Glossary of zoological terms

Abdomen: The posterior body region of an animal, located behind the thorax, often containing digestive, excretory, and reproductive systems.

Albumen: The clear, protein-rich substance surrounding the yolk in an egg; it cushions the embryo and provides water and nutrients during development.

Aquatic animal: An animal that lives predominantly or entirely in water, including freshwater and marine environments.

Amphibians: Cold-blooded vertebrate animals that typically begin life in water with gills and later develop lungs for breathing air; they have moist skin and include frogs, toads, salamanders, and newts.

Annelids: A group of invertebrate animals with elongated, segmented bodies and a true coelom, including earthworms, leeches, and polychaetes.

Antenna: A paired sensory organ found mainly in arthropods, used to detect chemical signals, touch, humidity, and sometimes sound.

Antennule: A small, paired preoral appendage found in some animals, typically used for sensory perception, balance, and sometimes food manipulation.

Anus: The posterior opening in the digestive tract of animals through which waste material is expelled from the body.

Appendage: A limb or projection extending from the main body of an animal, used for locomotion, manipulation, sensation, or other specialized functions. Common examples include, antennae, fins, and wings

Artery: A blood vessel that carry oxygen-rich blood away from the heart to the tissues of the body.

Arthropods: A diverse goup of invertebrate animals characterized by segmented bodies, jointed limbs, and an exoskeleton made of chitin; includes insects, arachnids, crustaceans, and myriapods.

Atrium: An upper heart chamber that receives blood from the body or lungs and passes it to a ventricle.

Auditory ossicles: The three tiny bones in the middle ear — the malleus (hammer), incus (anvil), and stapes (stirrup) — that transmit and amplify sound vibrations from the eardrum to the oval window of the inner ear.

Back: The dorsal surface of an animal’s body, extending from the neck or shoulders to the hips or tail, often providing protection and structural support.

Beak: A hard, pointed, keratin-covered structure forming the mouthparts of birds and some other animals, used for feeding, grooming, and manipulating objects.

Benthic animal: An organism that lives on, in, or near the bottom of aquatic environments such as oceans, seas, or lakes, often interacting with the substrate for feeding, shelter, or reproduction.

Bilaterally symmetrical animal: An organism whose body plan can be divided into mirror-image halves.

Biramous appendage: An appendage consisting of two branches arising from a single base. Typically, one branch serves locomotion or feeding, while the other may support respiration or additional functions.

Birds: Warm-blooded, egg-laying vertebrate animals characterized by feathers, a beak without teeth, a high metabolic rate, a lightweight skeleton, and the ability (in most species) to fly.

Blastocoel: The fluid-filled cavity within the blastula that provides space for cell movement during gastrulation and contributes to the early structural organization of the embryo.

Blastomere: Any of the individual cells produced by the rapid division of a fertilized egg during early embryonic development, which collectively form the blastula.

Blastopore: The opening that forms during the early stages of gastrulation in the embryo, giving rise either to the mouth or the anus.

Blastula: An early stage of embryonic development consisting of a hollow sphere of cells (called blastomeres) surrounding a fluid-filled cavity known as the blastocoel.

Blood: A specialized fluid circulating in the vessels of many animals, transporting oxygen, nutrients, hormones, and waste products throughout the body; typically composed of plasma, red and white blood cells, and platelets.

Bloodstream: The continuous flow of blood within the circulatory system of animals, transporting nutrients, gases, hormones, and waste products to and from cells.

Blood clotting: The biological process by which blood changes from a liquid to a gel, forming a clot to stop bleeding after injury.

Blood vessel: A tubular structure in animals that carries blood throughout the body, including arteries, veins, and capillaries.

Body cavity: A fluid- or air-filled internal space within an animal’s body that houses and protects organs, provides room for growth, and can aid in circulation or movement.

Bone: A rigid organ composed mainly of collagen and calcium phosphate, forming the structural framework of vertebrate skeletons and providing support, protection, and attachment points for muscles.

Book lung: A layered, internal respiratory organ in certain arthropods, consisting of stacked, leaf-like structures that maximize surface area for gas exchange.

Brain: The central organ of the nervous system in most animals, located in the head; it processes external information, controls body functions, and enables complex behaviors.

Branched gill: A gill structure characterized by a tree-like or filamentous branching pattern, which increases the surface area for respiration.

Brooding: The behavior or process in which an animal protects its eggs or young until they are capable of independent survival.

Capillary: A tiny blood vessel where the exchange of oxygen, carbon dioxide, nutrients, and waste products occurs between the blood and the body’s tissues.

Carapace: A hard, protective exoskeleton that covers the cephalothorax of many arthropods, providing structural support and defense while often housing important organs such as gills.

Cartilage: A flexible, semi-rigid tissue found in many animals, providing support and cushioning in joints, and forming parts of the skeleton in some species.

Cell: The basic unit of structure and function in all living organisms. In animals, each cell has a nucleus that contains genetic material and controls the cell’sa activities.

Cell membrane: The cell membrane is the thin, flexible barrier that surrounds a cell. It protects the cell and controls what goes in and out of it.

Central Nervous System (CNS): The part of the nervous system consisting of the brain and spinal cord, responsible for processing information and coordinating body activities.

Cephalon: The anterior body tagma in certain arthropods, comprising the head region and typically bearing sensory organs (eyes, antennules, antennae) and mouthparts.

Cephalothorax: The fused region formed by the cephalon and thorax in many crustaceans, often covered by a protective carapace.

Chalazae: Pair of spiral, cord-like structures made of twisted strands of albumen that anchor the yolk in the center of the egg. They help maintain the yolk’s position and protect the developing embryo by minimizing movement.

Chela (claw): A pincer-like appendage formed by the modification of the terminal segment of a limb, typically used for grasping, manipulating food, defense, or combat in many arthropods.

Chitin: A substance forming a primary component of the exoskeleton in many invertebrates providing rigidity, protection, and structural support.

Chordates: A group of animals characterized by having, at some stage of development, a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail.

Chromosomes: Structures found inside the nucleus of a cell, made of tightly coiled genetic material. They carry the information that determines an organism’s traits and guide cell activities.

Circulatory system: The group of organs in animals responsible for transporting blood or hemolymph throughout the body, delivering oxygen, nutrients, hormones, and removing waste.

Clot: A semi-solid mass of blood formed during the clotting process to stop bleeding and seal a wound.

Cochlea: A spiral-shaped, fluid-filled structure in the inner ear that converts sound vibrations into electrical signals. These signals are then transmitted to the brain via nerves, enabling hearing.

Coelom: A fluid-filled body cavity completely lined by mesoderm in triploblastic animals, which houses and protects internal organs, allows organ movement, and contributes to body shape and development.

Coelomic pouch: A sac-like structure in the embryo that contributes to the formation of the coelom, the main body cavity of many animals, by expanding and differentiating into various body compartments.

Cold-blooded animal: An animal whose internal body temperature varies with the temperature of the environment, typically relying on external sources to regulate its body heat; includes reptiles, amphibians, fish, and most invertebrates.

Cuticle: A protective, non-cellular layer covering the external surface of many invertebrates, often hardened or sclerotized, providing support, protection, and reducing water loss.

Cytoplasm: The cytoplasm is the thick, jelly-like substance inside a cell that fills the space between the cell membrane and the nucleus. It helps give the cell its shape and supports the movement of materials within the cell.

Decapodid: The post-zoeal larval stage of certain crustaceans, in which the organism resembles a miniature adult but still retains some larval features.

Digestive system: The group of organs in an animal’s body responsible for breaking down food, absorbing nutrients, and eliminating waste; it typically includes the mouth, esophagus, stomach, intestines, and some glands.

Digestive tract: The continuous tube in animals extending from the mouth to the anus, where food is ingested, digested, absorbed, and waste is expelled.

Dorsal hollow nerve cord: A tubular, hollow nerve cord located dorsally (along the back) in chordate embryos and adults, which develops into the central nervous system.

Ear: A sensory organ that detects sound waves and helps with balance; found in many animals with varying structures depending on species.

Ear canal: A tubular passageway in the external ear that extends from the pinna to the eardrum. It channels sound waves inward and protects the middle ear maintaining a controlled environment.

Eardrum (or tympanic membrane): It is a thin, semi-transparent membrane that vibrates in response to sound waves. It separates the external ear from the middle ear and transmits sound vibrations to the auditory ossicles.

Earlobe: The soft, fleshy lower part of the external ear, typically lacking cartilage.

Echinoderms: A group of aquatic invertebrate animals characterized by a radial symmetry, a water vascular system, and a calcareous endoskeleton; examples include sea stars, sea urchins, and sea cucumbers.

Ectoderm: The outermost germ layer of the gastrula, which develops into the nervous system, epidermis, and other external structures.

Egg: A biological structure produced by animals that contains an ovum. When fertilized, it provides a protected environment for the embryo to develop into an independent organism.

Egg-laying animal: An animal that reproduces by laying eggs, either in the environment or in a protected structure, where the embryo develops until hatching; typical of birds, reptiles, amphibians, fish, and many invertebrates.

Egg membranes: Thin, semi-permeable membrane located just beneath the eggshell, consisting of an outer and inner membrane. They provide a barrier against bacterial invasion, help regulate gas exchange, and support the structural integrity of the egg during development.

Embryo: The early stage of development after fertilization, when the organism begins to grow and differentiate before becoming a fetus.

Embryo’s incubation: The period during which an embryo develops in a protected environment—such as inside an egg or a uterus—under stable conditions until it reaches a more advanced stage of growth.

Embryonic tissue: Specialized layers of cells formed during early animal development that give rise to all tissues and organs in the body.

Endoderm: The innermost germ layer of the gastrula, which forms the lining of the digestive tract, respiratory system, and associated organs.

Endoskeleton: An internal supporting structure made of bone or cartilage found within many vertebrate and some invertebrate animals, providing shape, protection, and support for muscles.

Enzyme: A biological molecule produced by animals to speed up chemical reactions, such as the breakdown of food during digestion.

Epidermis: The outermost layer of the skin in vertebrates. It serves as a protective barrier against environmental damage, pathogens, and water loss.

Esophagus: A muscular tube in the digestive system that connects the mouth or pharynx to the stomach, moving food through peristaltic contractions.

Esthetasc: a specialized sensory seta located on the antennae of crustaceans, functioning primarily to detect chemical signals in the environment, such as food-related compounds.

Exoskeleton: A hard external skeleton that supports and protects an animal’s body, commonly found in arthropods and some other invertebrates.

External ear: The outer part of the ear, composed of the visible pinna (or auricle) and the ear canal. It functions to capture sound waves and direct them toward the eardrum, playing a key role in hearing and sound localization.

External gill: A respiratory structure projecting from the surface of an animal’s body, allowing gas exchange directly with the surrounding water.

Eye: A sensory organ that detects light and allows vision; its structure varies widely among animals, from simple light-sensitive spots to complex image-forming organs.

Feather: A lightweight, keratin-based structure that covers the body of birds, providing insulation, waterproofing, and enabling flight.

Fetus: A later stage of prenatal development following the embryo, during which the organism grows and develops the structures needed to survive independently after birth.

Fertilization: The biological process in which a sperm and an ovum merge, combining their genetic material to form a zygote, the first stage of a new organism’s development.

Filter feeder animal: An aquatic animal that obtains food by straining suspended particles, such as plankton or detritus, from the water using specialized feeding structures. They play a key role in nutrient cycling and maintaining water clarity in their ecosystems.

Fins: Flattened limbs found on aquatic animals such as fish and some amphibians, used for steering, balance, and propulsion in water.

Fish: Aquatic, cold-blooded vertebrate animals typically covered with scales, breathing through gills, and usually laying eggs; they inhabit freshwater and marine environments.

Front limb: One of the pair of limbs located at the front of the body in some animals, used for locomotion, grasping, or other functions depending on the species.

Fur: A dense coat of fine hair covering the body of many mammals, providing insulation and protection.

Furca: A rigid, often forked structure located at the posterior end of the trunk, serving as a stabilizer or propulsive aid during locomotion.

Gas exchange: The biological process by which gases such as oxygen and carbon dioxide are transferred between an organism and its environment, typically occurring in lungs, gills, or through the skin.

Gastrula: The stage following the blastula during gastrulation, characterized by the formation of a multilayered structure in which the three primary germ layers are established.

Gastrulation: A fundamental phase of embryonic development during which a simple blastula reorganizes into a multilayered structure called the gastrula. This process establishes the three primary germ layers—ectoderm, mesoderm, and endoderm—that give rise to all tissues and organs.

Genetic material: The substance inside the nucleus of a cell, made up of chromosomes, that carries the information needed to guide the cell’s functions and development.

Germ layers: The three primary cell layers—ectoderm, mesoderm, and endoderm—formed during gastrulation, which give rise to all tissues and organs of the organism.

Gill: A respiratory organ found in many aquatic animals that extracts oxygen from water and expels carbon dioxide.

Gland: An organ in an animal’s body that produces and secretes substances such as enzymes, hormones, or mucus, either into the bloodstream or onto a surface.

Gonopore: An opening on the body of an animal, particularly in arthropods and other invertebrates, through which gametes are released or received during reproduction.

Hair: A filamentous structure made of keratin that grows from the skin of mammals; it provides insulation, sensory function, and protection.

Hatching: The process by which a young animal breaks out of its egg after development, emerging into the external environment.

Head: The front part of an animal’s body that typically contains the brain, mouth, and major sensory organs such as eyes, ears, and nose or antennae.

Heart: An organ in animals that pumps blood through the circulatory system, supplying oxygen and nutrients to tissues and removing waste products.

Heart chamber: A cavity within the heart that receives or pumps blood; most vertebrates have multiple chambers, such as atria (which receive blood) and ventricles (which pump blood).

Hemocoel: The primary body cavity of many invertebrates, especially arthropods, through which blood (hemolymph) circulates freely around organs, serving circulatory function.

Hemoglobin: An iron-containing protein found in red blood cells that binds to oxygen and transports it from the lungs o gills to the body’s tissues.

Hemolymph: The circulatory fluid found in many invertebrates (such as arthropods and mollusks) that combines the functions of blood and an other fluid, transporting nutrients, hormones, and waste without being confined to vessels.

Heteronomous metamerism: A type of metameric organization in which metameres are morphologically and functionally specialized, leading to differentiation into distinct regions or tagmata.

Hind limb: A posterior pair of limbs in animals used for support, locomotion, or specialized functions like jumping or swimming.

Hip: The region on each side of the body where the hind limb connects to the pelvis.

Homonomous metamere: A body segment in which metameres are morphologically similar and not specialized, exhibiting uniform structure along the length of the body.

Hormone: A chemical substance produced by specialized glands in animals that travels through the bloodstream to regulate various physiological processes, such as growth, metabolism, reproduction, or behavior.

Immune cell: A type of cell involved in detecting, attacking, or remembering pathogens as part of the immune system; it includes white blood cells.

Immune system: A complex network of cells, tissues, and organs that defends the body against harmful microorganisms, toxins, and other foreign substances.

Incus (anvil): The second auditory ossicle, situated between the malleus and stapes. It receives vibrations from the malleus and passes them to the stapes.

Inner ear: The innermost part of the ear, containing the cochlea, vestibule, and semicircular canals. It is responsible for converting sound vibrations into neural signals for hearing and for maintaining balance and spatial orientation.

Intestines: A long, tubular part of the digestive system in animals, where most nutrient absorption and water reabsorption occur; typically divided into small and large sections.

Invertebrate animal: An animal that lacks a spinal column, typically having a soft body, and often protected by an exoskeleton or shell.

Jaws: A pair of bony or cartilaginous structures in the head of many animals used for grasping, biting, or chewing food.

Joint: A point where two or more bones or body parts meet, allowing movement and flexibility.

Jointed limb: A limb composed of multiple segments connected by joints, allowing for flexible and precise movement; characteristic of arthropods and vertebrates.

Lamellar gill: a type of respiratory structure in aquatic animals, especially crustaceans, composed of thin, plate-like filaments that maximize surface area for efficient gas exchange.

Large intestine: The wider, shorter tubular organ in animals responsible for absorbing water and electrolytes from indigestible food residues and forming feces.

Larva: The immature form of an animal that undergoes metamorphosis before reaching its adult stage, often differing significantly in structure, habitat, and feeding habits from the adult.

Larynx: A cartilaginous structure in the respiratory tract of many vertebrate animals that houses the vocal cords and plays a role in breathing, sound production, and protecting the airway.

Laying: The process by which an animal deposits eggs outside its body, usually after fertilization, to allow development in the external environment.

Leathery eggs: An egg with a soft, flexible shell made of tough, leathery material; commonly found in reptiles and some amphibians.

Leg: A paired appendage used by an animal for locomotion, support, or manipulation of the environment.

Limb: A paired appendage of an animal’s body used for movement, manipulation, or support, such as wings or fins.

Live-birth animal: An animal that gives birth to fully developed offspring rather than laying eggs; most commonly found in mammals, but also occurs in some reptiles, amphibians, and fish.

Locomotor appendage: A body appendage specialized for movement, allowing an animal to walk, swim, crawl, or climb.

Lung: A respiratory organ in terrestrial vertebrates that facilitates gas exchange by absorbing oxygen from the air and releasing carbon dioxide.

Lymph: A clear fluid that circulates through the lymphatic system, helping to transport white blood cells and remove waste and pathogens.

Lymph node: A small, bean-shaped organ of the lymphatic system that filters lymph and houses immune cells that detect and respond to pathogens.

Lymphatic system: A network of vessels, nodes and organs that transports lymph, supports immune system, and helps maintain fluid balance in the body.

Malleus (hammer): The first of the three auditory ossicles in the middle ear, attached to the eardrum. It transmits sound vibrations from the eardrum to the incus.

Mammals: Warm-blooded vertebrate animals characterized by the presence of mammary glands (in females), hair or fur, and typically live birth. Most mammals also have an hearth with four chambers and a well-developed brain.

Mammary gland: A specialized gland in female mammals that produces and secretes milk to nourish offspring.

Maxilliped: A thoracic appendage of crustaceans modified to assist in feeding and often involved in manipulating and transporting food to the mouth.

Membrane: A thin, flexible layer of tissue or material that acts as a barrier or interface between different environments or structures. In biological contexts, membranes regulate the passage of substances, provide protection, and support cellular or anatomical functions.

Mandible: A jaw-like, often paired, mouthpart in mandibulate animals, used for biting, cutting, or grinding food.

Mandibulate animal: An animal possessing mandibles—jaw-like mouthparts used for biting, cutting, or grinding food.

Maxilla: a paired jaw structure found in many arthropods, positioned just behind the mandibles, serving mainly to manipulate, cut, or hold food and, in some groups, also involved in sensory or respiratory functions.

Mesoderm: The middle germ layer of the gastrula, giving rise to the muscles, skeleton, circulatory system, and other internal organs.

Metabolic rate: The speed at which an organism converts food into energy to maintain vital functions and activities.

Metabolism: The set of life-sustaining chemical reactions in animals that convert food into energy and building blocks for growth, repair, and maintenance.

Metamere: A single segment of a metamerically organized animal, representing one of the repeated structural units that collectively form the body. Each metamere may contain components of the various systems.

Metamerism: The condition in which an animal’s body is divided into a series of repeated segments, each containing similar sets of organs or structures, often enhancing flexibility, locomotion, and specialization of functions.

Metamorphosis: A biological process in which an animal undergoes a series of dramatic physical and physiological changes during development, transforming from a larval stage into its adult form, often with different morphology, behavior, or habitat.

Mesodermal tissue: The middle layer of embryonic tissue in animals that gives rise to muscles, the circulatory system, bones, and other internal structures.

Middle ear: The air-filled cavity located between the external ear and the inner ear, containing the auditory ossicles. It functions to transmit and amplify sound vibrations from the eardrum to the inner ear.

Milk: A nutrient-rich fluid produced by the mammary glands of female mammals to feed their young; it contains proteins, fats, sugars, vitamins, and antibodies.

Moist skin: A type of skin that remains damp to aid in respiration and prevent dehydration, commonly found in amphibians.

Molting: The biological process in which some animals (as arthropods) sheds its external covering, such as an exoskeleton or cuticle, to allow growth, development, or replacement of worn structures. During this process, a new protective layer forms beneath the old one, which hardens or matures after shedding.

Mollusks: A diverse group of invertebrate animals characterized by a soft, unsegmented body, often protected by a hard calcium carbonate shell; includes snails, clams, squids, and octopuses.

Mouth: The opening through which an animal takes in food, often also used for vocalization, respiration, or sensory functions depending on the species.

Mouthpart: Any specialized structure or appendage associated with the mouth of an animal, used for feeding, manipulating food, or sensory functions.

Mucus: A slippery, viscous secretion produced by specialized cells or glands in animals, used to protect, lubricate, or trap particles on body surfaces such as the skin, gills, or digestive and respiratory tracts.

Muscle: A tissue composed of fibers capable of contracting to produce movement or maintain posture in animals.

Muscle contraction: The process by which muscle fibers shorten and generate force, allowing movement of body parts or internal structures in animals.

Muscle fiber: A long, cylindrical cell that contracts in response to nerve signals, enabling movement and force generation.

Naupliar stage: Any of the early phases in the development of certain crustaceans, characterized by a simple body structure, a single eyeeye, and limited appendages, during which the organism undergoes growth and differentiation before reaching more advanced stages such as zoea.

Neck: The part of an animal’s body that connects the head to the trunk, containing structures such as the trachea, esophagus, blood vessels, and part of the spinal column.

Nerve: A bundle of fibers made of neurons that transmits electrical signals between different parts of the body and the central nervous system.

Nerve cord: A major bundle of nerve fibers running along the body of many animals, transmitting signals between the brain and the rest of the body.

Nervous system: A complex network of cells and organs that detects stimuli, processes information, and coordinates responses; it typically includes the brain, spinal cord, and nerves in vertebrates.

Neural signal: An electrical impulse generated by a cell and transmitted along nerve fibers to the brain for processing and interpretation.

Neuron: A specialized cell that transmits electrical impulses within the nervous system, enabling communication between the brain, spinal cord, and other body parts.

Nose: A sensory organ responsible for detecting smells and, in many animals, involved in respiration as part of the respiratory system.

Notochord: A flexible, rod-shaped supportive structure found in the embryos of all chordates and in some adult chordates, serving as a primary axial support.

Nucleus: The control center of the cell. It contains the genetic material and directs the cell’s growth, division, and activities.

Oolemma: The outer cell membrane of an egg cell (ovum). It serves as a protective boundary and plays a key role in fertilization by regulating the entry of sperm.

Open circulatory system: A type of circulatory system in which blood or hemolymph flows freely through body cavities (such as the hemocoel) and bathes organs directly, rather than being confined entirely within vessels.

Opisthosoma: The posterior tagma of some animals, located behind the prosoma, containing digestive, reproductive, and sometimes respiratory organs.

Organ: A specialized structure within the body made up of different tissues that performs a specific function essential to the organism’s survival.

Oval window: A membrane-covered opening between the middle ear and the cochlea of the inner ear. It receives vibrations from the stapes and transmits them into the fluid-filled cochlea.

Ovary: The ovary is a female reproductive organ in animals that produces eggs (ova) and hormones necessary for reproduction.

Oviparous animal: An animal that reproduces by laying eggs, with embryos developing and hatching outside the mother’s body; common among birds, reptiles, amphibians, fish, and many invertebrates.

Ovulation: The process in which a mature egg is released from an ovary in female animals, making it available for fertilization.

Ovum (egg cell): The female reproductive cell in animals, which can be fertilized by a sperm cell to begin the development of a new organism.

Oxygen-poor blood: Blood that contains a low concentration of oxygen and is typically returning to the heart or lungs for re-oxygenation.

Oxygen-rich blood: Blood that contains a high concentration of oxygen, typically transported from the lungs or gills to the tissues through arteries.

Pedunculated eye: An eye of certain arthropods, mounted on a movable stalk that allows independent rotation and extended visual range.

Pelvis: A bony structure in the lower part of the trunk in vertebrate animals that supports the hind limbs and protects some organs.

Pereopod: A thoracic appendage of crustaceans attached to a pereionite, mainly adapted for locomotion but sometimes specialized for other functions such as feeding or defense.

Pereion: The thoracic region of certain crustaceans, consisting of multiple pereionites that typically bear pereopods. It is located between the cephalothorax and the pleon.

Pereionite: A single thoracic segment of the pereion in crustaceans, each typically bearing one pair of pereopods.

Peristaltic contraction: A series of rhythmic, wave-like muscle contractions that occur in tubular organs (such as the esophagus or intestines) of animals to move contents like food or waste in a single direction.

Pharyngeal slit: Openings or grooves located in the pharynx region of chordate embryos that may develop into gills or other structures depending on the species.

Pharynx: A muscular passage in many animals located behind the mouth and nose, connecting them to the esophagus and (in some species) the respiratory tract; involved in both feeding and breathing.

Pigment: A natural substance that produces color by selectively absorbing and reflecting specific wavelengths of light. In zoology, pigments are responsible for coloration in tissues such as skin, eyes, and eggshells, often serving functions in camouflage, signaling, or protection from UV radiation.

Pinna (or auricle): The external, visible part of the ear made of cartilage and skin, shaped to collect and funnel sound waves into the ear canal. It also helps in determining the direction of sounds.

Plasma: The liquid component of blood in animals, mostly water, that carries cells, nutrients, hormones, and waste products throughout the body.

Platelet (Thrombocyte): A small cell fragment in the blood of animals involved in blood clotting to prevent bleeding.

Pleon: The abdominal region of crustaceans, located posterior to the pereion, composed of pleonites and typically bearing pleopods and the telson.

Pleonite: An individual abdominal segment of the pleon in crustaceans, usually bearing a pair of pleopods.

Pleopod: A biramous abdominal appendage of crustaceans attached to a pleonite, primarily used for swimming, respiration, or brooding eggs.

Pleura: Flexible cuticle regions that connect sclerites in an arthropod’s exoskeleton, allowing movement despite the rigidity of the surrounding plates.

Pore: A small opening on the surface of an organism’s body or structure that allows the passage of substances such as gases, liquids, or cells.

Post-anal tail: An extension of the body past the anal opening present in the embryos and often adults of chordates, used for locomotion or balance in many species.

Pregnancy: The state in which a fertilized egg develops inside the uterus of an animal, leading to the growth of an embryo or fetus until birth or hatching.

Preoral appendage: An appendage located in front of the mouth in an animal, often involved in sensory perception, manipulation, or processing of food.

Prosoma: The anterior tagma of chelicerates, formed by the fusion of several metameres, bearing the eyes, mouthparts, and legs, and functioning primarily in sensory perception and locomotion.

Protostomes: A major clade of bilaterally symmetrical animals characterized by embryonic development in which the mouth forms before the anus from the blastopore.

Radial symmetry: A body plan in which the parts are arranged around a central axis, allowing the animal to interact with the environment from all sides equally.

Red blood cell (Erythrocyte):A type of cell in animals responsible for transporting oxygen from the lungs or gills to tissues, containing the pigment hemoglobin.

Reproductive system: A group of organs and structures responsible for producing offspring. It includes organs that produce eggs or sperm, facilitate fertilization, and support the development of embryos or offspring.

Reptiles: Cold-blooded, egg-laying vertebrate animals typically covered with scales, lungs for breathing air, and typically laying leathery eggs; includes snakes, lizards, turtles, and crocodilians.

Respiratory system: The system in animals responsible for the intake of oxygen and the release of carbon dioxide, involving organs such as lungs, gills, or skin.

Respiratory tract: The series of organs and structures in animals involved in the intake of oxygen and release of carbon dioxide, including nose, trachea, lungs, or gills.

Scales: Small, rigid plates that cover the skin of many fish and reptiles, providing protection and sometimes aiding in movement.

Scapula: A flat, triangular bone in the shoulder region of vertebrate animals that connects the front limb to the body and provides attachment for muscles.

Scavenger: An animal that feeds on dead or decaying organic matter, playing a key role in ecosystems by recycling nutrients and preventing the accumulation of carcasses.

Sclerite: A hardened plate or section of an arthropod’s exoskeleton, forming part of the protective armor. Sclerites are connected by more flexible regions of cuticle to allow movement.

Segmented body: A body structure divided into repeated sections or segments, each of which may have specialized functions; common in arthropods, annelids, and some other invertebrates.

Semicircular canals: Three looped tubes in the inner ear arranged perpendicularly to each other. They detect rotational movements of the head and help maintain dynamic balance.

Sensory organ: A specialized structure in animals that detects specific types of stimuli from the environment, such as light, sound, smell, taste, or touch (e.g., eyes, ears, nose, skin, tongue).

Sensory seta: a hair-like structure on the body or appendages of arthropods and other invertebrates, specialized for detecting environmental stimuli such as touch, vibration, chemical cues, or temperature.

Setose leg: An appendage of an animal, typically an aquatic invertebrate, that is covered with bristle-like structures used for feeding, filtering particles, or sensing the environment.

Shell: A hard, protective outer covering secreted by an animal, found as the external skeleton in invertebrates, or as the rigid protective layer surrounding eggs in oviparous animals.

Shoulder: The region of the body where the front limb attaches to the trunk; in vertebrates, it includes the scapula and associated muscles and joints that allow limb movement.

Skeleton: The internal or external framework of bones or other rigid structures that supports and protects an animal’s body.

Skin: The outer covering of an animal’s body that protects internal organs, helps regulate temperature, and can aid in respiration or sensing the environment, depending on the species.

Skull: The bony structure in vertebrates that forms the head, protects the brain, and supports sensory organs such as the eyes, ears, and nose.

Small intestine: The long, coiled tubular organ in animals where most digestion and nutrient absorption occurs after the stomach.

Sperm: The male reproductive cell designed to fertilize the ovum, carrying half of the genetic material needed to form a zygote, the first stage of a new organism’s development.

Spermatophore: A package of sperm produced by male animals, especially invertebrates and some fish, which is transferred to the female for fertilization.

Spinal column: The series of vertebrae extending from the skull to the lower back, enclosing and protecting the spinal cord while providing structural support and flexibility to the body.

Spinal cord: A long, cylindrical structure that runs inside the vertebral column, transmitting signals between the brain and the rest of the body.

Spiracle: A small external opening on the body of some terrestrial arthropods that allows air to enter the tracheal system for respiration.

Stapes (stirrup): The third and smallest auditory ossicle, connecting to the oval window of the inner ear. It transmits sound vibrations from the incus to the inner ear.

Sterile egg: A non-viable egg that cannot develop into an embryo, typically because it has not been fertilized.

Stomach: An organ found in many animals, where food is stored and broken down by enzymes before passing to the intestines.

Syncelom (mixocoel): A mixed body cavity formed partly from coelomic pouches and partly from remnants of the blastocoel, combining features of both coelom and hemocoel.

Tail: A posterior, flexible extension of the body found in many animals, used for balance, communication, locomotion, or defense.

Tail fan: The flattened, fan-like structure at the posterior end of many crustaceans, formed by the telson and uropods, used for rapid backward swimming and maneuvering.

Tagma: A single functional or morphological body region formed by the fusion or specialization of metameres in some animals.

Telson: The terminal, unsegmented part of the body in crustaceans and other arthropods, located after the last pleonite and often forming, with the uropods, the tail fan.

Tendon: A flexible but strong band of tissue that attaches muscle to bone or other rigid structures, transmitting the force generated by the muscle to produce movement.

Terrestrial animal: An animal that lives primarily or entirely on land rather than in water or air.

Thorax: The middle body region of an animal, typically situated between the head and abdomen, which often bears locomotor appendages such as legs or wings and contains muscles and organs associated with movement and respiration.

Tissue: A group of similar cells that work together to perform a specific function in the body.

Tongue: An organ in the mouth used for tasting, manipulating food, and aiding in swallowing and vocalization.

Tooth: A hard, calcified structure embedded in the jaws of vertebrates, used for biting, chewing, and processing food.

Trachea: A tubular structure in the respiratory system of many animals that connects the larynx to the lungs, allowing the passage of air; in insects, it refers to a system of air-filled tubes used for breathing.

Triploblastic animal: An animal whose embryo develops three primary germ layers—ectoderm, mesoderm, and endoderm—which give rise to all tissues and organs.

True coelom: A fluid-filled body cavity completely lined by mesodermal tissue, allowing for the development of complex organs and greater body flexibility; found in annelids, mollusks, arthropods, echinoderms, and chordates.

Trunk: The central part of an animal’s body excluding the head, neck, and limbs; it contains major internal organs such as the heart, lungs, and digestive system organs.

Unfertilized egg: Egg laid by a female without being fertilized by a male. It does not contain a viable embryo, but may still be produced as part of natural reproductive cycles.

Unsegmented body: A body structure in animals that is continuous and not divided into repeated segments or sections.

Uropod: A paired appendage attached to the last pleonite of crustaceans, often forming part of the tail fan and aiding in swimming and steering.

Uterus: A muscular, hollow organ in female mammals where a fertilized egg cell implants and develops into an embryo and then a fetus until birth.

Vein: A blood vessel that carries oxygen-poor blood from the body tissues back toward the heart.

Ventricle: A lower heart chamber that pumps blood out of the heart to the lungs or the rest of the body.

Vertebra: One of the bones that form the spinal column in vertebrate animals, providing structural support and protecting the spinal cord.

Vertebrate animal: An animal belonging to the subphylum Vertebrata, characterized by having a spinal column, a well-developed endoskeleton, and a complex nervous system.

Vessel: A tubular structure in animals that carries fluids such as blood or lymph throughout the body; includes arteries, veins, and capillaries in vertebrates.

Vestibule: The central part of the inner ear located between the cochlea and semicircular canals. It contains sensory organs that detect changes in head position and linear acceleration, contributing to balance.

Vitelline membrane: A thin, transparent, proteinaceous layer that surrounds the yolk of an egg, particularly in oviparous animals such as birds, reptiles, and some invertebrates. It functions as a protective barrier against physical damage and microbial invasion, and it plays a crucial role in fertilization and the regulation of substances between the yolk and the surrounding environment.

Vocal cords: Paired folds of tissue located in the larynx of many vertebrate animals that vibrate to produce sounds when air passes through them.

Warm-blooded animal: An animal that maintains a constant internal body temperature regardless of external conditions, typically through metabolic heat production; includes birds and mammals.

Water vascular system: A network of fluid-filled canals unique to echinoderms that aids in locomotion, feeding, and respiration.

Wings: Paired limbs adapted for flight, found in birds, bats, insects, and some other animals; they may also serve other functions such as gliding or display.

White blood cell (Leukocyte): A type of cell in animals that defends the body against infections and foreign substances.

Yolk: The nutrient-rich part of an egg that provides energy and materials for the developing embryo; it contains proteins, fats, and vitamins.

Zoea larva: An early developmental stage of many decapod crustaceans, emerging from fertilized egg. During this stage, the larva is typically free-swimming, with specialized appendages for feeding and locomotion, and undergoes several molts before transforming into a juvenile form.

Zygote: The single cell formed when a sperm fertilizes an ovum, containing a complete set of genetic material that will develop into a new organism.