abiotic(Gr. a, without, + biotos, life, livable). Characterized by the absence of life.
abomasum(L. ab, from, + omasum, paunch).Fourth and last chamber of the stomach of ruminant mammals.
aboral(L. ab, from, + os, mouth) .A region of an animal opposite the mouth.
abscess(L. abscessus, a going away). Dead cells and tissue fluid confined in a localized area, causing swelling.
acanthodians(Gr. akantha, prickly, thorny). A group of the earliest known true jawed fishes from Lower Silurian to Lower Permian.
acanthor(Gr. akantha, spine or thorn, + or). First larval form of acanthocephalans in the intermediate host.
acclimatization(L. ad, to, + Gr. klima, climate). Gradual physiological adaptation in response to relatively long-lasting environmental changes.
acetabulum(L. a little saucer for vinegar). True sucker, especially in flukes and leeches; the socket in the hip bone that receives the thigh bone.
acicula(L. acicula, a small needle). Needlelike supporting bristle in parapodia of some polychaetes.
acidA molecule that dissociates in solution to produce a hydrogen ion (Hus++).
acinus, pl. acini (L. grape). A small lobe of a compound gland or a saclike cavity at the termination of a passage.
acoelomate(Gr. a, not, + koiloma, cavity). Without a coelom, as in flatworms and proboscis worms.
acontium, pl. acontia (Gr. akontion, dart). Threadlike structure bearing nematocysts located on mesentery of sea anemone.
acquired immuune deficiency syndromeAn eventual consequence of infection with the human immunodeficiency virus in which the immune response is severely disabled. The disease is ultimately fatal, and no cure has been discovered.
acrocentric(Gr. akros, tip, + kentron, center). Chromosome with centromere near the end.
acron(Gr. akron, moakros, tip). Preoral region of an insect.
actin(Gr. aktis, ray). A protein in the contractile tissue that forms the thin myofilaments of striated muscle.
actinotroch(Gr. aktis, ray, beam, + trochos, wheel). Larval form found in Phoronida.
active transportMediated transport in which a permease transports a molecule across a cell membrane against a concentration gradient; requires expenditure of energy; contrast with facilitated diffusion.
adaptation(L. adaptatus, fitted). 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 radiationEvolutionary 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 valueDegree to muscle that draws the two valves of a mollwhich a characteristic helps an organism to survive and reproduce or lends greater fitness in its environment; selective advantage.
adaptive zoneA characteristic reaction and mutual relationship between environmment and organism ("way of life") demonstrated by a group of evolutionarily related organisms.
adductorA muscle that draws a part toward a median axis, or a musc shell together.
adenine(Gr. aden, gland, + ine, surine base; component of nucleotides and nucleic acids.
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.
adipose(L. adeps, fat). Fatty tiss
adrenaline(L. ad, to, + renalis, pertaining to kidneys). A hormone produced by the adrenal, or suprarenal, gland; epinephrine.
adsorption(L. ad, to, + sorbeo, to absorb). The adhesion of molecules to solid bodies.
aerobic(Gr. aer, air, + bios, life). Oxygen-dependent form of respiration.
afferent(L. ad, to, + ferre, to bear). 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.
aggression(L. aggressus, attack). An offensive action or procedure.
agonistic behavior(Gr. agonistes, combatant). An offensive action or threat directed toward another organism.
alate(L. alatus, wing). Winged.
albumin(al-byallumen, white of egg). 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.
alimentary(L. aliment food, noutrition or nourishment.
allantois(Gr. allas, saeidos, form). 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.
allele(Gr. allelon, of one another). Alternative forms of genes coding for the same trait; situated at the same locus in homologous chromosomes.
allograft(Gr. allos, other, + graft). A piece of tissue or an organ transferred from one individual to another individual of the same species, not identical twins; homograft.
allometry(Gr. allos, other,+ metry, meas
allopatric(Gr. allos, other, + patra, native land). In separate and mutually exclusive geographical regions.
alpha-helix(Gr. alpha, first, + L. helix, spiral). Literally the first spiral arrangement of the genetic DNA molecular coiled arrangement of polypeptide chain in proteins; secondary structure
altricial(L. altrices, noung animals (especially birds) having the young hatched in an immature, dependent condition.
alula(L. dim. of ala, wing). The first digit or thumb of a bird's wing, much reduced in size.
alveolus(L. dim. of alveus, cavity, hollow). A small cavity or pit, such as a microscopic air sac of the lungs, terminal part of an alveolar gland, or bony socket of a tooth.
ambulacra(L. ambulare, to walk). In echinoderms, radiating grooves where podia of water-vascular system characteristically project to outside.
amebocyte(Gr. amoibe, change, + kytos, hollow vessel). Cell in metazoan invertebrate, often functioning in defense against invading particles.
ameboid(Gr. amoibe, change, + oid, like). Ameba-like in putting forth pseudopodia.
amictic(Gr. a, withouce only diploid eggs that cannot be fertilized, or to the eggs produced by such females. Compare with mictic.
amino acid(amine, an organic compoup (-NH2). Makes up the structure of proteins.
amitosis(Gr. a, not, + mitos, thread). A form of cell division in which mitotic nuclear changes do not occur; cleavage without separation of daughter chromosomes.
amniocentesis(Gr. amnion, membrane around the fetus, + centes, puncture). Procedure for withdrawing a sample of fluid around the developing embryo for examination of chromosomes in the embryonic cells and other tests.
amnion(Gr. amnion, membrane around the fetus). The innermost of the extraembryonic membranes forming a fluidfilled sac around the embryo in amniotes.
amnioteHaving an amnion; as a no
amphiblastula(Gr. amphi, on both sides, + blastos, germ, + L. small). 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(Gr. amphidea, anything that is bound around). One of a pair of anterior sense organs in certain nematodes.
amphipathic(Gr.amphi,on both sides, + pathos, sule with one part soluble in water (polar) and another part insoluble in water (nonpolar).
amplexus(L. embrace). The copulatory embrace of frogs or toads.
ampulla(am-pus vesicle; dilation at one end of each semicircular canal containing sensory epithelium; muscular vesicle above tube foot in water-vascular system of echinoderms.
amylase(L. amyl starch, + ase, suffix meaning enzyme). An enzyme that breaks down starch into smaller units.
anabolism(Gr. ana, bol, to throw, + ism, suffix meaning state of condition). Constructive metabolism.
anadromous(Gr.anadromos,running upward). Refers to fishes that migrate up streams from the sea to spawn.
anaerobic(Gr. an, not, + aer, air, + bios, life). Not dependent on oxygen for respiration.
analogy(L. analog ratio). Similarity of function but not of origin.
anaphylaxis(Gr. ana-, phylax, guard). A systemic (whole body) immediate hypersensitivity reaction.
anapsid(Gr. an,without apsis,arch).Amniotes in which the skull lacks temporal openings, with turtles the only living representatives.
anastomosis(Gr. ana, again, + stoma, mouth).A union of two or more blood vessels, fibers, or other structures to form a branching network.
androgen(Gr. aner, andros, man, + genes, born). Any of a group of vertebrate male sex hormones.
androgenic gland(Gr. aner, male, + gennaein, to produce).Gland in Crustacea that causes development of male characteristics.
aneuploidy(Gr.an,without not,+eu,good,well,+ploid, multiple of). 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).
angiotensin(Gr. angeion, vessel,+ L. tensio, to stretch). Blood protein formed from the interaction of renin and a liver protein, causing increased blood pressure and stimulating release of aldosterone and ADH.
Angstrom(Swedish physicist). A unit of one ten-millionth of a millimeter (one ten-thousandth of a micrometer); it is represented by the symbol Å.
anhydrase(Gr. an, not, + hydor, water, + ase, enzyme suffix). An enzyme involved in the removal of water from a compound. Carbonic anhydrase promotes the conversion of carbonic acid into water and carbon dioxide.
anisogametes(Gr. anisos, unequal, + gametes, spouse). Gametes of a species that differ in form or size.
anlage(Ger. laying out, foundation).Rudimentary form; primordium.
annulus(L. ring). Any ringlike structure, such as superficial rings on leeches.
antenna(L. sail yard). A sensory appendage on the head of arthropods, or the second pair of the two such pairs of structures in crustaceans.
antennal glandExcretory gland of Crustacea located in the antennal metamere.
anterior(L. comparative of ante, before). The head end of an organism, or (as an adjective) toward that end.
anthracosaurs(Gr. anthrax, coal, carbon,+ sauros, lizard). A group of Paleozoic labyrinthodont amphibians.
antibodiesProteins (immunoglobulins) in cell surfaces and dissolved in blood, capable of combining with the antigens that stimulated their production.
anticodonA sequence of three nucleotides in transfer RNA that is complementary to a codon in messenger RNA.
antigenAny substance capable of stimulating an immune response, most often a protein.
antigenic determinantSee epitope.
aperture(L. apertura from aperire, to uncover). An opening; the opening into the first whorl of a gastropod shell.
apex(L. summit). Highest or uppermost point; the lower pointed end of the heart.
apical(L. apex, tip). Pertaining to the tip or apex.
apical complexA certain combination of organelles found in the protozoan phylum Apicomplexa.
apocrine(Gr. apo, away, + krinein, to separate). Applies to a type of mammalian sweat gland that produces a viscous secretion by breaking off a part of the cytoplasm of secreting cells.
apoptosis(Gr. apo-, prefix meaning away from, + ptosis, a falling). Genetically determined cell death, "programmed" cell death.
apopyle(Gr. apo, away from, + pyle, gate). In sponges, opening of the radial canal into the spongocoel.
appendicular(L. ad, to, + pendere, to hang). Pertaining to appendages; pertaining to vermiform appendix.
arboreal(L. arbor, tree). Living in trees.
archaeocytes(Gr. archaios, beginning, + kytos, hollow vessel). Ameboid cells of varied function in sponges.
archenteron(Gr. arche, beginning,+ enteron, gut). The main cavity of an embryo in the gastrula stage; it is lined with endoderm and represents the future digestive cavity.
archinephros(Gr. archaois, ancient, + nephros, kidney). Ancestral vertebrate kidney, existing today only in the embryo of hagfishes.
archosaur(Gr. archon, r sauros, lizard). Advanced diapsid vertebrates, a group that includes the living crocodiles and the extinct pterosaurs and dinosaurs.
areolar(L. areola, small space). A small area, such as spaces between fibers of connective tissue.
arginine phosphatePhosphate storage compound (phosphagen) found in many invertebrates and used to regenerate stores of ATP.
Aristotle's lanternMasticating apparatus of some sea urchins.
arteriole(L. arteria, artery). A small arterial branch that delivers blood to a capillary network.
artery(L. arteria, artery). A blood vessel that carries blood away from the heart and toward a peripheral cavity.
artiodactyl(Gr. artios, even, + daktylos, toe). One of an order of mammals with two or four digits on each foot.
asconoid(Gr. askos, bladder). Simplest form of sponges, with canals leading directly from the outside to the interior.
asexualWithout distinct sexual organs; not involving formation of gametes.
assimilation(L. assimilatio, bringing into conformity). Absorption and building up of digested nutriments into complex organic protoplasmic materials.
atherosclerosis(Gr. atheroma, tumor full of gruel-like material, + skleros, hard). Disease characterized by fatty plaques forming in the inner lining of arteries.
atoke(Gr. a, withotokos, offspring). Anterior, nonreproductive part of a marine polychaete, as distinct from the posterior, reproductive part (epitoke) during the breeding season.
atoll(Maldivian, atol<). A coral reef or island surrounding a lagoon.
atomThe smallest unit of an element, composed of a dense nucleus of protons and (usually) neutrons surrounded by a system of electrons.
ATPAdenosine triphosphate. In biochemistry, an ester of adenosine and triphosphoric acid.
atrium(L. atrium, vestibule). 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(L. auricula, dim. of auris, ear). One of the less muscular chambers of the heart; atrium; the external ear, or pinna; any earlike lobe or process.
auricularia(L. auricula, a small ear). A type of larva found in Holothuroidea.
autogamy(Gr. autos, self, + gamos, marriage). Condition in which the gametic nuclei produced by meiosis fuse within the same organism that produced them to restore the diploid number.
autosome(Gr. autos, self, + soma, body). Any chromosome that is not a sex chromosome.
autotomy(Gr. autos, self, + tomos, a cutting). The breaking off of a part of the body by the organism itself.
autotroph(Gr. autos, self, +trophos, feeder). An organism that makes its organic nutrients from inorganic raw materials.
autotrophic nutrition(Gr. autos, self, + trophia, denoting 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.
avicularium(L. avicula, small bird, + aria, like or connected with). Modified zooid that is attached to the surface of the major zooid in Ectoprocta and resembles a bird's beak.
axial(L. axis, axle). Relating to the axis, or stem; on or along the axis.
axocoel(Gr. axon, an axle, + koilos, hollow). The most anterior of three coelomic spaces that appear during larval echinoderm development.
axolotl(Nah atl, water, + xolotl, doll, servant, spirit). Larval stage of any of several species of the genus Ambystoma (such as Ambystoma tigrinum) exhibiting neotenic reproduction.
axon(Gr. axon). Elongate extension of a neuron that conducts impulses away from the cell body and toward the synaptic terminals.
axoneme(L. axis, axle, + Gr. nema, thread). The microtubules in a cilium or flagellum, usually arranged as a circlet of nine pairs enclosing one central pair; also, the microtubules of an axopodium.
axopodium(An axis,+ podion, small foot). Long, slender, more or less permanent pseudopodium found in certain sarcodine protozoa. (Also axopod)
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