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The adaptation of an animal or plant through breeding in captivity to a life intimately associated with and advantageous to humans. Domesticated plants and animals differ genetically and morphologically from their wild counterparts, often in ways…

Typically, an aquatic vertebrate with scales, gills, and paired fins. Fishes (Fig. 1) are found throughout the world's waterways, from saltwater oceans to freshwater streams and ponds. About 35,000 species of fishes have been described to date. Many…

A biological organism whose genetic material has been manipulated artificially through genetic or genomic editing techniques. In contrast to the long history of artificial selection (domestication), in which humans selectively breed organisms to…

A space-traversing unmanned vehicle which contains the means for controlling its flight path and that delivers an explosive warhead with high accuracy. Various kinds of missiles are guided in the sense that they can aerodynamically adjust after…

A large aquatic reptile of the family Alligatoridae. Alligators belong to the family Alligatoridae in the reptilian order Crocodylia (Crocodilia). Common usage generally restricts the name alligator to the two extant species of the genus Alligator…

Any protozoan moving by means of protoplasmic flow. In their entirety, ameboid protozoans (or ameboid protists) include naked amebas, those enclosed within a shell or test, and more highly developed representatives, such as heliozoians,…

The use of live nonhuman animals for scientific research and experimentation. Animal testing has been a cornerstone of biomedical research, having contributed greatly to the understanding of human biology, to the unraveling of the complex underlying…

One of the three living orders of the class Amphibia, encompassing the frogs and toads. Anura is the largest order of amphibians (class Amphibia), containing more than 7000 species of frogs (Fig. 1) and toads. Because members of the order Anura…

A natural order of the class Arachnida, commonly known as the spiders. Members of the order Araneae (also called Araneida)—the spiders (Fig. 1)—are widespread over most of the land areas of the world and are well adapted to many different habitats.…

The application of chemical techniques to the study of archeological materials in order to elucidate aspects of the human past. The materials used in archeological chemistry might be anthropogenic (for example, pottery or metal) or natural (from…

The study of skeletal remains from archeological sites by biological (or physical) anthropologists. Bioarcheology differs in several ways from traditional skeletal research (see illustration). Previous work focused on individual case studies (for…

The name for two species within the genus Bison of the cattle family Bovidae in the mammalian order Artiodactyla found in North America and Europe. Bison are artiodactyl mammals belonging to the genus Bison (family Bovidae). The North American bison…

A taxonomic order of insects comprising the cockroaches and termites. The order Blattodea, also referred to as Blattaria, contains at least 4500 species of cockroaches (Fig. 1) and 3000 species of termites. Previously, termites were assigned to a…

The name for several members of the family Bovidae in the mammalian order Artiodactyla. The buffalo is an Old World artiodactyl that resembles oxen in general appearance. The North American bison is often called a buffalo, but it is not related to…

Any terrestrial or aquatic animal that is capable of excavating holes in the ground for protection from adverse environmental conditions, as well as for storing food. Burrowing animals are uniquely adapted species, having the capacity to dig burrows…

A member of the mammalian family Camelidae in the order Artiodactyla. Camels are even-toed, hoofed mammals (ungulates). There are three extant camel species: the Bactrian camel (Camelus bactrianus; Fig. 1), the wild Bactrian camel (C. ferus), and…

A class in the phylum Mollusca that is exclusively marine and includes squids, octopuses, and cuttlefishes. The molluscan class Cephalopoda is one of the major taxonomic groupings of marine animals and contains both extant members—squids (Fig. 1),…

An order of the class Reptilia, including the turtles, terrapins, and tortoises; also termed Testudines. The order Chelonia (often referred to as Testudines) comprises one of the oldest groups of reptiles on Earth. Members of the Chelonia…

The common name for any of 25 species of rodents in the mammalian family Sciuridae. Chipmunks are assigned to three different genera within the mammalian family Sciuridae: Tamias, with 1 species; Eutamias, with 1 species; and Neotamias, with 23…

An order of Insecta comprising the beetles. Beetles belong to the insect order Coleoptera. This is the largest animal order, containing approximately 400,000 species. With so many species, beetles show very great diversity in size, form, color,…

The common name used for the large aquatic reptiles included in the family Crocodylidae. Crocodile (see illustration) is the common name used to describe about 17 species of large aquatic reptiles assigned to the family Crocodylidae, which is one of…

An order of insects commonly known as earwigs, in reference to an unfounded superstition that they crawl into the ears of sleeping persons. Earwigs are elongate and flattened, with a pair of forceps-like appendages, or cerci, at the end of the…

An order of mammals comprising one family (Cynocephalidae), one genus (Cynocephalus), and two species—C. variegatus (Malayan colugo) and C. volans (Philippine colugo). These mammals inhabit the rainforests of Southeast Asia from the Malay Peninsula…

An order of the class Insecta comprising the true flies. Members of the order Diptera (class Insecta, phylum Arthropoda) are known as dipterans or flies (Fig. 1). The name Diptera is derived from the Greek di-ptera, for "two wings," because the…

Any member of the carnivorous mammalian family Canidae. All breeds of domestic dogs (Fig. 1), the wild dogs, and related species belong to the family Canidae. Despite the various breeds of domestic dogs that are known, the scientific name for all…

The biological study of insects that have a direct influence on humanity. Economic entomology is a subdiscipline of entomology (the biological science that deals with the investigation of insects) and focuses on the effects of insects on human…

The common name for three living species of mammals in the family Elephantidae (order Proboscidea). Elephants are extant members of the mammalian family Elephantidae, which is one of several families included in the order Proboscidea. The remaining…

An active and intentional act or practice of putting to death, in a relatively painless way, of persons suffering severely from medical conditions that are incurable, untreatable, or irreversible. Euthanasia is an act by a person, usually a…

The test of a hypothesis under controlled conditions. The experiment is one of the distinctive tools of the scientist. It enables the scientist to put questions to nature and receive answers. These answers lead to new problems whose solutions…

The common name for any of the numerous small lizards in the infraorder Gekkota. Members of the reptilian infraorder Gekkota (order Squamata) are called geckos (see illustration). These small lizards are distinguished by a flattened body [never…

An artiodactyl member of the genus Giraffa in the mammalian family Giraffidae. The giraffe (see illustration) is the only extant member of the genus Giraffa (family Giraffidae, class Mammalia). Giraffes were once thought to consist of a single…

Optical instruments that establish an optical line or axis for the purpose of aiming a weapon. The axis includes the observer's eye, a suitable mark in the instrument, and the target. For a pistol, the traditional iron sights employ a rear notch and…

An African carnivore belonging to the family Hyaenidae. The mammalian family Hyaenidae (order Carnivora) comprises three species of hyenas (also spelled hyaenas): one species (spotted hyena) in the genus Crocuta (see illustration), one species…

A tentative supposition with regard to an unknown state of affairs, the truth of which is thereupon subject to investigation by any available method, either by logical deduction of consequences which may be checked against what is known, or by…

The study of the functional properties of insect tissues and organs. Insect physiology is a branch of biology that investigates how insects live and function. Insects are one of the most diverse groups of animals on Earth and have evolved…

An arachnid taxon comprising the ticks. Ticks are arachnids (Fig. 1). Specifically, they are members of the Ixodida, which is considered to be an order or suborder (depending on the exact classification) of the subclass Acari (Acarina) in the class…

Intentional generation of interfering signals in the electromagnetic spectrum by powerful transmitters as a countermeasure intended to block a communication, radar, or infrared system or to impair its effectiveness appreciably. Radio broadcasts or…

The common name for a member of the agnathan fishes. The fish superclass Agnatha (subphylum Vertebrata, phylum Chordata) comprises the jawless vertebrates (agnathans). Members include cyclostomes (modern lampreys and hagfishes), various extinct…

A single species, Phascolarctos cinereus, which is the sole extant member of the family Phascolarctidae in the mammalian order Marsupialia (pouch-bearing animals). The koala (see illustration) is a small, clean, affectionate animal that weighs from…

Lama glama, a member of the camel family (Camelidae) found only in South America. The llama (see illustration) is an artiodactyl, or even-toed ungulate, belonging to the mammalian family Camelidae. It has two toes on each foot. The llama is a…

A lineage of true elephants in the extinct genus Mammuthus that occupied Eurasia and North America in the late Cenozoic Era. Mammoths (genus Mammuthus) are extinct elephants assigned to the mammalian order Proboscidea (Fig. 1). They inhabited…

The names associated with a great number of species of mammals in a number of different families of the order Rodentia. The rodents constitute the largest order (Rodentia) of mammals, and they include approximately 2300 species, or about 42% of the…

The body of knowledge that concerns the fundamental nature, function, development, and use of formal models in science and technology. In its most general sense, a model is a proxy. A model is one entity used to represent some other entity for some…

A major phylum of the animal kingdom comprising a diverse group of unsegmented, soft-bodied invertebrates. The phylum Mollusca is one of the most significant groupings of invertebrate animals. Members of the Mollusca, termed mollusks or molluscs,…

A mammalian order containing the egg-laying platypus and echidnas. Members of the order Monotremata (monotremes) are egg-laying mammals belonging to the subclass Prototheria (in some alternative classifications, the monotremes have been assigned to…

An even-toed, bovid ungulate, Ovibos moschatus. The musk-ox (Ovibos moschatus) is a member of the family Bovidae in the mammalian order Artiodactyla. The single species of musk-ox (see illustration) is the northernmost representative of the family,…

Weapons that arm warships. Of the wide variety of such weapons, three important categories are exclusively naval: antiship missiles, torpedoes, and mines. Other weapons all correspond to land-based types, but submarine-launched ballistic missiles…

The Nobel Prizes for 2021 included the following awards for scientific disciplines. Chemistry The chemistry prize was awarded jointly to Benjamin List of the Max-Planck-Institut für Kohlenforschung, Mülheim an der Ruhr, Germany, and David W.C.…

A small extinct order of articulated brachiopods that ranges in age from Early to Middle Cambrian and includes the earliest known calcitic brachiopods. Classification The order Obolellida is included within the class Obolellata, subphylum…

A subclass of Anthozoa consisting of colonial cnidarians. Members of the subclass Octocorallia (Alcyonaria) are benthic as adults, living attached to firm substrata or burrowed into soft sediments, from the intertidal zone to great depths. As in all…

An order of coleoid cephalopods, characterized by eight appendages (which encircle the mouth), a saclike body, and an internal shell that is much modified or reduced from that of its ancestors. Members of the order Octopoda (subclass Coleoidea,…

An order of the class Insecta, commonly known as dragonflies and damselflies. Odonates (Fig. 1) generally are conspicuous insects with strong flying ability and a complex range of behaviors. They occur on all continents except Antarctica, although…

A former primitive order of brittle stars (class Ophiuroidea) reassigned to the order Ophiurida. The order Oegophiurida is an invalid taxon of brittle stars. Formerly comprising a single living genus, Ophiocanops, the members of this former order…

A suborder of trilobites (a class of extinct arthropods) and a diverse clade consisting of roughly 100 distinct species. The trilobites constituting the suborder Olenellina belong to the order Redlichiida in the class Trilobita. These extinct…

A subclass of worms, including the earthworms, within the class Clitellata of the phylum Annelida. Oligochaeta is an important annelid subclass of worms. The most notable worm types belonging to the Oligochaeta are earthworms (Fig. 1). In general,…

An extinct order of irregular echinoids in the superorder Neognathostomata resembling clypeasteroids but lacking the accessory ambulacral pores characteristic of that group. Oligopygoids have well-developed petals, and there are characteristic small…

An order of nematodes comprising the single superfamily Oncholaimoidea. Members of the order Oncholaimida are principally marine and brackish-water forms with alleged predaceous and carnivorous feeding habits. The external amphidial aperture is an…

A small phylum of soft-bodied terrestrial invertebrates, commonly known as velvet worms, living in tropical, subtropical, and temperate forests. The Onychophora is an archaic group of animals that has been described for some time as a missing or…

An order of extant echinoderms known as the brittle stars. Living brittle stars comprise two orders: Ophiurida or brittle stars (see illustration), and Euryalida or basket stars. In Ophiurida, the arms are unbranched and they lack rings of hooks.…

A class of the subphylum Asterozoa (phylum Echinodermata), comprising the brittle stars and basket stars, in which the arms are usually clearly demarcated from a central disk and perform whiplike locomotor movements, and the tube feet are…

An order of the class Arachnida, whose members are commonly known as the harvestmen or daddy longlegs. The order Opiliones is sometimes referred to as Phalangida. More than 6400 species are known. They are common in temperate and tropical climates.…

A major group of largely marine heterobranch gastropod mollusks, containing more than 6000 living species. Most species of the Opisthobranchia (opisthobranchs) are chemically defended by toxins that are derived from their prey. As a result, the…

A small order of birds containing a single family, the Opisthocomidae. The order Opisthocomiformes contains one species, Opisthocomus hoazin, which is commonly known as the hoatzin (see illustration). It is restricted to South America and has been…

A New World marsupial belonging to the family Didelphidae. Didelphidae consists of one North American species, the Virginia opossum (Didelphis virginiana), and 65 Central and South American species, including the mouse opossums (Marmosa,…

An order of sharks, commonly known as carpet sharks. Members of the order Orectolobiformes share the following characteristics: two dorsal fins without spines; a short snout that leaves the mouth terminal or almost terminal; and each nostril…

A small phylum (about 25 named species) of microscopic animals that parasitize a wide variety of marine invertebrates: turbellarians, nemerteans, polychaete annelids, gastropod and bivalve mollusks, brittle stars, and ascidians. Four genera of…

An order that includes over 20,000 species of terrestrial insects, including most of the “singing” insects, some of the world's largest insects, and some well-known pests. Most species of Orthoptera (from “orthos,” meaning straight, and “pteron,”…

A very large superorder of actinopterygian (ray-finned) fishes containing close to 30% of all known fishes of the world and accounting for about 70% of the freshwater species. Although the perciforms dominate modern seas, the members of the…

A taxonomic fish group comprising the bony fishes. The Osteichthyes grouping (often considered a superclass) consists of the classes Actinopterygii (ray-finned fishes) and Sarcopterygii (lungfishes and lobefins, or coelacanths, but excluding…

An order of ray-finned teleost fishes consisting of two monophyletic clades, the suborders Osteoglossoidei and Notopteroidei. The members of the order Osteoglossiformes belong to the class Actinopterygii (ray-finned fishes) and are divided into the…

A major taxon of the Crustacea containing small bivalved animals 0.004–1.4 in. (0.1–33 mm) long, with most between 0.04 and 0.08 in. (1 and 2 mm). Members of the Ostracoda inhabit aquatic environments in nearly all parts of the world.…

Any of various carnivorous mammalian members of the family Mustelidae having a long thin body, short legs, a somewhat flattened head, webbed toes, and a broad flattened tail, as well as being adapted to aquatic life. Otters (Fig. 1) belong to the…

An order of class Zoomastigophorea in the phylum Protozoa. These are colorless flagellate symbionts in the digestive tract of the roach Cryptocercus and of certain termites. They are xylophagous; that is, they ingest wood particles taken in by the…

A class of the Acanthocephala, the adults of which are parasitic worms found in fishes, aquatic birds, and mammals. They have the following characteristics. The nuclei of the hypodermis are fragmented and the chief lacunar vessels are lateral. The…

A peculiar arthropod, evidently related to the Pycnogonida and represented by a number of well-preserved fossils from the Devonian Hunsrück shales. Palaeoisopus was formerly considered by a number of paleontologists to have an anterior jointed…

The oldest archeological cultures of the New World, the ancestors of modern Native Americans. The Paleoindian colonizing populations of the Americas were Homo sapiens sapiens who arrived during the late Pleistocene (Ice Age) from Asia, though…

The study of disease in the ancient past. Paleopathology is the investigation of ancient diseases and their origins. The term applies to any evidence of disease in any form of life, including both plants and animals. However, the research emphasis…

An order of rare arachnids from tropical and warm temperate regions. Members of the order Palpigradi, comprising approximately 80 known species, are often referred to as microscorpions or miniature whip scorpions. American species occur in Texas and…

A medium-sized class of extinct blastozoan echinoderms containing about 17–19 genera, ranging from the late Early Ordovician to the Early Silurian throughout North America and in northern Europe. Their maximum standing diversity (13 genera) occurred…

A class of hexapod (six-legged) arthropods consisting of the orders Collembola and Protura. Class Parainsecta (also known as Ellipura) is one of several classifications of those hexapods, which, while similar to insects, are sufficiently different…

A single-celled ciliated protozoan belonging to the genus Paramecium. The genus Paramecium (family Parameciidae) comprises a group of widely distributed, free-living ciliated protozoans, which are commonly termed paramecium organisms (see…

The scientific study of parasites and parasitism. Parasitology is the branch of biology dealing with parasitic organisms (see illustration), with particular focus being directed toward understanding the relationship between the parasite and the…

A large crustacean superorder. Members of the superorder Peracarida, commonly termed peracarids, comprise a major category of eumalacostracan crustaceans (subclass Eumalacostraca, class Malacostraca, subphylum Crustacea, phylum Arthropoda). The most…

An order of herbivorous, odd-toed, hoofed mammals, including the living horses, zebras, donkeys, rhinoceroses, tapirs, and their extinct relatives. Members of the order Perissodactyla, or perissodactyls, are odd-toed ungulates (mammals with hooves).…

An order of jawless fishes commonly known as the lampreys. Members of the order Petromyzontiformes (class Petromyzontida or Hyperoartia) are known as lampreys. Characteristics of these jawless fishes include the following combination of features…

The order of insects comprising the leaf insects and the stick insects, also known as walking sticks. There are about 2800 species of phasmids (Phasmatodea is alternatively known as Phasmida) cataloged into six families, the majority of which live…

The flamingos, a small monotypic order of wading birds that includes the family Phoenicopteridae, which has five species in one genus Phoenicopterus found worldwide in tropical to temperate marine and fresh waters; four species are found in South…

An order of mammals comprising the living pangolins and their poorly known fossil predecessors. All living members of the mammalian order Pholidota, that is, the pangolins (also termed scaly anteaters), belong to the family Manidae and are assigned…

A small phylum of marine animals; in the recent past, they have been grouped with Brachiopoda and Bryozoa into the Lophophorata. The Phoronida phylum or class has no intermediate hierarchical level until the generic level. Two genera, Phoronis and…

A former order of brittle stars (class Ophiuroidea). The order Phrynophiurida is an invalid taxon of brittle stars. The members of this former order have been incorporated into the orders Euryalida and Ophiurida (class Ophiuroidea). In the…

An order of small wingless insects, permanently parasitic on mammals and birds. The Phthiraptera are one of approximately 30 major groups of insects and can be divided into four suborders, three of which (the Amblycera, Ischnocera, and…

A class of the phylum Bryozoa. Phylactolaemates have lophophores (crowns of tentacles surrounding the mouth) which are markedly U-shaped (or rarely nearly circular but still kidney-shaped) in basal outline, and relatively short wide zooecia…

A subclass of the crustacean class Malacostraca containing the extant order Leptostraca and the fossil order Archiostraca (see illustration). The Phyllocarida has a long fossil record, and many early fossil taxa were referred to this subclass.…

A large order of land birds, second in number of species only to the Passeriformes, that is found throughout the world except for the Australian region and is concentrated in tropical areas. The relationships of the suborder Galbulae to the other…

Any of the species of freshwater fishes that compose the family Esocidae in the order Esociformes. Members of the family Esocidae (order Esociformes) belong to one extant genus, Esox, and are commonly termed pike. They also are known by other names,…

The most primitive metazoan animal phylum. At present, this phylum harbors a single named species, the enigmatic Trichoplax adhaerens. Animal kingdom Metazoa Description In 1883, the German zoologist Franz Eilhard Schulze discovered this microscopic…

Protozoa composing an order of Mycetozoia, endoparasitic in plants and primarily causing, for example, club root of cabbage, powdery scab of potatoes, and galls in Ruppia. Underground portions of host plants are invaded by young parasites, often a…

A Paleozoic (Ordovician and Devonian) order of sea stars of the class Asteroidea; only two genera have been recognized. Platyasteridans have relatively elongate flat arms; broad, transversely aligned ventral arm plates; and paxilliform aboral plates…

An order of the Podocopa (Ostracoda) containing a single family, Cytherellidae, and two extinct superorders. Platycopids are small marine or brackish-water ostracods with an asymmetrical shell that is oblong, nearly rectangular, and laterally…

An order of the phylum Ctenophora comprising four families (Ctenoplanidae, Coeloplanidae, Tjalfiellidae, Savangiidae) and six genera. All species are highly modified from the planktonic ctenophores. The platyctenes are fairly small (1–6 cm or…