#vertebra

• Acalepha — An archaic or scientific zoological term used to designate large marine jellyfish or, broadly, organisms belonging to the class Scyphozoa (phylum Cnidaria). These gelatinous, skeletonless organisms are characterized by their umbrella- or bell-shaped bodies and the presence of stinging cells (cnidocytes) on their tentacles, used for self-defense and paralyzing prey.
• 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.
• Acrocyst — A temporary spherical anatomical structure or protective vesicular sac formed during the reproduction of certain colonial hydrozoans, protecting eggs or planulae.
• 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.
• 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.
• Actinozoa — A class of marine invertebrates within the coelenterate phylum, characterized by a sac-like body with a pronounced radial symmetry, featuring a mouth surrounded by tentacles and a gastric cavity partitioned by vertical septa. This structural grouping includes corals and sea anemones, most being sessile organisms that secrete a calcareous exoskeleton.
• Adelopod — An organism or anatomical segment characterized by hidden, invisible feet upon superficial examination, or completely retracted inside body cavities; in zoology, it describes invertebrate species or larvae whose locomotor appendages are rudimentary or structurally masked by skin folds. The term defines an evolutionary adaptation specific to fossorial or parasitic organisms.
• Adelostomatous — Having a hidden, invaginated mouth opening or one protected by special anatomical folds; an organism or anatomical structure characterized by a retracted mouth apparatus, specific to certain groups of invertebrates or marine larvae. This adaptive morphological trait provides enhanced protection against predators or abrasive environmental factors, while optimizing the filter-feeding mechanism.
• Adenopodous — Having feet or locomotor appendages provided with special secretory glands; in zoology, it describes invertebrate organisms (such as certain arthropods or annelids) whose limbs include glandular structures that eliminate adhesive, silky, or toxic substances. This structural term highlights a unique locomotor and defensive adaptation in the animal kingdom.
• Aglea — A genus of sea slugs (opisthobranch gastropods) belonging to the family Aglajidae, inhabiting tropical and temperate benthic marine environments. These marine creatures possess a soft, often vividly colored body and are specialized predators that hunt other small invertebrates on sandy substrates.
• Amphiarthrosis — A type of semi-movable bony joint characterized by the presence of a structural fibrocartilaginous disc situated between the articular surfaces, bound by strong peripheral ligaments that limit movement to a simple sliding or minimal flexion. Classic examples in human anatomy are the joints between vertebral bodies (intervertebral discs) and the pubic symphysis.
• Amoebocyte — A free migratory cell with amoeboid movements, found in the blood, hemolymph, or interstitial tissues of many invertebrate animals (such as sponges, coelenterates, or mollusks); it features a flexible structure and performs fundamental functions of intracellular digestion, nutrient transport, excretion, and immune defense via phagocytosis.
• Ammothea — A taxonomic genus comprising species of sea spiders (pycnogonids) belonging to the Ammotheidae family, predominantly distributed in the cold and deep waters of the world's oceans, including Antarctic regions. These marine invertebrates are structurally characterized by an extremely reduced body and unusually long legs relative to the trunk, possessing a well-developed proboscis used to absorb nutrients from the tissues of soft organisms, such as cnidarians or sea sponges.
• Amniote — An organism or higher vertebrate belonging to the Amniota clade, characterized by embryonic development protected by complex cellular membranes, including the amnion. In an adjectival sense, it describes an egg or an evolutionary stage equipped with an amniotic sac, allowing reproduction completely independent of the external aquatic environment.
• Anamniote — A lower vertebrate animal (such as fish or amphibians) whose embryo develops directly in the aquatic environment and lacks complex protective embryonic membranes, specifically the amnion and allantois. As an adjective, it describes a species, embryonic stage, or anatomical structure specific to these non-amniotic vertebrate organisms, which entirely depend on the presence of water for reproduction and egg survival.
• Ankylochely — A structural malformation or morphological feature specific to certain arthropods or mollusks, characterized by the fusion, ankylosis, or partial to total immobilization of the claws, chelicerae, or shell fold margins, preventing normal grasping movement or functional articulation of the appendages.
• Ankylosing — Causing, promoting, or accompanying ankylosis; causing progressive stiffening and permanent immobilization of the joints through fibrous or bony fusion. It is commonly used to describe severe forms of chronic autoimmune arthritis affecting the axial skeleton.
• Annelid — An invertebrate animal from the phylum Annelida, structurally characterized by segmented rings (metamerism), with the body composed of a series of similar ring-like segments. As an adjective, it describes any organism, anatomical trait, or biological process displaying ringed or metameric structures.
• Annelid — The feminine singular form of 'anelid' or a designation used for any invertebrate belonging to the phylum Annelida. In its plural form (anelide), it represents the broad taxonomic group gathering segmented worms (oligochaetes, polychaetes, and hirudineans).
• Annelids — A major phylum of bilaterally symmetrical invertebrate animals (Annelida), comprising segmented worms such as earthworms, leeches, and marine polychaetes. Structurally, their body is characterized by homonomous metamerism, being divided externally and internally (by transverse septa) into a series of repeating ring-like segments, possessing a closed circulatory system and a ventral ganglionic nerve cord.