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A diverse group of early jawed fishes from the Paleozoic Era, usually classified as a distinct class of vertebrates. Most acanthodians have tiny body scales with a cross section like that of an onion, the layers reflecting incremental scale growth.…

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…

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…

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).…

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…

An approach to management of natural resources that emphasizes how little is known about the dynamics of ecosystems and that, as more is learned, management will evolve and improve. Adaptive management is a systematic strategy that seeks to improve…

Habituation to a specific practice, such as drinking alcoholic beverages or using drugs (for example, heroin, cocaine, or amphetamines), and disorders characterized by the chronic use of a drug, alcohol, or similar substance. Addiction and addictive…

Aeronomy concentrates on the regions above the tropopause or upper part of the atmosphere. The region of the atmosphere below the tropopause is the site of most of the weather phenomena that so directly affect all life on the planet; this region has…

A variety of chalcedonic quartz that is distinguished by the presence of color banding in curved or irregular patterns (Fig. 1). Most agate used for ornamental purposes is composed of two or more tones or intensities of brownish-red, often…

Behavior that is intended to threaten or inflict physical injury or emotional harm on another person or organism. Aggression is a type of behavior (physical or verbal, or both) displayed by an individual who aims or is motivated to harm another…