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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…

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…

The principle that there is an upward fluid force on a body submerged or floating in a stationary fluid that is equal to the weight of the fluid displaced by the body. The concept of Archimedes' principle (Fig. 1), perhaps the oldest stated…

The science of sound as it pertains to buildings. There are three major branches of architectural acoustics. (1) Room acoustics involves the design of the interior of buildings to project reflected and diffused sound at appropriate levels and time…

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…

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…

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 uniform and unique direction associated with the apparent inevitable flow of time into the future. There appears to be a fundamental asymmetry in the universe. Yet herein lies a paradox, for all the laws of physics, whether they are the…

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…

Substitutes for sugar (sucrose) in foods and beverages that provide intense sweetness with few to no calories. Artificial sweeteners are sometimes called low-calorie sweeteners, high-intensity sweeteners, noncaloric sweeteners, non-nutritive…

Manufactured, reproducibly layered structures having layer thicknesses approaching interatomic distances. Modern thin-film techniques are at a stage at which it is possible to fabricate these structures, also known as artificial crystals or…

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 white, crystalline compound, 1-aspartyl-1-phenylalanine methyl ester (APM), with formula (1). It is slightly soluble in water. Its sweetening properties were discovered accidentally in 1965 when the compound, a dipeptide, was produced as an…

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…

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 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…

An order of actinopterygian (ray-finned) fishes consisting of only one family, Ateleopodidae, known as jellynose fishes. Ateleopodiformes along with the order Stomiiformes constitute a teleost superorder called the Stenopterygii. Jellynose fishes…

An order of Granuloreticulosia in which the naked amebas form branched, threadlike, interconnected pseudopodia (reticulopodia). Species are known from freshwater, salt water, and brackish water. General characteristics of the order Athalamida are…

An order of actinopterygian (ray-finned) fishes, including the silversides, rainbowfishes, and blue eyes. The members of the Atheriniformes, together with those of the Beloniformes and Cyprinodontiformes, comprise the series Atherinomorpha.…

A constituent of matter consisting of z negatively charged electrons bound predominantly by the Coulomb force to a tiny, positively charged nucleus consisting of Z protons and (A − Z) neutrons. Z is the atomic number, and A is the mass or nucleon…

Clusters are aggregates of atoms (or molecules) containing between three and a few thousand atoms that have properties intermediate between those of the isolated monomer (atom or molecule) and the bulk or solid-state material. The study of such…

An atom laser is a device that generates an intense coherent beam of atoms through a stimulated process. It does for atoms what an optical laser does for light. The atom laser emits coherent matter waves, whereas the optical laser emits coherent…

The use of laser light and nanofabricated structures to manipulate the motion of atoms in the same manner that rudimentary optical elements control light. The term refers to both an outlook in which atoms in atomic beams are thought of and…

Unidirectional streams of neutral atoms passing through a vacuum. These atoms are virtually free from the influence of neighboring atoms but may be subjected to electric and magnetic fields so that their properties may be studied. The technique of…

A device that uses an internal resonance frequency of atoms (or molecules) to measure the passage of time. The terms atomic clock and atomic frequency standard are often used interchangeably. A frequency standard generates pulses at regular…

A gas of atoms, generally ultracold, in which the individual atoms possess half-integer spin (that is, are fermionic). Since the 1980s, researchers have developed powerful new techniques for cooling atoms to ultralow temperatures. Among the most…

The mass of an atom or molecule on a scale where the mass of a carbon-12 (12C) atom is exactly 12.0 atomic mass units. The mass of any atom is approximately equal to the total number of its protons and neutrons multiplied by the atomic mass unit, u…

A unit of mass equal to exactly 1/12 of the mass of an atom of carbon-12 (12C), the predominant isotope of carbon. The unit, also known as the dalton, is often abbreviated amu, and is designated by the symbol u. The relative atomic mass of a…

The positively electrically charged central core of the atom. Atoms are analogous to the solar system, with an extremely dense central core that provides the electrical positive charge to hold the negatively charged electrons in orbits through the…