiralia
Spiral cleavage in a sea snail embryo Spiral cleavage in Trochus.png
Spiral cleavage in a sea snail embryo

The Spiralia are a large group of protostomes that develop by spiral cleavage in the early embryo. [160] 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. [161] [162]

The Lophotrochozoa includes the molluscs, annelids, brachiopods, nemerteans, bryozoa and entoprocts. [161] [163] [164] 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. [165] [166]

History of classification

Jean-Baptiste de Lamarck led the creation of a modern classification of invertebrates, breaking up Linnaeus's "Vermes" into 9 phyla by 1809. Jean-Baptiste de Lamarck.jpg
Jean-Baptiste de Lamarck led the creation of a modern classification of invertebrates, breaking up Linnaeus's "Vermes" into 9 phyla by 1809.

In the classical era, Aristotle divided animals, [lower-alpha 5] 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. [168]

In 1758, Carl Linnaeus created the first hierarchical classification in his Systema Naturae . [169] 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) [lower-alpha 6] 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. [167]

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). [171] 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. [172]

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. [173] [172] The protozoa were later moved to the former kingdom Protista, leaving only the Metazoa as a synonym of Animalia. [174]

In human culture

Practical uses

Sides of beef in a slaughterhouse Carni bovine macellate.JPG
Sides of beef in a slaughterhouse

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. [175] [176] Marine fish of many species are caught commercially for food. A smaller number of species are farmed commercially. [175] [177] [178] Humans and their livestock make up more than 90% of the biomass of all terrestrial vertebrates, and almost as much as all insects combined. [179]

Invertebrates including cephalopods, crustaceans, and bivalve or gastropod molluscs are hunted or farmed for food. [180] Chickens, cattle, sheep, pigs, and other animals are raised as livestock for meat across the world. [176] [181] [182] 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. [183] Dyestuffs including carmine (cochineal), [184] [185] shellac, [186] [187] and kermes [188] [189] 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. [190]

Animals such as the fruit fly Drosophila melanogaster serve a major role in science as experimental models. [191] [192] [193] [194] Animals have been used to create vaccines since their discovery in the 18th century. [195] Some medicines such as the cancer drug trabectedin are based on toxins or other molecules of animal origin. [196]

A gun dog retrieving a duck during a hunt Hebbuz.JPG
A gun dog retrieving a duck during a hunt

People have used hunting dogs to help chase down and retrieve animals, [197] and birds of prey to catch birds and mammals, [198] while tethered cormorants have been used to catch fish. [199] Poison dart frogs have been used to poison the tips of blowpipe darts. [200] [201] A wide variety of animals are kept as pets, from invertebrates such as tarantulas, octopuses, and praying mantises, [202] reptiles such as snakes and chameleons, [203] and birds including canaries, parakeets, and parrots [204] all finding a place. However, the most kept pet species are mammals, namely dogs, cats, and rabbits. [205] [206] [207] There is a tension between the role of animals as companions to humans, and their existence as individuals with rights of their own. [208] A wide variety of terrestrial and aquatic animals are hunted for sport. [209]

Symbolic uses

Artistic vision: Still Life with Lobster and Oysters by Alexander Coosemans, c. 1660 Alexander Coosemans - Still Life with Lobster and Oysters.jpg
Artistic vision: Still Life with Lobster and Oysters by Alexander Coosemans, c. 1660

Animals have been the 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 . [210] Insects, birds and mammals play roles in literature and film, [211] such as in giant bug movies. [212] [213] [214]

Animals including insects [215] and mammals [216] are featured in mythology and religion. In both Japan and Europe, a butterfly was seen as the personification of a person's soul, [215] [217] [218] while the scarab beetle was sacred in ancient Egypt. [219] Among the mammals, cattle, [220] deer, [216] horses, [221] lions, [222] bats, [223] bears, [224] and wolves [225] are the subjects of myths and worship. The signs of the Western and Chinese zodiacs are also based on animals. [226] [227]

See also

Notes

  1. ↑ Henneguya zschokkei does not have mitochondrial DNA or utilize aerobic respiration. [18]
  2. ↑ The application of DNA barcoding to taxonomy further complicates this; a 2016 barcoding analysis estimated a total count of nearly 100,000 insect species for Canada alone, and extrapolated that the global insect fauna must be in excess of 10 million species, of which nearly 2 million are in a single fly family known as gall midges (Cecidomyiidae). [74]
  3. ↑ Not including parasitoids. [70]
  4. ↑ Compare File:Annelid redone w white background.svg for a more specific and detailed model of a particular phylum with this general body plan.
  5. ↑ In his History of Animals and Parts of Animals .
  6. ↑ The French prefix une espèce de is pejorative. [170]

Related Research Articles

<span class="mw-page-title-main">Cnidaria</span> Aquatic animal phylum having cnydocytes

Cnidaria is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in fresh water and marine environments, including jellyfish, hydroids, sea anemones, corals and some of the smallest marine parasites. Their distinguishing features are a decentralized nervous system distributed throughout a gelatinous body and the presence of cnidocytes or cnidoblasts, specialized cells with ejectable flagella used mainly for envenomation and capturing prey. Their bodies consist of mesoglea, a non-living, jelly-like substance, sandwiched between two layers of epithelium that are mostly one cell thick. Cnidarians are also some of the only animals that can reproduce both sexually and asexually.

<span class="mw-page-title-main">Invertebrate</span> Animals without a vertebral column

Invertebrates is an umbrella term describing animals that neither develop nor retain a vertebral column, which evolved from the notochord. It is a paraphyletic grouping including all animals excluding the chordate subphylum Vertebrata, i.e. vertebrates. Well-known phyla of invertebrates include arthropods, mollusks, annelids, echinoderms, flatworms, cnidarians and sponges.

<span class="mw-page-title-main">Sponge</span> Animals of the phylum Porifera

Sponges, the members of the phylum Porifera, are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through them, consisting of jelly-like mesohyl sandwiched between two thin layers of cells.

<span class="mw-page-title-main">Bilateria</span> Animals with embryonic bilateral symmetry

Bilateria is a large clade or infrakingdom of animals called bilaterians, characterized by bilateral symmetry during embryonic development. This means their body plans are laid around a longitudinal axis with a front and a rear end, as well as a left–right–symmetrical belly (ventral) and back (dorsal) surface. Nearly all bilaterians maintain a bilaterally symmetrical body as adults; the most notable exception is the echinoderms, which extend to pentaradial symmetry as adults, but are only bilaterally symmetrical as an embryo. Cephalization is also a characteristic feature among most bilaterians, where the special sense organs and central nerve ganglia become concentrated at the front/rostral end.

<span class="mw-page-title-main">Ctenophora</span> Phylum of gelatinous marine animals

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<span class="mw-page-title-main">Ecdysozoa</span> Superphylum of protostomes including arthropods, nematodes and others

Ecdysozoa is a group of protostome animals, including Arthropoda, Nematoda, and several smaller phyla. The grouping of these animal phyla into a single clade was first proposed by Eernisse et al. (1992) based on a phylogenetic analysis of 141 morphological characters of ultrastructural and embryological phenotypes. This clade, that is, a group consisting of a common ancestor and all its descendants, was formally named by Aguinaldo et al. in 1997, based mainly on phylogenetic trees constructed using 18S ribosomal RNA genes.

<span class="mw-page-title-main">Eumetazoa</span> Basal animal clade as a sister group of the Porifera

Eumetazoa, also known as diploblasts, Epitheliozoa or Histozoa, are a proposed basal animal clade as a sister group of Porifera (sponges). The basal eumetazoan clades are the Ctenophora and the ParaHoxozoa. Placozoa is now also seen as a eumetazoan in the ParaHoxozoa. The competing hypothesis is the Myriazoa clade.

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<span class="mw-page-title-main">Lophotrochozoa</span> Superphylum of animals

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<span class="mw-page-title-main">Cephalization</span> Evolutionary trend of a head region developing

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<span class="mw-page-title-main">Triploblasty</span> State of having three germ layers in embryonic development

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<span class="mw-page-title-main">Marine life</span> Organisms that live in salt water

Marine life, sea life, or ocean life is the plants, animals, and other organisms that live in the salt water of seas or oceans, or the brackish water of coastal estuaries. At a fundamental level, marine life affects the nature of the planet. Marine organisms, mostly microorganisms, produce oxygen and sequester carbon. Marine life, in part, shape and protect shorelines, and some marine organisms even help create new land.

<span class="mw-page-title-main">Marine invertebrates</span> Marine animals without a vertebrate column

Marine invertebrates are the invertebrates that live in marine habitats. Invertebrate is a blanket term that includes all animals apart from the vertebrate members of the chordate phylum. Invertebrates lack a vertebral column, and some have evolved a shell or a hard exoskeleton. As on land and in the air, marine invertebrates have a large variety of body plans, and have been categorised into over 30 phyla. They make up most of the macroscopic life in the oceans.

<span class="mw-page-title-main">Phylum</span> High level taxonomic rank for organisms sharing a similar body plan

In biology, a phylum is a level of classification or taxonomic rank below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature for algae, fungi, and plants accepts the terms as equivalent. Depending on definitions, the animal kingdom Animalia contains about 31 phyla, the plant kingdom Plantae contains about 14 phyla, and the fungus kingdom Fungi contains about 8 phyla. Current research in phylogenetics is uncovering the relationships among phyla within larger clades like Ecdysozoa and Embryophyta.

The Cambrian explosion is an interval of time approximately 538.8 million years ago in the Cambrian period of the early Paleozoic when there was a sudden radiation of complex life, and practically all major animal phyla started appearing in the fossil record. It lasted for about 13 to 25 million years and resulted in the divergence of most modern metazoan phyla. The event was accompanied by major diversification in other groups of organisms as well.

<span class="mw-page-title-main">Deuterostome</span> Superphylum of bilateral animals

Deuterostomes are bilaterian animals of the superphylum Deuterostomia, typically characterized by their anus forming before the mouth during embryonic development. Deuterostomia is further divided into 4 phyla: Chordata, Echinodermata, Hemichordata, and the extinct Vetulicolia known from Cambrian fossils. The extinct clade Cambroernida is also thought to be a member of Deuterostomia.

<span class="mw-page-title-main">Spiralia</span> Clade of protosomes with spiral cleavage during early development

The Spiralia are a morphologically diverse clade of protostome animals, including within their number the molluscs, annelids, platyhelminths and other taxa. The term Spiralia is applied to those phyla that exhibit canonical spiral cleavage, a pattern of early development found in most members of the Lophotrochozoa.

The evolution of nervous systems dates back to the first development of nervous systems in animals. Neurons developed as specialized electrical signaling cells in multicellular animals, adapting the mechanism of action potentials present in motile single-celled and colonial eukaryotes. Primitive systems, like those found in protists, use chemical signalling for movement and sensitivity; data suggests these were precursors to modern neural cell types and their synapses. When some animals started living a mobile lifestyle and eating larger food particles externally, they developed ciliated epithelia, contractile muscles and coordinating & sensitive neurons for it in their outer layer.

<span class="mw-page-title-main">ParaHoxozoa</span> Clade of all animals except sponges and comb jellies

ParaHoxozoa is a clade of animals that consists of Bilateria, Placozoa, and Cnidaria. The relationship of this clade relative to the two other animal lineages Ctenophora and Porifera is debated. Some phylogenomic studies have presented evidence supporting Ctenophora as the sister to Parahoxozoa and Porifera as the sister group to the rest of animals. Other studies have presented evidence supporting Porifera as the sister to Parahoxozoa and Ctenophora as the sister group to the rest of animals, finding that nervous systems either evolved independently in ctenophores and parahoxozoans, or were secondarily lost in poriferans. If ctenophores are taken to have diverged first, Eumetazoa is sometimes used as a synonym for ParaHoxozoa.

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