#animal anatomy

• Acanthopterous — An organism or anatomical feature specific to fishes of the superorder Acanthopterygii, characterized by the presence of hard, sharp, and unbranched rays in the anterior part of the dorsal and anal fins. As an adjective, it describes a fin structure equipped with rigid bony spines used for self-defense and efficient hydrodynamic stabilization during swimming.
• 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.
• Agnathostome — An organism or marine animal structurally characterized by the total absence or complete atrophy of the jaws, having a simple or circular oral opening adapted for fixation or suction. In archaic zoological taxonomy, the term described primitive species of invertebrates or jawless fish that lacked a developed maxillary bone structure.
• Apterous — Having no wings; an organism, bird, or insect structurally lacking wings or a flight apparatus. This anatomical trait can be primitive, unchanged throughout evolution, or secondarily acquired through the regression or atrophy of wings as a specific adaptation to terrestrial, subterranean, or parasitic environments.
• Ardennais horse — The feminine singular form of the adjective 'ardenez' (specific to the Ardennes region); as a noun, it denotes a breed of heavy draft horses (the Ardennais horse), originating from the Ardennes mountain range, structurally recognized for its massive musculature, short, powerful legs, and exceptional resistance to heavy physical labor and low temperatures. In a geographical or linguistic sense, it can reference a regional dialect variant or geomorphological features specific to the Franco-Belgian plateau.
• Knucklebone — A small, short, and hard bone from the hind leg joints of small horned animals (astragalus), historically used by children and adults as a basic piece in a traditional game of chance and skill that bears its name. Each of the four long faces of the bone has a specific name and scoring value (such as blank, notch, king, or janissary), and the game's objective is to throw and flip the pieces according to popular rules.