#atom

• Protective shell — A rigid anatomical structure resembling a shield, shell, test, or protective skeletal frame, composed of calcareous, chitinous, or siliceous matter, which partially or fully encloses the body of certain invertebrates (such as some crustaceans, mollusks, or protozoans). It serves an essential function as a predator barrier, structural support for soft tissues, and defense against dehydration.
• Ratite — A scientific term designating any flightless running bird of the order of ratites (Struthioniformes), anatomically characterized by severely reduced wings and, specifically, a flat sternum completely lacking a keel (carina), a bony structure essential for anchoring the pectoral muscles required for flight. As an evolutionary consequence, these birds (such as the ostrich, emu, cassowary, or kiwi) permanently lost the capacity for flight, developing powerful legs specialized for high-speed running instead.
• Acrodolichomelia — A rare congenital structural malformation or anatomical feature characterized by disproportionate and excessive length of the distal segments of the limbs (hands and feet relative to arms and thighs, or unusually long digits).
• Acrodolichomelia — A spelling variant or medical transcription error for acrodolichomelia, describing a rare congenital structural malformation characterized by disproportionate and excessive length of the distal segments of the limbs.
• Acrodont — Regarding animals or dentitions; having the teeth fused directly to the free upper margin or crest of the jawbones, without being implanted in bony sockets (alveoli).
• Acromial — Pertaining to the acromion (the superior bony process of the scapula) or located in the upper shoulder region; referring to the anatomical, vascular, or nervous structures associated with this skeletal prominence.
• Acromioclavicular — Pertaining to or relating simultaneously to the acromion and the clavicle; describing the articular, ligamentous, or capsular elements uniting the bony process of the scapula with the external extremity of the clavicle at the superolateral aspect of the shoulder.
• Acromion — A flattened and broadened bony projection forming the superior extremity of the scapular spine, articulating directly with the clavicle and representing the highest structural point of the shoulder skeleton.
• Acropodium — The distal anatomical region of the upper or lower limb in vertebrates, comprising the finger or toe bones (phalanges) and their associated musculo-tendinous structures, representing the terminal segment for support or manipulation (the hand or foot proper, excluding the metapodium and basipodium). In archaeology and art history, the term designates a high pedestal, a structural base, or an ornate plinth upon which a statue or monumental column is placed.
• Acrosincarpy — A structural condition or plant morphological anomaly characterized by the fusion, welding, or partial joining of individual carpels or fruits exclusively at their upper (apical) extremities, while the lower portions or bases remain distinct and separate during development.
• Acrospire — The first embryonic shoot, germ, plumule, or rudimentary stem developing from the embryo of a cereal seed (especially barley) during the germination process. In the malting and brewing industry, monitoring the length and development of this anatomical structure beneath the grain husk (the malted grain) is an essential technical parameter for evaluating the degree of modification and solubilization of starch within the endosperm.
• Sacrum — An obsolete, rare graphic variant or faulty transcription for the sacrum bone (os sacrum); the large triangular bony piece located at the base of the vertebral column, formed by the structural fusion of the five sacral vertebrae, articulating laterally with the iliac bones to close the pelvic girdle.
• Actinella — Romanian name adapted from the taxonomic genus Actinella. In botany, it defines a genus of herbaceous plants (often associated with Hymenoxys) in the Asteraceae family, or a genus of unicellular algae (diatoms) characterized by asymmetric, arched siliceous shells that expand radially at one extremity. In micropaleontology, these fossilized structures are used as environmental indicators.
• Actinenchyma — A structural plant parenchymal tissue, characterized by star-shaped cells with large radiating extensions that leave voluminous intercellular air spaces (lacunae) between them. This special type of tissue is specific to aquatic and marsh plants, having the primary physiological role of facilitating gas circulation and ensuring the buoyancy and respiration of submerged organs in oxygen-deficient environments.
• Actinodromous — Relating to leaf venation; characterized by a structure where three or more primary veins diverge radially from a single central point located at the base of the blade (at the petiole insertion) and run toward the margin or apex of the leaf. This structural anatomical pattern is characteristic of many dicotyledonous plant families (such as maple or grapevine leaves), ensuring an even distribution of raw sap across the entire foliar surface.
• Actinogenesis — A physical process, natural or artificial, through which rays or radiations are generated, produced, and emitted, particularly within the short-wave electromagnetic spectrum, such as ultraviolet radiation, X-rays, or gamma rays. This structural phenomenon constitutes a subject of study in nuclear physics and radiology, being essential for understanding the emission mechanisms of radiant sources, celestial bodies, or technological devices used in industrial medical imaging.
• Actinomere — An anatomical segment or structural zone within a body exhibiting radial symmetry, corresponding to one of the repetitive sections arranged around the central axis. In invertebrate zoology, the term describes each of the similar radial segments making up the body of certain echinoderms (such as the arms of starfish) or coelenterates, including the internal organs and skeletal structures associated with that section.
• Actinomorphic — Relating to flowers or invertebrate organisms; characterized by perfect radial symmetry, allowing the body or organ to be divided into two perfectly symmetrical (mirrored) halves by two or more distinct vertical planes passing through the central axis. In structural botany, an actinomorphic flower (such as that of Rosaceae or Liliaceae) displays floral parts arranged in regular concentric circles, facilitating pollinator access from any direction.
• Actinopterygian — A fish belonging to the class Actinopterygii, structurally characterized by having fins supported by bony or cartilaginous rays (lepidotrichia) arranged radially, without fleshy lobes at the base. This vast group represents the dominant class of extant bony fishes, encompassing over 95% of all known fish species worldwide, adapted to diverse aquatic environments, from mountain freshwaters to abyssal ocean basins.
• Actinostele — An archaic type of vascular cylinder (stele) characteristic of the stems and roots of primitive vascular plants (such as pteridophytes), representing a structural variety of protostele. In cross-section, the central xylem is not circular but features ridges or arms arranged in a radiating (star-like) manner, being fully encircled by a peripheral band of phloem, a configuration optimizing mechanical resistance and sap conduction.