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aboral

Overview page. Subjects: Zoology and Animal Sciences — Earth Sciences and Geography.

Away from the mouth; on the opposite side of the body from the mouth in animals that lack clear-cut dorsal and ventral surfaces.

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abyssal

Overview page. Subjects: Ecology and Conservation — Earth Sciences and Geography.

Applied to the deepest part of the ocean, below about 2000 m. The abyssal zone covers approximately 75% of the ocean floor. Compare bathyal; neritic.

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Acanthodii

Overview page. Subjects: Earth Sciences and Geography — Zoology and Animal Sciences.

Class of primitive, fossil fish that had a true bony skeleton, a heterocercal tail fin, a persistent notochord, ganoid scales, and stout spines in front of the fins. The acanthodians lived...

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acid soil

Overview page. Subjects: Biological Sciences — Earth Sciences and Geography.

Soil with a pH under 7, such as podzols and brown earths. Acidity in a soil may be due to the leaching out of cations when precipitation exceeds evapotranspiration. Other factors include...

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acidophile

Overview page. Subjects: Ecology and Conservation — Earth Sciences and Geography.

An extremophile (domain Archaea) that thrives in environments where the pH is below 5.0.

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acme zone

Overview page. Subjects: Earth Sciences and Geography — Ecology and Conservation.

Informally, a body of strata that contains the maximum abundance of a particular fossil taxon occurring within the stratigraphic range of that taxon, and after which the zone is named.

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Acrisols

Overview page. Subjects: Biological Sciences — Earth Sciences and Geography.

A reference soil group in the soil classification scheme used by the FAO. Acrisols are acid soils with an argic B horizon (see argic horizon) having a cation-exchange capacity of less than...

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acritarchs

Overview page. Subjects: Earth Sciences and Geography — Genetics and Genomics.

Spherical bodies thought to represent the earliest eukaryotic cells, estimated to begin in the fossil record about 1.6 billion years ago. Most acritarchs were probably thick-walled,...

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Actinopterygii

Overview page. Subjects: Zoology and Animal Sciences — Earth Sciences and Geography.

; class Osteichthyes)

A subclass of ray-finned fish, that includes the majority of living bony fish of sea and fresh water. The fins are composed of a membranous web of skin...

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active layer

Overview page. Subjects: Earth Sciences and Geography — Biological Sciences.

Seasonally thawed surface layer between a few centimetres and about 3 m thick, lying above the permanently frozen ground in a periglacial environment. It may be subject to considerable...

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