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Archaebacteria, Archaeobacteria

(ar″kē-bak-tēr′ē-ă, ar″kē-ō-) [Gr. archaios, ancient + bacteria] A group of single-celled organisms, classified by some microbiologists as a type of bacteria and by others as a separate kingdom or domain of life. Unlike Eubacteria, they lack peptidoglycans in their cell walls. Their ribosomal RNA base sequences also differ from those found in Eubacteria. There are three divisions of archaebacteria: methanogens (organisms that produce methane), thermophiles (organisms that can live in extremely hot, acidic environments, e.g., sulfur springs), and halophiles (organisms that can live only in bodies of concentrated salt water, e.g., the Dead Sea).


(ar-ken′tĕ-rŏn″) [archi- + enteron] The primitive digestive cavity of the gastrula, which is lined with endoderm. Its opening to the outside is the blastopore.


(ar′kĕ-tīp″) [archi- + type] 1. The original type, from which other forms have developed by differentiation. 2. An ideal or perfect anatomical type; used as a theoretical standard in judging other individuals. archetypal (ar″kĕ-tīp′ăl), adj.

archi-, arche-, arch-

[Gr. archē, beginning] Prefixes meaning first, principal, beginning, or original.

architectural distortion

(ar″kĭ-tek′chŭ-răl dis-tor′shŏn) Abnormal compression or displacement of anatomical structures seen on an imaging study. It indicates a disease process, e.g., bleeding, scarring, or a tumor pulling or pushing on tissue.


(ar′kīv″) [Fr. fr. L. fr. Gr. archeia, public offices] 1. A database; a bank of stored information. 2. A location in which documents, images, or records are preserved. archival (ar-kī′văl), adj. archivist (ar′kĭ-vist), n.

arch width

The measured distance between the canines, bicuspids, and the first molars. These distances establish the shape and size of the dental arch.


(ar′sĭ-form″) [arcus + -form] Arcuate.


(ar″kō-bak′tĕr) A genus of gram-negative bacteria of the family Campylobacteraceae. Members of the genus can cause diarrhea.

Arctium lappa

(ark′shē-ŭm lap′ă) SEE: greater burdock.


(ar′kū-ăt, -āt″) [L. arcuatus, bowed] Shaped like an arc; bowed. SYN: arciform. arcuation (ar″kū-ā′shŏn), n.


(ar′kŭs) pl. arcus [L. arcus, a bow] In anatomy, an arch. SEE: arch.

a. senilis An opaque white ring about the periphery of the cornea, seen in the aged. It is caused by the deposit of fat granules in the cornea or by hyaline degeneration. SEE: illus.






(ar′dŏr) [L. ardor, heat] Burning; great heat.

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