Welcome to the Integrative Biology Virtual Glossary!

A virtual online glossary derived from leading McGraw-Hill textbooks in zoology, botany, environmental science and marine biology.

A

abdomen The portion of a tetrapod's body between the thorax and pelvic girdle.The region of an arthropod's body behind the thorax. It contains the visceral organs.

abducens A nerve.

abduct An action.

abductor A muscle that performs the action.

abioticfactor Nonliving component in the environment.

abiotic A non-living (physical or chemical) component of the environment. Compare biotic.

abomasum Fourth and last chamber of the stomach of ruminant mammals.

aboral The part of the body farthest from the mouth.

aboral surface The surface opposite the mouth (or oral surface) in echinoderms.

abrasion The mechanical process of gradually breaking down a hard layer, as in a seed coat.

abscess Dead cells and tissue fluid confined in a localized area, causing swelling.

abscisic acid A plant hormone associated with dormancy, abscission of organs, and water stress.

abscission The detachment of leaves, flowers, or fruit from a plant, usually at a mechanically weak location, termed the abscission zone.

absorption spectrum Graph of absorbance values for different wavelengths of light.

absorption The process of taking in, as uptake by roots.

absorptive feeding A means of taking up dissolved foodmaterial through specialized organs or across the body wall.

absorptive heterotroph Mode of nutrition in which organisms secrete digestive enzymes into a substrate and then absorb the products of digestion; nutritional mode of fungi.

abyssal plains Flat, sediment-covered areas in the ocean basin usually 3,000 to 5,000 m deep.

abyssal zone The bottom from a depth of approximately 4,000 m (13,000 ft) to 6,000 m (13,100 to 20,000 ft).

abyssopelagic zone The pelagic environment from a depth of 4,000 m (13,000 ft) to 6,000 m (20,000 ft) .

acanthella Developing acanthocephalan larva, between an acanthor and a cystacanth, in which the definitive organ systems are developed.

Acanthocephala The phylum of aschelminths commonly called the spiny-headed worms.

acanthodians A group of the earliest known true jawed fishes from Lower Silurian to Lower Permian.

acanthor Acanthocephalan larva (first larval stage) that hatches from the egg. The larva has a rostellum with hooks that are used in penetrating the host's tissues.

accessory buds Those buds adjacent to a primary bud and usually smaller in size.

accessory fruit Most or part of the fruit is derived from tissue other than the ovary of a flower.

accessory pigment One of several nongreen photosynthetic pigments found in marine plants that absorbs light energy from the center of the visible light spectrum and transfers it to the green pigment chlorophyll.

accidental myiasis Presence within a host of a fly not normally parasitic.Also called pseudomyiasis.

accidental parasite Parasite found in other than its normal host. Also called an incidental parasite.

acclimation The change in tolerance of an animal for a condition in its environment.

acclimatization Gradual physiological adaptation in response to relatively long-lasting environmental changes.

accommodation The adjustment of the eye for various distances.

acetabulum True sucker, especially in flukes and leeches; the socket in the hip bone that receives the thigh bone.

Acetospora The protozoan phylum characterized by members having multicellular spores; all parasitic in invertebrates. Examples: acetosporans (Paramyxa, Halosporidium).

acetylcholine A neurotransmitter liberated by certain neurons. It is excitatory at neuromuscular junctions and inhibitory at other synapses.

achene A small, dry, one-seeded indehiscent fruit; the pericarp is easily separated from the seed coat.

acicula Needlelike supporting bristle in parapodia of some polychaetes.

acid rain The combination of sulfur dioxide and nitrogen oxides with water in the atmosphere. This combination produces acidicprecipitation called acid rain. The burning of fossil fuels is a major contributor to acid rain.

acid H+(proton) donor; a substance that associates to release H+ and thus cause the pH of the solution to be less than 7.0.

acidic Possessing a relatively large number of hydrogen ions; having a pH less than 7.0.

acinar Cellular arrangement of some glands.

acinus A small lobe of a compound gland or a saclike cavity at the termination of a passage.

acoelomate Lacking a body cavity between the gut and the outer body wall musculature,as in flatworms and proboscis worms.

acontium Threadlike structure bearingnematocysts located on mesentery of sea anemone.

acorn worms (or enteropneusts)See hemichordates.

acoustic to hear.

acquired immunity Immunity arising from a specific immune response, stimulated by antigen in the host's body (active) or in the body of another individual with the antibodies or lymphocytes transferred to the host (passive).

acquisition Gaining a response in a learning paradigm.

acridine dyes Organic pigment molecules that are capable of causing permanent genetic changes (mutations).

acrocentric Chromosome with centromere near the end.

acromion A process on the scapula.

acron Preoral region of an insect.

acrorhagi Nematocyst-armed defensive structures of anemones.

acrosome The enzyme-filled cap on the head of a sperm. Used in egg penetration.

actin A protein in a muscle fiber that, together with myosin, is responsible for contraction and relaxation.

actinomorphic Regular or radially symmetric flowers.

actinotroch Larval form found in Phoronida.

action potential The sequence of electrical changes occurring when a nerve cell membrane is exposed to a stimulus that exceeds its threshold.

activational effects Effects of hormones that act as triggering influences on the expression of particular behavior patterns, response is rapid, in hours or days. Contrast with organizational effects.

activational effects of hormones Events that occur in the behavior of an animal where an external stimulus triggers a hormonally mediated response by the organism.

active avoidance learning A form of operant conditioning where the animal has to act in order to avoid some noxious consequence (e.g., shock).

active continental margin A continental margin that is colliding with another plate and as a result is geologically active.Compare passive continental margin.

active transport A process that requires an expenditure of ATP energy to move molecules across a cell membrane; usually moved against the concentration gradient with the aid of specific transport proteins.

adaptation An anatomical structure, physiological process, or behavioral trait that evolved by natural selection and improves an organism's ability to survive and leave descendants.

adaptive behavior Behavior patterns that make an organism more fit to survive and reproduce in comparison with other members of the same species.

adaptive radiation Evolutionary diversification that produces numerous ecologically disparate lineages from a single ancestral one, especially when this diversification occurs within a short interval of geological time.

adaptive value Degree to which a characteristic helps an organism to survive and reproduce or lends greater fitness in its environment; selective advantage.

adaptive zone A characteristic reaction and mutual relationship between environment and organism ("way of life") demonstrated by a group of evolutionarily related organisms.

addictive drug Substance that induces an addiction by causing physiological dependence, psychological dependence, and/or tolerance.

adduct An action.

adductor A muscle that draws a part toward a median axis, or a muscle that draws the two valves of a mollusc shell together.

adenine A purine base; component of nucleotides and nucleic acids. A nitrogen base found in both DNA and RNA.

adenohypophysis An endocrine gland.

adenophorea The class of nematodes formerly called Aphasmidia. Examples: Trichinella, Trichuris.

adenosine diphosphate (ADP) The building block for ATP; by adding a terminal phosphate group and a large amount of energy, ATP can be formed.

adenosine monophosphate AMP molecule; created when the terminal phosphate is lost from a molecule of adenosine diphosphate.

adenosinetriphosphate (ATP) A complex organic compound composed of the molecule adenosine and three phosphates, which serves in short-term energy storage and conversion in all organisms.

adenosine (di-, tri) phosphate (ADP and ATP). A nucleotide composed of adenine, ribose sugar, and two (ADP) or three (ATP) phosphate units; ATP is an energy-rich compound that, with ADP, serves as a phosphate bond-energy transfer system in cells.

adhesio Interthalamica Part of the brain.

adhesion Attraction between unlike molecules. The polar nature of water molecules causes them to adhere to a surface.

adhesion The attraction of unlike particles; water particles adhere to the surface of clays.

adhesive disc Suckerlike circular organ or organelle used for attachment.

adhesive gland Attachment glands in Turbellaria that produce a chemical that attaches part of the turbellarian to a substrate.

adipose Fatty tissue; fatty.

adipose tissue Fat-storing tissue.

adoptive immunity Immune state conferred by inoculation of lymphocytes, not antibodies, from an immune animal rather than by exposure to the antigen itself.

ADP (adenosine diphosphate) A nucleotide diphosphate that is often phosphorylated to form ATP.

adrenal glands Paired endocrine glands, located next to the kidneys in the abdomen. The adrenal cortex produces steroid hormones involved in water balance, glucose metabolism, and electrolyte balance. The adrenal medulla produces adrenaline and noradrenaline, which are involved in glucose metabolism, heart rate, and blood pressure.

adrenaline A hormone produced by the adrenal, or suprarenal, gland; epinephrine.

Adrenocorticotrophic A hormone secreted by the pituitary.

adsorption The adhesion of molecules to solid bodies.

adventitious A structure arising at some location not usually expected, such as on a stem.

adventitous root A root produced by a stem or leaf rather than by a root.

aeciospore Binucleate spore produced in an aecium.

aecium Cup-shaped structure in rust fungi composed of binucleate hyphae; aeciospores are formed here.

aedeagus Copulatory organ or penis in insects and acarines.

aerate To supply with oxygen.

aerial Pertaining to being in the air, such as a root projecting from an aboveground stem.

aerobic Having molecular oxygen present; an oxygen-dependent form of respiration.

aerobic respiration Respiration in the presence of oxygen.

aerobic Oxygen-dependent form of respiration.

aestivation The condition of dormancy or torpidity during the hot summer months.

afferent Adjective meaning leading or bearing toward some organ, for example, nerves conducting impulses toward the brain or blood vessels carrying blood toward an organ; opposed to efferent.

aflatoxin A complex of four mycotoxins produced by Aspergillus flavus; often found in peanut products.

afterbirth The placental and fetal membranes expelled from the uterus after childbirth.

agar Gelatinous product extracted from the walls of some red algae; used in growth media for microorganisms and in tissue culture.

age structure The proportion of a population that is in preproductive, reproductive, and postreproductive classes.

agent orange A herbicide used as a defoliant in the Vietnam War; composed of 2,4-D and 2,4,5-T.

agregate fruit A fruit derived from a single flower with several separate ovaries; example: blackberry.

aggregated Describes the concentration of most parasites of a single species in a minority of hosts.Also called overdispersed.

aggression Behavior that appears to be intended to inflict noxious stimulation or destruction on another organism.

aggressive mimicry A technique for capturing prey in which the predator uses lures or other means to misinform the prey.

Agnatha A superclass of vertebrates whose members lack jaws and paired appendages and possess a cartilaginous skeleton and a persistent notochord. Lampreys and hagfishes.

Agonist The "prime mover" or muscle of reference.

agonistic behavior A suite of behavior patterns used during conflict with a conspecific, usually indicating whether an individual is going to submit to the other animal or fight if the other does not submit.

agricultural infrastructure The complexities of processing, storing, packaging, and transporting farm products.

ahermatypic coral A coral that does not build reefs.

air sacs Lateral branches of the nasal passages of smaller toothed whales; sources of echolocation sounds.

airfoil A surface, such as a wing, that provides lift by using currents of air it moves through.

ala Term often applied to winglike structure on plants or animals: the lateral winglike expansions of the branchiuran carapace to form respiratory alae, cuticular winglike expansions of nematodes, and others.

Alare Usually a bone process shaped like a wing.

alate Winged.

albedo A description of a surface's reflective properties.

albumin Any of a large class of simple proteins that are important constituents of vertebrate blood plasma and tissue fluids and also present in milk, whites of eggs, and other animal substances.

alcoholic fermentation A form of anaerobic respiration in which sugar is degraded to alcohol and CO₂ and energy is released.

aleurone layer A group of cells rich in protein granules and located as the outer layer of the endosperm of many grain seeds.

aleurone Protein-rich, outermost layer of endosperm in a cereal grain.

alga A photosynthetic protist containing plastids. Any of several groups of autotrophs that lack the structural features (true leaves, roots, and stems) of the higher plants.

algal bloom A proliferation of algae due to a nutrient-rich medium, usually resulting in a green scum on the water surface.

algal grazer An animal that consumes either large algae or thin algal films.

algal ridge A ridge of coralline algae that is found on the outer edge of some coral reefs.

algal turf A dense growth of often filamentous algae.

algin A polysaccharide derived from brown algae and used for many industrial processes.

alginic acid Gelatinous material extracted from the wall of certain brown algae; used in a variety of commercial and industrial products.

alien species (exotic species)A species introduced by humans.

alimentary Having to do with nutrition or nourishment.

alisphenoid A bone. (Note: Latin and Greek roots are sometimes combined in the same term.)

alkaline Denoting substances that release hydroxyl (OH_) ions into solution; see basic.

alkaloid A group of nitrogen-containing compounds having diverse structures; many alkaloids have medicinal, hallucinogenic, or toxic properties.

all-or-none law The phenomenon in which a muscle fiber contracts completely when it is exposed to a stimulus of threshold strength. Also, the principle that states a neuron will "fire" at full power or not at all.

allantois One of the extraembryonic membranes of the amniotes that functions in respiration and excretion in birds and reptiles and plays an important role in the development of the placenta in most mammals.

allee effect The unfavorable consequences of undercrowding, as when populations fail to breed when numbers are below some critical density.

allele Alternative forms of genes coding for the same trait; situated at the same locus in homologous chromosomes.

allelopathy The release of chemicals by certain plants that inhibit the growth of competing plants.

allergens Substances that activate the immune system and cause an allergic response; may not be directly antigenic themselves but may make other materials antigenic.

allograft A piece of tissue or an organ transferred from one individual to another individual of the same species, not identical twins; homograft.

allometry Relative growth of a part in relation to the whole organism.

allopatric speciation The formation of new species by a process involving geographic barriers.

allopatric In separate and mutually exclusive geographical regions.

allopatry Populations or species with nonoverlapping geographic distributions. Contrast with sympatry.

alpha-helix Literally the first spiral arrangement of the genetic DNA molecule; regular coiled arrangement of polypeptide chain in proteins; secondary structure of proteins.

alpine The high, treeless biogeographic zone of mountains that consists of slopes above the timberline.

alternate arrangement One leaf borne per node.

alternate host The alternate plant required to complete the life cycle of some microorganisms. For example, for Puccinia graminis tritice, wheat is the primary host and Berberis vulgaris is the alternate host.

alternate Leaf arrangement in which there is only one leaf.

alternation of generations A reproductive cycle in which a sexual stage alternates with an asexual one, as in the case of a gametophyte alternating with a sporophyte.

altricial An animal that is helpless at hatching or birth.

altricial young Young that are born or hatched in a relatively immature or helpless condition. Compare with precocial.

altruism The principle or practice of unselfish concern for, or devotion to, the welfare of others.

altruistic preservation A philosophy of preserving nature for its own sake.

alula A group of feathers on the wing of a bird that is supported by the bones of the medial digit. The alula reduces turbulent airflow over the upper surface of the wing.

amebocyte Cell in metazoan invertebrate, often functioning in defense against invading particles.

ameboid Ameba-like in putting forth pseudopodia.

ameboma Granuloma containing active trophozoites, occasionally resulting from a chronic amebic ulcer; rare except in Central and South America.

amebula Daughter cell resulting from mitosis and cytokinesis of an encysted ameba.

ametabolous metamorphosis Development in which the number of molts is variable; immature stages resemble adults, and molting continues into adulthood.

amictic Pertaining to female rotifers, which produce only diploid eggs that cannot be fertilized, or to the eggs produced by such females. Compare with mictic.

amino acid An organic molecule including one or more amino (_NH₂) and acid (_COOH) groups; one of the 20 nitrogen-containing molecules that make up proteins.

amino group (_NH₂) A chemical part of a molecule that imparts basic properties to an amino acid.

amitosis A form of cell division in which mitotic nuclear changes do not occur; cleavage without separation of daughter chromosomes.

ammonotelic excretion Having ammonia as the chief excretory product of nitrogen metabolism; occurs in freshwater fishes.

amniocentesis Procedure for withdrawing a sample of fluid around the developing embryo for examination of chromosomes in the embryonic cells and other tests.

amnion One of the extraembryonic membranes of the embryos of reptiles, birds, and mammals. The amnion encloses the embryo in a fluid-filled sac.

amniote lineage The evolutionary lineage of vertebrates leading to modern reptiles, birds, and mammals.

amniote Having an amnion; as a noun, an animal that develops an amnion in embryonic life, that is, reptiles, birds, and mammals.

amniotic egg The egg of reptiles, birds, and mammals. It possesses a series of extraembryonic membranes that help prevent desiccation, store wastes, and promote gas exchange. These adaptations allowed vertebrates to invade terrestrial habitats.

amoeboid movement A form of movement similar to that found in amoebae. Fluid endoplasm (plasmasol) flows forward inside the cell and changes state to viscous ectoplasm (plasmagel) on reaching the tip of a pseudopodium. At the opposite end of the cell, ectoplasm is converted into endoplasm.

amphibia The class of vertebrates whose members are characterized by skin with mucoid secretions, which serves as a respiratory organ. Developmental stages are aquatic and are usually followed by metamorphosis to an amphibious adult. Frogs, toads, and salamanders.

amphiblastula Free-swimming larval stage of certain marine sponges; blastula-like but with only the cells of the animal pole flagellated; those of the vegetal pole unflagellated.

amphid One of a pair of chemosensory organs found on the anterior end of certain nematodes.

amphipathic Adjective to describe a molecule with one part soluble in water (polar) and another part insoluble in water (nonpolar).

amphipods A group of small, laterally compressed crustaceans that includes beach hoppers and others.

amphistome Fluke with the ventral sucker located at the posterior end.

amplexus The positioning of a male amphibian dorsal to female amphibian, his forelimbs around her waist. During amplexus, the male releases sperm as the female releases eggs.

amplitude The maximum absolute value of a periodically varying quantity, as the peak in activity in a circadian or circannual cycle.

ampulla of Lorenzini One of several sensory structures in the head of sharks that detect weak electric fields.

ampulla Membranous vesicle; dilation at one end of each semicircular canal containing sensory epithelium; muscular vesicle above tube foot in water-vascular system of echinoderms.

ampullary organ A receptor that can detect electrical currents. These electroreceptors are found in most fishes, some amphibians, and the platypus.

amygdaloid A part of the brain.

amylase An enzyme that breaks down starch into smaller units.

amylopectin Component of starch consisting of highly branched chains of repeating glucose units; insoluble in water.

amyloplast Starch-storing plastid.

amylose Component of starch consisting of unbranched chains of repeating glucose units; soluble in water.

anabolic Chemical reactions that synthesize and require energy.

anabolism Any constructive metabolic process by which organisms convert substances into other components of the organism's chemical architecture. System of biosynthetic reactions in a cell by which large molecules are made from smaller ones.

anadromous Refers to fishes that migrate up streams from the sea to spawn.

anaerobic The phase of cellular respiration that occurs in the absence of oxygen; lacking oxygen.

anaerobic bacteria Bacteria that do not need oxygen.

anaerobic respiration The incomplete intracellular breakdown of sugar or other organic compounds in the absence of oxygen that releases some energy and produces organic acids and/or alcohol.

anaerobic Not dependent on oxygen for respiration.

anal fin Each of the last pair of ventral fins of fishes.

analgesic Pain-relieving remedy.

analogous Structures that have similar functions in two organisms but have not evolved from a common ancestral form.

analogy Similarity of function but not of origin.

anamnestic response Immune response to a challenge or secondary antigen inoculation, marked by more rapid and stronger manifestation of the immune reaction (specifically, antibody titer) than after the primary immunizing dose.

anamorphic The asexual or imperfect stage in a fungus life cycle.

anaphase The stage in mitosis and meiosis, following metaphase, in which the centromeres divide and the chromatids, lined up on the mitotic spindle, begin to move apart toward the poles of the spindle to form the daughter chromosomes.

anaphylaxis A systemic (whole body) immediate hypersensitivity reaction.

anapolysis Detachment of a senile proglottid after it has shed its eggs.

anapsid Amniotes in which the skull lacks temporal openings, with turtles the only living representatives.

anastomose Coming together; usually of blood vessels.

anastomosis A union of two or more blood vessels, fibers, or other structures to form a branching network.

anatomy The study of the structure of an organism and its parts.

anautogeny In some Diptera the necessity of a blood meal before eggs can develop within the female.

Anconeus elbow.

androecium A collective term referring to the stamens within a flower.

androgen Any substance that contributes to masculinization, such as the hormone testosterone.

androgenic gland Gland located near the vas deferens in many Crustacea. Its secretions are responsible for development of male secondary sexual characteristics.

androsterone An animal hormone not synthesized by plants.

anecdysis Ecdysis in which successive molts are separated by quite long intermolt phases; referred to as terminal anecdysis when maximal size is reached and no more ecdyses occur.

anemia Low levels of hemoglobin due to iron deficiency or lack of red blood cells.

aneuploidy Loss or gain of a chromosome, cells of the organism have one fewer than normal chromosome number, or one extra chromosome, for example, trisomy 21 (Down syndrome).

angiosperm The group of plants characterized by having flowers as their sexual reproductive structures.

angiotensin Blood protein formed from the interaction of renin and a liver protein, causing increased blood pressure and stimulating release of aldosterone and ADH.

Angstrom A unit of one ten-millionth of a millimeter (one ten-thousandth of a micrometer); it is represented by the symbol Å.

Angularis sharply bent.

anhydrase An enzyme involved in the removal of water from a compound. Carbonic anhydrase promotes the conversion of carbonic acid into water and carbon dioxide.

animal behavior Activities animals perform during their lifetime.

animal pole The region of a fertilized egg where meiosis is completed. It contains less yolk and is more metabolically active than the opposite vegetal pole.

animalia The kingdom of organisms whose members are multicellular, eukaryotic, and heterotrophic. The animals.

animals Members of the kingdom Animalia, which consists of heterotrophic, eukaryotic, and multicellular organisms.

anisogametes Outwardly dissimilar male and female gametes. Anisogamy is the condition of having dissimilar male and female gametes.

anisogamy The condition in which the female gamete (ovum) is larger than the male gamete (sperm).

anlage Rudimentary form; primordium.

annelida The phylum of triploblastic, coelomate animals whose members are metameric (segmented) and wormlike. Annelids have a complete digestive tract and a ventral nerve cord.

annelids See segmented worms.

annual ring Ring of xylem in woody stem composed of springwood and summerwood that corresponds in temperate regions to a chronological year.

annual A plant that lives for a single growing season.

annulation External division of a worm-shaped body into a series of conspicuous rings.

annuli Rings on the body of a parasite; not necessarily indicative of internal segmentation.

annulus Any ringlike structure, such as superficial rings on leeches.

annulus In fern sporangia, a row of cells with both thin and thickened cell walls which facilitate the opening of the sporangia and the release of spores because of the unequal expansion and tension produced as they are moistened and dried.

anoxic Lacking oxygen.

antagonism The condition of being an opposing principle, force, or factor, as when two hormones have opposite effects on target tissues.

antagonist The muscle or muscles opposing the agonist.

ante before [a prefix].

antebrachium anti = against; brachium = the arm.

antenna A sensory appendage on the head of arthropods, or the second pair of the two such pairs of structures in crustaceans.

antennae (second antennae of crustaceans) Second pair of appendages in Crustacea, with bases usually immediately posterior to antennules; primarily sensory but sometimes adapted for other functions; derived from appendages on primitive third preoral somite; no homologous appendage in insects.

antennal gland The excretory organ in some crustaceans (crayfish). Called antennal glands because of their location near the base of each second antenna and their green color; also called green glands.

antennules (first antennae) Anteriormost pair of appendages of Crustacea; primarily sensory but often adapted for additional or other functions in particular species; derived from appendages on primitive second preoral somite; homologous to antennae of insects.

anterior station Development of a protozoan in the middle or anterior intestinal portions of its insect host, such as the section Salivaria of Trypanosomatidae.

anterior The head end of an organism, or (as an adjective) toward that end.

anther The male reproductive organ enclosing and containing the pollen grains.

antheridiophore in some liverworts, a stalk that bears antheridia embedded on an elevated and expanded tip.

antheridium The multicellular male sex structure of plants other than seed plants.

anthocyanins A group of water-soluble red to blue flavonoid pigments found in certain plants; especially important pigmentation in flower petals.

anthozoa The class of cnidarians whose members are solitary or colonial polyps. Medusae absent; gametes originate in the gastrodermis; mesenteries divide the gastrovascular cavity. Sea anemones and corals.

anthozoans (class Anthozoa)Cnidarians whose life cycle consists of a complex polyp and no medusa.

anthracosaurs A group of Paleozoic labyrinthodont amphibians.

anthropocentric The belief that humans hold a special place in nature; being centered primarily on humans and human affairs.

anthropomorphism The attribution of human characteristics to nonhuman beings and objects.

antibiosis The inhibition of growth of a microorganism by a substance produced by another microorganism.

antibiotic Substance that inhibits the growth of microorganisms, antibodiesProteins (immunoglobulins) in cell surfaces and dissolved in blood, capable of combining with the antigens that stimulated their production.

antibody Immunoglobulin protein, produced by B cells (or plasma cells derived from B cells), that binds with a specific antigen.

antibody titer Measure of the amount of antibody present, usually given in units per milliliter of serum.

anticoagulant Substance that prevents blood clotting.

anticodon A sequence of three bases on transfer RNA that pairs with codons of messenger RNA to position amino acids during protein synthesis.

anticodon loop The portion of a tRNA molecule responsible for the anticodon triplet, which pairs with the codon of mRNA.

antigen A foreign (nonself) substance (such as a protein, nucleoprotein, polysaccharide, and some glycolipids) to which lymphocytes respond; also known as an immunogen because it induces the immune response.

antigen challenge Dose or inoculation with an antigen given to an animal some time after primary immunization with that antigen has been achieved.

antigenic determinant Area on an antigen molecule that binds with antibody or specific receptor sites on the sensitized lymphocyte; it "determines" the specificity of the antibody or lymphocyte. See epitope.

antiparallel Refers to opposing strands of DNA that are oriented in opposite directions.

antipodal One of several cells (usually three) of the embryo sac (female gametophyte) of angiosperms. They are located opposite the egg and synergids.

antipodal cell The three haploid cells at the end of the embryo sac away from the micropyle.

antrum antrum = a cave.

aorta aorta = great artery. Large artery carrying blood away from the heart

aorta aperture An opening; the opening into the first whorl of a gastropod shell.

apex Highest or uppermost point; the lower pointed end of the heart. The tip of a structure; a leaf apex, for example, is the tip of the leaf.

aphotic zone The portion of the ocean where the absence of sunlight prohibits plant growth.

apical complex Dense ring and conelike structure, along with associated microtubules, micronemes, and rhoptries, at the anterior end of an apicomplexan sporozoite. (apical = at the apex.)A certain combination of organelles found in the protozoan phylum Apicomplexa.

apical dominance The phenomenon leading to controlled growth of lateral shoots; growth occurs primarily at the top of the plant.

apical meristem A group of cells at the tip of the stem and root that give rise by cell division to the primary tissues and are ultimately responsible for the structural organization of the entire primary plant body.

apical organ Organ of unknown function at the apex of a cestode's scolex.

apical Pertaining to the tip or apex.

apicomplexa The protozoan phylum characterized by members having an apical complex used for penetrating host cells; cilia and flagella lacking, except in certain reproductive stages. Examples include the gregarines (Monocystis), coccidians (Eimeria, Isospora, Sarcocystis, Toxoplasma), Pneumocystis, and Plasmodium.

aplacophora The class of molluscs whose members lack a shell, mantle, and foot. Wormlike, burrowing animals with head poorly developed. Some authors divide this group into two classes: Caudofoveata and Solengasters.

apneustic breathing Breathing pattern exhibited by marine mammals in which several rapid breaths alternate with a prolonged cessation of breathing.

apocrine Applies to a type of mammalian sweat gland that produces a viscous secretion by breaking off a part of the cytoplasm of secreting cells.

apodeme Spinelike inward projection of the cuticle in arthropods on which a muscle inserts; a ridgelike projection is an apophysis.

apodous larva Larva with no legs and with reduced head; usual in Hymenoptera, Diptera, some Coleoptera; requires maternal care or deposition in or on food source.

apolysis Disintegration or detachment of a gravid tapeworm segment; also, the detachment of the hypodermis from the old procuticle in arthropods before molting.

apomorphic Adjective that refers to the form of characters, in particular characters whose form differs from that of the same character in an outgroup.

aponeurosis Dense connective tissue forming a broad tendon.

apoplast Pathway of water entering a root that flows along intercellular spaces and cell walls.

apoplastic Pertaining to the movement of water in the free space of tissue; free space includes cell walls and intercellular spaces.

apoptosis Genetically determined cell death, "programmed" cell death.

apopyle In sponges, opening of the radial canal into the spongocoel.

aposematic coloration Sharply contrasting colors of an animal that warn other animals of unpleasant or dangerous effects.

appendicular skeleton The bones of the upper and lower extremities; includes the shoulder and pelvic girdles.

appendicular Pertaining to appendages; pertaining to vermiform appendix.

appendicularians See larvaceans.

appendix Refers to the appendix vermiformis of the colon.

appetitive behavior The flexible introductory phase of a behavior sequence during which the organism is searching to obtain something to meet a need, as in seeking food, a mate, stimulation of a specific type, etc. See consummatory behavior.

applied animal behavior A subdiscipline concerned with all aspects of the behavioral biology of pets, domestic animals, animals in zoos and circuses, and animals in aquaria.

applied research Research conducted to address a specific problem for the purpose of application for productivity or commercial gain.

aquaculture The farming of marine and freshwater organisms. Also see mariculture.

aquifer Underground water reservoir.

aquifers Porous, water-bearing layers of sand, gravel, and rock below the earth's surface; reservoirs for groundwater.

arachnida The class of chelicerate arthropods whose members are mostly terrestrial, possess book lungs, or tracheae, and usually have four pairs of walking legs as adults. Spiders, scorpions, ticks, mites, and harvestmen.

arachnoid The weblike middle covering (meninge) of the central nervous system.

arachnoid Part of the membranes covering the brain.

arbor arbor = a tree.

arboreal Living in trees.

archaea (archebacteria) Early prokaryotes, differing from bacteria in having unusual types of metabolism, membrane lipids, and amino acid and DNA and RNA base sequences. Inhabit extreme environments.

archaeocytes Ameboid cells of varied function in sponges.

archegonium Female gametangium in which the female gamete or egg is produced and housed.

archenteron The main cavity of an embryo in the gastrula stage; it is lined with endoderm and represents the future digestive cavity.

archinephric archi = beginning, original; nephros = kidney.

archinephros Ancestral vertebrate kidney, existing today only in the embryo of hagfishes.

archosaur Advanced diapsid vertebrates, a group that includes the living crocodiles and the extinct pterosaurs and dinosaurs.

arcuate arcus = an arch, bow.

areolar A small area, such as spaces between fibers of connective tissue.

areolus A unit of diatom frustules.

arginine phosphate Phosphate storage compound (phosphagen) found in many invertebrates and used to regenerate stores of ATP.

aril Thick, fleshy seed covering around some seeds.

arista Flagellumlike appendage on the antenna of a fly of the suborder Brachycera and some members of the Nematocera.

Aristotle's lantern The set of jaws and associated muscles used by sea urchins to bite off food.

arithmetic growth A pattern of growth that increases at a constant amount per unit time, such as 1, 2, 3, 4 or 1, 3, 5, 7.

arrhenotoky Parthenogenetic production of males. See haplodiploidy.

arrhythmic activity Activity that does not exhibit any clear cyclical pattern.

arrow worms (chaetognaths; phylum Chaetognatha)Planktonic invertebrates characterized by a streamlined, transparent body.

arteriole (A small artery that delivers blood to capillaries.

artery A blood vessel that carries blood away from the heart and toward a peripheral cavity.

artesian well The result of a pressurized aquifer intersecting the surface or being penetrated by a pipe or conduit, from which water gushes without being pumped; also called a spring.

arthroidial arthrodia = gliding. A type of joint.

arthropoda The phylum of animals whose members possess metamerism with tagmatization, a jointed exoskeleton, and a ventral nervous system. Insects, crustaceans, spiders, and related animals.

arthropodization Evolutionary development of the combination of characteristics associated with the Arthropoda, including a firm cuticular exoskeleton containing chitin.

arthropods (phylum Arthropoda)Invertebrates that have jointed appendages and a chitinous, segmented exoskeleton.

articular articularis = a joint.

articulate articulatus = jointed.

Articulation A joint or union of two bones.

artificial selection Selective breeding as practiced by humans on domesticated plants and animals.

artificially acquired active immunity The type of immunity that results from immunizing an animal with a vaccine.

artificially acquired passive immunity The type of immunity that results from introducing antibodies that have been produced either in another animal or by specific in vitro methods into an animal.

artiodactyl One of an order of mammals with two or four digits on each foot.

arytenoid Cartilages in the larynx.

ascaridine Protein of unknown function in the sperm of Ascaris.

ascaroside Glycoside found in Ascaris, made of the sugar ascarylose and a series of secondary monol and diol alcohols.

aschelminth Any animal in the phyla Gastrotricha, Rotifera, Kinorhyncha, Nematoda, Nematomorpha, Acanthocephala, Loricitera, Priapulida, or Entoprocta.

Aschoff's Rule Pertaining to circadian rhythms, the rule stating that when nocturnal animals are held in constant darkness, their free-running period becomes slightly shorter each day and when a diurnal animal is held in constant darkness, the free-running period becomes slightly longer.

ascidiacea A class of urochordates whose members are sessile as adults, and solitary or colonial.

ascites Edema, or accumulation of tissue fluid, in the mesenteries and abdominal cavity.

ascocarp The dense, compact mass of hyphae constituting the fruiting body of the Ascomycetes.

ascogonium A female reproductive structure in some ascomycetes.

Ascomycetes A group of fungi whose spores are borne in an ascus or bag.

ascon The simplest of the three sponge body forms. Asconoid sponges are vaselike, with choanocytes directly lining the spongocoel.

asconoid Simplest form of sponges, with canals leading directly from the outside to the interior.

ascospore Sexual spore of an ascomycete fungus that is produced by meiosis, followed by mitosis.

ascus Saclike reproductive structure of ascomycetes in which meiosis, followed by mitosis, produces eight haploid cells called ascospores.

asexual Lacking sexual reproduction; vegetative reproduction.

asexual (vegetative) reproduction The type of reproduction that takes place without the formation of gametes. Compare sexual reproduction.

aspartate Four-carbon amino acid found with malate to be the first products of the C4.

aspect ratio Index of propulsive efficiency obtained by dividing the square of a fish's fin height by the fin area.

asphyxiants Chemicals that exclude oxygen or actively interfere with oxygen uptake and distribution; includes inert chemicals, such as nitrogen gas or halothane, that can displace oxygen and fill enclosed spaces.

assimilation Absorption and building up of digested nutriments into complex organic protoplasmic materials.

aster The star-shaped structure seen in a cell during the prophase of mitosis; composed of a system of microtubules arranged in astral rays around the centrosome; may emanate from a centrosome or from a pole of a mitotic spindle.

asteroidea The class of echinoderms whose members have rays that are not sharply set off from the central disk; ambulacral grooves with tube feet; and suction disks on tube feet. Sea stars.

asthma A distressing disease characterized by shortness of breath, wheezing, and bronchial muscle spasms.

asymmetry Without a balanced arrangement of similar parts on either side of a point or axis.

asynchronous flight See indirect flight.

atherosclerosis Disease characterized by fatty plaques forming in the inner lining of arteries.

athlete's foot A disease caused by imperfect fungi that flourish under warm, wet conditions. Usually dissappears if the feet are kept dry.

atmosphere A unit of pressure equal to 14.7 lbs/in.2 and equivalent to the pressure created by a 10 m column of water.

atmospheric deposition Sedimentation of solids, liquids, or gaseous materials from the air. Snow, rain, and dust are the most familiar examples, but acids, metals, and toxic organic chemicals also can be transported by winds and deposited far from the source of origin.

atmospheric pressure Ambient pressure created near the earth's surface by large air cells that circulate around the globe.

atoke Anterior, nonreproductive part of a marine polychaete, as distinct from the posterior, reproductive part (epitoke) during the breeding season.

atoll A ring-shaped chain of coral reefs from which a few low islands project above the sea surface.

atom The basic unit of matter; the smallest complete unit of the elements, consisting of protons, neutrons, and electrons.

atomic mass A mass unit determined by arbitrarily assigning the carbon-12 isotope a mass of 12 atomic mass units.

atomic nucleus Most of the mass of an atom; composed of protons and neutrons.

atomic number Number of protons in the atomic nucleus; there is a characteristic number for each type of element.

atomic weight The weight of an atom determined by adding the number of protons and neutrons (the mass of electrons is usually considered to be negligible).

atomic weight unit The mass of one proton or neutron.

ATP (adenosine triphosphate) A molecule that stores energy and releases it to power chemical reactions in organisms.

ATP synthase A membrane-bound enzyme in mitochondria and chloroplasts that phosphorylates ADP to form ATP using energy from the passage of protons through the enzyme.

Atria Chambers of the heart.

atrium One of the chambers of the heart; also, the tympanic cavity of the ear; also, the large cavity containing the pharynx in tunicates and cephalochordates.

auricle The portion of the external ear not connected within the head. Also used to designate an atrium of a heart. In the class Turbellaria, the sensory lobes that project from the side of the head.

auricularia A type of larva found in Holothuroidea.

autogamy Condition in which the gametic nuclei produced by meiosis fuse within the same organism that produced them to restore the diploid number.

autoimmunity Immune response to ones' own proteins or other antigens.

autoinfection Reinfection by a parasite juvenile without its leaving the host.

autosome Any chromosome that is not a sex chromosome.

autotomy The self amputation of an appendage. For example, the casting off of a section of a lizard's tail caught in the grasp of a predator. The autotomized appendage is usually regenerated.

autotroph An organism that makes its organic nutrients from inorganic raw materials by using an external energy source, such as light energy.

autotrophic Having the ability to synthesize food from inorganic compounds.

autotrophic nutrition Nutrition characterized by the ability to use simple inorganic substances for the synthesis of more complex organic compounds, as in green plants and some bacteria.

auxin A hormone that causes the bending response in Avena coleoptiles.

auxospore The resistant stage of diatoms that restores the maximum size characteristic to the species.

Aves A class of vertebrates whose members are characterized by scales modified into feathers for flight, endothermy, and amniotic eggs. The birds.

avicularium Modified zooid that is attached to the surface of the major zooid in Ectoprocta and resembles a bird's beak.

awn A slender bristle.

axial cells Central cells of a dicyemid mesozoan.

axial skeleton Portion of the skeleton that supports and protects the organs of the head, neck, and trunk.

axial Relating to the axis, or stem; on or along the axis.

axil The upper angle between a stem

Axilla axillae = armpit; beneath the axis.

axillary bud A bud, consisting of an apical meristem and leaf primordia, situated in the leaf axil.

Axis The second cervical vertebra; any axial part.

axocoel The most anterior of three coelomic spaces that appear during larval echinoderm development.

axolotl Larval stage of any of several species of the genus Ambystoma (such as Ambystoma tigrinum) exhibiting neotenic reproduction.

axon A fiber that conducts a nerve impulse away from a neuron cell body.

axon Elongate extension of a neuron that conducts impulses away from the cell body and toward the synaptic terminals.

axoneme The axial thread of the chromosome in which is located the axial combination of genes. The central core of a cilium or flagellum, consisting of a central pair of filaments surrounded by nine other pairs; also called axial filament.

axopodium Fine, needlelike pseudopodium that contains a central bundle of microtubules. Also called axopod. Found in certain sarcodine protozoa.

axostyle Tubelike organelle in some flagellate protozoa, extending from the area of the kinetosomes to the posterior end, where it often protrudes.

Azygous A blood vessel.