Search Results

A mathematical entity consisting of a set of values (the carrier set) and a collection of operations that manipulate them. For example, the Integer abstract data type consists of a carrier set containing the positive and negative whole numbers and…

A class of the phylum Radiozoa in the Actinopoda. The kingdom Protozoa contains 18 phyla. One of the parvkingdoms (a hierarchical classification between kingdom and superphylum that is controversial and not officially recognized as such) is the…

An order of ectoparasitic annelid worms consisting of one species, Acanthobdella peledina, which feeds on the dermal tissues of freshwater salmonid fishes in the far regions of Northern Europe. The order Acanthobdellida is taxonomically odd in that…

A distinct phylum of helminths (parasitic worms), commonly known as the spiny- or thorny-headed worms. The phylum Acanthocephala comprises three classes: Archiacanthocephala, Palaeacanthocephala, and Eoacanthocephala (a suggested fourth class,…

A taxon of marine protozoans belonging to the Acantharea. Acanthometrida (also known as Arthracanthida) is a taxonomic grouping (sometimes considered as an order or subclass) placed within the Acantharea (Acantharia) taxon. All members are…

A taxon of arachnids (traditionally considered a subclass of the Arachnida), comprising the mites and ticks. The Acari (also Acarina) consist of two superorders of small to minute arachnids. They have two body regions (Fig. 1)—a headlike gnathosoma…

An order of actinopterygian (ray-finned) fishes represented by the sturgeons and paddlefishes. The order Acipenseriformes, the only extant order of about 10–12 orders of chondrosteans (subclass Chondrostei), probably originated from palaeonisciform…

A group of small, soft-bodied, ciliated worms with the form (“morph”) of acoel turbellarians. The Acoelomorpha is now ranked as a phylum, distinct from other turbellarian worms of the phylum Platyhelminthes. Its relation to other animals, however,…

An autonomous undersea vehicle that can be launched from submarines, surface ships, or aircraft to attack enemy submarines and surface ships. An acoustic torpedo is a sophisticated weapon. Its main components are a guidance and control system, a…

A subtaxon of burrowing barnacles in the subclass Thecostraca. All members of the Acrothoracica burrow into shells of mollusks and thoracican barnacles, echinoderm tests, bryozoans, dead coral, and limestone. The entrance to the burrow is slitlike.…

An order of inarticulated brachiopods. Together with Lingulida and Siphonotretida, Acrotretida is one of the three orders of inarticulated brachiopods currently classified in class Lingulata in subphylum Linguliformea (phylum Brachiopoda).…

An order of the class Anthozoa, commonly known as the sea anemones. Actiniaria contains approximately 1100 species. They are strictly marine, occurring from northern to southern polar latitudes and inhabiting regions that range from the intertidal…

An order of Heliozoia. These protozoans are characterized by stiff arms radiating from the central cell mass. Each arm is supported internally by an array of microtubules which terminate on the surface of nuclei. The organisms are found in…

A class of Sarcodina; or in some modern schemes a superclass (Actinopoda) within the phylum Sarcomastigophora. Some of these protozoans have more or less permanent pseudopodia, composed of a central shaft of microtubules surrounded by a thin…

A class of teleostom fishes commonly known as the ray-finned fishes, which are distinguished by the structure of the paired fins that are supported by dermal rays. The class Actinopterygii comprises the subclasses Cladistia [Polypteriformes (bichirs…

The science of optical systems in which a controllable optical element, usually a deformable mirror, is used to optimize the performance of the system, for example, to maintain a sharply focused image in the presence of wavefront aberrations. A…

Signal processing is a discipline that deals with the extraction of information from signals. The devices that perform this task can be physical hardware devices, specialized software codes, or combinations of both. In recent years the complexity of…

A large, rabbit-sized rodent, belonging to the family Dasyproctidae and inhabiting South America and Central America, which is well adapted for speed because of its elongated hind legs. The agouti is a large rodent that resembles the rabbit or hare…

Radar equipment carried by commercial and military aircraft. These aircraft use airborne radar systems to assist in weather assessment and navigation. Military systems also provide other specialized capabilities such as targeting of hostile aircraft…

The efficiency of a surface in reflecting light. White surfaces have albedos close to 1; black surfaces have albedos close to 0. Because this efficiency can vary with the direction to the light source, the direction to the observer, and the color…

An order of actinopterygian fishes in the subdivision Elopomorpha, along with Elopiformes, Anguilliformes, and Saccopharyngiformes, all of which have a leptocephalous larval stage. Albuliformes, comprising three families, eight genera, and 30…

An order of the class Anthozoa (phylum Cnidaria), containing 2500–3000 species, commonly referred to as soft corals, sea fans, sea whips, and precious coral. Members of the Alcyonacea are strictly marine. They occur from northern to southern polar…

A well-defined procedure to solve a problem. As such, an algorithm describes how to solve a problem (Fig. 1). The study of algorithms is a fundamental area of computer science. In writing a computer program to solve a problem, a programmer expresses…

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.…

One of the four subclasses of Mammalia, containing a single order, the Multituberculata. The Allotheria first appeared in the Late Jurassic and survived well into the Cenozoic, a period of at least 100,000,000 years. Fossils are known from North…

A book that contains astronomical or meteorological data arranged according to days, weeks, and months of a given year and may also include diverse information of a nonastronomical character. This article is restricted to astronomical and…

Vicugna pacos, a member of the camel family (Camelidae), which has been domesticated for more than 2000 years and is economically important for its long, fine wool. The alpaca (Vicugna pacos; formerly, Lama pacos) [see illustration] belongs to the…

Two-way radio communications by individuals as a leisure-time activity. Amateur, or “ham,” radio is defined by international treaty as a “service of self-training, intercommunications, and technical investigation carried on by amateurs; that is, by…

A fatty substance formed in the intestinal tract of the sperm whale (Physeter catodon). There is a question as to whether the origin of ambergris is normal or pathological, but it does serve as protection from the horny indigestible portions of the…

The tailless whip scorpions or whip spiders, an order of the class Arachnida. There are about 80 species in the tropics and subtropics. They are flattened, red to brown, and range from 5 to 45 mm (0.2 to 1.8 in.) in body length. All are nocturnal,…

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

An order of actinopterygian fishes in the subclass Neopterygii. Amiiformes plus several related fossil orders comprise the Halecomorphi, a well-developed group known from the middle Mesozoic. The Amiiformes comprise several families that persisted…

A collective term for the classes Reptilia (reptiles), Aves (birds), and Mammalia (mammals) of the subphylum Vertebrata. The remaining vertebrates, including the several classes of fishes and the amphibians, are grouped together as the Anamnia.…

An order of Lobosia without protective coverings (tests). These protozoa range in size from about 4 micrometers to 0.08–0.12 in. (2–3 mm). Pellicles may be thin, as in Amoeba proteus, or thicker and less flexible, as in Thecamoeba verrucosa (Fig.…

One of four classes (the others are Reptilia, Aves, and Mammalia) composing the superclass Tetrapoda of the subphylum Vertebrata. The living amphibians number over 4700 species and are classified in three orders: Salientia or Anura (frogs and toads,…

The process or result of the process whereby the amplitude of a carrier wave is changed in accordance with a modulating wave. This broad definition includes applications using sinusoidal carriers, pulse carriers, or any other form of carrier, the…

A device for moving the frequency of an information signal, which is generally at baseband (such as an audio or instrumentation signal), to a higher frequency, by varying the amplitude of a mediating (carrier) signal. The motivation to modulate may…

A device for recovering information from an amplitude-modulated (AM) electrical signal. Such a signal is received, usually at radio frequency, with information impressed in one of several forms. The carrier signal may be modulated by the information…

Radio communication employing amplitude modulation (AM) of a radio-frequency (RF) carrier wave as the means of conveying the desired intelligence. In AM, the amplitude of the carrier wave is made to vary in response to the fluctuations of a sound…

A computer or computational device in which the problem variables are represented as continuous, varying physical quantities. An analog computer implements a model of the system being studied. The physical form of the analog may be functionally…

A device for converting the information contained in the value or magnitude of some characteristics of an input signal, compared to a standard or reference, to information in the form of discrete states of a signal, usually with numerical values…

A collective term, also referred to as Anamniota, for the vertebrate animals that lack an amnion in development, including the Agnatha, Chondrichthyes, Osteichthyes, and Amphibia. The amnion is a protective embryonic envelope that encloses the…

Formerly, a group of reptiles that was recognized in rank-based classifications. Amniotes were once divided into subclasses, based on the patterns of temporal fenestrae (openings in the skull roof posterior to the openings for the eyes). Subclass…

An extant order of the crustacean superorder Syncarida confined to freshwater and recorded from both surface and subterranean waters. Members of the order Anaspidacea (anaspidaceans) lack a carapace. The first thoracomere (thoracic somite) is…

A prominently located constellation in the northern sky (see illustration), named for the daughter of Cassiopeia in Greek mythology: When Cassiopeia bragged that her daughter Andromeda was more beautiful than Poseidon's daughters, the Nereids,…

The giant spiral galaxy closest to Earth. The Andromeda Galaxy is seen in the constellation of Andromeda, which is visible from Earth in the autumn skies. The galaxy is also known as M31 and NGC 224, based on its entries in two widely used catalogs.…

Modulation in which the instantaneous angle varies in proportion to the modulating waveform. When used in an analog manner, it is referred to as either frequency modulation (FM) or phase modulation (PM), depending upon whether the instantaneous…

The true eels, a large order of actinopterygian fishes, also known as the Apodes. The order Anguilliformes is related to Saccopharyngiformes (sackpharynx fishes), Elopiformes (tarpons), and Notacanthiformes (spiny eels and halosaurs); all members…

An animal's movement through the atmosphere sustained by aerodynamic reaction or other forces. Animal flight includes gliding and flapping flight. Gliding flight does not involve the generation of thrust, whereas flapping flight does generate thrust…

The classification rank comprising the animals. Animals are taxonomically assigned to the classification kingdom known as Animalia. (The other kingdoms for classification of organisms include Plantae, Fungi, Protista, and Monera; in certain…

The phylum comprising the multisegmented, invertebrate wormlike animals, of which the most numerous are the marine bristle worms and the most familiar the terrestrial earthworms. The Annelida (meaning little annuli or rings) include the Polychaeta…

An order of branchiopod crustaceans, known as fairy shrimps and, in some cases, brine shrimps (Artemia). Members of the order Anostraca (anostracans) range up to about 100 mm (4 in.) in length but usually are much smaller. The trunk consists of…

An order of birds comprising two families, the screamers (Anhimidae) of South America and the worldwide waterfowl (Anatidae). They are closely related to the Galliformes (fowls), with the screamers being a rather intermediate group. The waterfowl…

α Scorpii, a cool supergiant star of spectral type M1Ib, whose red color stands out in the midsummer sky. With an effective temperature of approximately 6000°F (3600 K), Antares resembles Betelgeuse, the brightest of the red supergiants, and would…

Any of several living mammals in the suborder Vermilingua in the order Pilosa that feed on ants and termites. The common name of anteater is assigned to four species of living mammals classified in the suborder Vermilingua in the order Pilosa…

The name given to a group of African and Asian hoofed ruminants of the order Artiodactyla. The 91 species in 31 genera are divided into five subfamilies (Bovinae, Cephalophinae, Hippotraginae, Antilopinae, and Caprinae) in the family Bovidae. The…

The device that couples the transmitter or receiver network of a radio system to space. Radio waves are used to transmit signals from a source through space. The information is received at a destination which in some cases, such as radar, can be…

A class of phylum Cnidaria (formerly known as Coelenterata) whose members are marine, live exclusively as polyps, and occur as solitary individuals or in clonal or colonial groups. Most adult anthozoans are attached to a firm object or the sea…

An order of cnidarians (subclass Hexacorallia) commonly known as black, or thorny, corals. Antipatharians have been identified in all of the world's oceans at depths of 4 to 8600 m (13 to 28,200 ft), but most known species live between 100 and 1000…

One of the three living orders of the class Amphibia, encompassing the frogs and toads. Because members of the order Anura (anurans) lack a tail, they differ most obviously from salamanders (order Urodela; also called Caudata). Usually, frogs and…

Any of the vermiform (wormlike) marine mollusks covered by a cuticle invested with innumerable calcareous scales or spines (sclerites) forming a scleritome. There are two groups that together are known as the Aplacophora: (1) Chaetodermomorpha or…

The smallest order of the class Amphibia, consisting of the wormlike caecilians. The order Apoda is sometimes called Gymnophiona. Its members are known commonly as caecilians, which are wormlike, legless amphibians with indistinct or even hidden…

An order of birds consisting of two dissimilar groups, the swifts (Apodi) and the hummingbirds (Trochili). These birds have been placed together because of several anatomical specializations of the wings and feathers, although an alternative…

A little-studied group of ciliates comprising an order of the Holotrichia. The majority occur as commensals on marine crustaceans. This association is generally more intimate and complex than that of the chonotrichs found on related hosts. Their…

A class of marine, planktonic animals in the subphylum Tunicata. The class Appendicularia (alternatively known as Larvacea) is characterized by the persistence of a tail, notochord, gill slits, and dorsal nerve cord throughout life and by a unique…

In astronomy, the two points in an elliptical orbit that are closest to, and farthest from, the primary body about which the secondary revolves. In the orbit of a planet or comet about the Sun, the apsides are, respectively, perihelion and aphelion.…

The Water Bearer, in Greek mythology, is a zodiacal constellation, meaning that the path of the Sun and planets passes through it. (The Sun actually passes through 13 of the modern constellations each year, not the traditional 12.) It is almost…

The largest class of the subphylum Chelicerata in the phylum Arthropoda. Most familiar of the included orders in the class Arachnida are the spiders (Fig. 1), scorpions (Fig. 2), harvestmen, and mites and ticks. Members of the class Arachnida…

An order of nematodes in which the amphids are simple spirals that appear as elongate loops, shepherd's crooks, question marks, or circular forms. The cephalic sensilla are often separated into three circlets: the first two are papilliform or the…

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

An order of regular sea urchins (class Echinoidea) that have a stirodont Aristotle's lantern (jaw apparatus) with the epiphyses not meeting at the top of the lantern, and with keeled teeth that are T-shaped in cross section. The ambulacral plates of…

An order of Lobosia. The shell (test) of these protozoa has a single, well-defined aperture through which slender fingerlike pseudopodia (lobopodia) can be extended. The test often has an internal chitinous layer known as a glycocalyx and an…

An extinct group of mainly Lower Cambrian marine sponges which, although lacking spicules, possessed an intricate, highly porous skeleton of calcite. It was probably a monophyletic group; that is, all representatives were derived from a single…

An order of ancestrally wingless insects, commonly known as bristletails, in the subclass Apterygota; about 350 species are known in the two families, Machilidae and Meinertellidae. All have slender, spindle-shaped bodies of about 0.4–0.8 in. (1–2…

A class of the phylum Acanthocephala. The adult worms are parasitic in terrestrial vertebrates. The body wall and lemnisci of species in this class have numerous amitotically fragmented nuclei or a few ameboid giant nuclei. The main trunks of the…

A diversified group of diapsid reptiles, comprising dinosaurs (and their descendants, birds), crocodylians, pterosaurs (flying reptiles), and various forms closely related to these groups. In the past, the primitive archosaurs from the Triassic…

The brightest star in the northern sky, apparent magnitude −0.05, also known as α Boötis. It is a yellow giant star of spectral type K1.5, one of the nearest giants to the Earth at a distance of 11.25 parsecs (2.16 × 1014 mi or 3.47 × 1014 km).…

An order of teleost fishes, comprising the argentines or herring smelts, in the superorder Protacanthopterygii. Members of the order Argentiniformes are marine, deep-water fishes. They are identifiable by the following combination of characters: a…

A suborder of leeches (order Hirudinida) characterized by the absence of a protrusible proboscis and including both blood-feeding and macrophagous (predatory) varieties. Most notorious among the blood-feeding leeches in the Arhynchobdellida is the…

The Ram, a northern zodiacal constellation, meaning that the path of the Sun and planets passes through it (see illustration). In Greek mythology, the golden fleece of this ram was pursued by Jason and the Argonauts, whose ship is memorialized in…

A heavily armored mammal in the order Cingulata. The order Cingulata (formerly Edentata or Xenarthra) comprises the armadillos, which are divided into two extant families: Chlamyphoridae and Dasypodidae. These creatures are characterized by bony…

The largest phylum in the animal kingdom, including the well-known insects, spiders, ticks, and crustaceans, as well as many smaller, less well-known groups. Members of the phylum Arthropoda, that is, arthropods, are the dominant metazoans…

The only surviving subclass of the echinoderm class Crinoidea. The Articulata include all living members of the echinoderm class Crinoidea and are generally regarded as the post-Paleozoic crinoids. Crinoids are exclusively marine organisms, as are…

An order comprising the even-toed ungulates (hoofed mammals). There are two main radiations: the predominantly omnivorous Bunodontia, including suoids (such as pigs, peccaries, and hippos); and the more herbivorous Selenodontia, including camels and…

An order of nematodes in which the oral opening is generally surrounded by three or six labia; in some taxa, labia are absent, but the cephalic sensilla are always evident. Usually there are eight cephalic or labial sensilla; the submedians may be…

A class of Tunicata (Urochordata) which occurs as solitary zooids or, by a process of asexual budding, develops into colonies. Zooids of the class Ascidiacea vary in length from a few millimeters to 10 in. (25 cm). Individuals or colonies are…

A subtaxon of marine crustaceans in the subclass Thecostraca. Members of the Ascothoracica are ectoparasites or endoparasites of cnidarians and echinoderms. The body is enclosed in a voluminous saclike mantle that is up to 20 mm (0.8 in.) in length;…

A small group of trematodes (flukes) with about 80 freshwater and marine species in 4 families and 13 genera, infecting fishes (chondrichthyans and teleosts) and turtles. The Aspidogastrea vary in length from about 2 mm (0.08 in.) to several…

One of the many thousands of small planets (minor planets) revolving around the Sun, mainly between the orbits of Mars and Jupiter. The presence of a gap in J. A. Bode's empirical law of planetary spacings motivated a search for the missing planet.…

An order of protozoans, subclass Holotrichia, in which all species are mouthless. All species are parasitic in other animals, typically oligochaete annelids. Many astomatids possess an elaborate holdfast organelle, of value in attachment to the…

The scientific discipline that studies the origin, evolution, distribution, and future of life in the universe. Astrobiology is an interdisciplinary science integrating contributions from biology, geology, astronomy, paleontology, and planetary…

That part of astronomy dealing with the position, distance, and motion of celestial objects, including solar system objects, stars, radio sources, and galaxies. New astrometry Astrometry has changed since the 1980s. Very long baseline interferometry…

Books, CD-ROMs, or computer-accessible sets showing stars, constellations, or other astronomical phenomena and their locations in the sky. Astronomical atlases provide two-dimensional views of the distributions of objects.The first sets of…

Lists or enumerations of astronomical data relevant to astronomy, navigation, geodesy, and space science applications. Astronomical catalogs vary a great deal in form and content depending upon the type of the data and the objects to which they are…

Schemes for locating astronomical objects in space. To an observer on the Earth's surface, the stars of the night sky appear to be placed upon a spherical shell of infinite radius with the observer at the center. Celestial objects appear to move…

The production of a permanent two-dimensional record of a scene of interest to astronomy. In direct imaging, the scene is a section of the sky optically imaged with a telescope. In other types of imaging, the scene is the output of an instrument,…

A telescope or telescopes, their protective enclosures (if any), support and headquarters buildings, and the staff of astronomers, engineers, technicians, and other support personnel. The telescopes can be optical or infrared (reflecting or…

The use of spectroscopy (the analysis of light as a function of wavelength) as a tool for obtaining observational data on the chemical compositions, physical conditions, and radial velocities of astronomical objects. Astronomical applications of…

A telescope adapted to the observation of the passage, or transit, of an astronomical object across the meridian of the observer. The astronomical transit instrument is the classic instrument of positional astronomy, which is the study of the…

The basic unit of length in the solar system. The astronomical unit (AU) is also used to a limited extent for interstellar distances through the definition of the parsec (1 pc = 206,265 AU). It is nearly equal to the mean distance a between the…