N

nacre Innermost lustrous layer of mollusc shell, secreted by mantle epithelium. Adj., nacreous.

NAD A coenzyme that acts as an electron acceptor; particularly important in respiration.

NADP A molecule that acts as an electron intermediate during photosynthesis. It is reduced during the light reactions and oxidized during the Calvin Cycle.

nagana Disease of ruminants caused by Trypanosoma brucei brucei or T. congolense.

naiad The aquatic immature stage of any hemimetabolous insect.

nail The horny cutaneous plate of the dorsal surface of the distal end of a finger or toe.

naked grain A grain that separates easily from the surrounding bracts.

nanoplankton The component of the plankton that consists of very small organisms 2 to 20 mm (0.002 to 0.2 mm) in size; they are too small to catch in a standard plankton net.

narcotic Any psychoactive compound that is dangerously addictive; a compound that induces central nervous system depression resulting in numbness, lethargy, and/orsleep.

nares Openings into the nasal cavity, both internally and externally, in the head of a vertebrate.

nasal gland Salt-excreting gland in the nostrils of seabirds and reptiles

natal Pertaining to birth.

natality The production of new individuals by birth, hatching, germination, or cloning.

native species A local species that has not been introduced.

natural history The study of where and how organisms carry out their life cycles.

natural increase Crude death rate subtracted from crude birth rate.

natural killer (NK) cell A lymphocyte present in nonimmunized individuals that exhibits independent cytolytic activity against tumor cells; also called cytotoxic T cells.

natural landscaping A landscaping design that uses carefully contrived meadows, forests, and natural-looking ponds and marshes to create an illusion of wild nature.

natural law A theory that has been tested many thousands of times and found always to be true, e.g., the law of gravity.

natural resources Goods and services supplied by the environment.

natural selection The mechanism for evolutionary change in which environmental pressures cause certain genetic combinations in a population to become more abundant; genetic combinations best adapted for present environmental conditions tend to become predominant.

naturally acquired active immunity The type of immunity that develops when an individual's immunologic system comes into contact with an appropriate antigenic stimulus during the course of normal activities; it usually arises as the result of recovering from an infection.

nauplius Typically the earliest larval stage(s) of crustaceans; has only three pairs of appendages: antennules, antennae, and mandibles_all primarily of locomotive function.

Navicular navicul = a little ship. A carpal bone shaped like a boat.

navigation The process by which an animal uses various cues to determine its position in reference to a goal.

neap tides The tides with a small tidal range. They occur around the times when the moon is in quarter. Compare spring tides.

neascus Strigeoid metacercaria with a spoon-shaped forebody.

necrosis Cell or tissue death.

nectar guide Color patterns present on petals that direct insects toward the nectar; often not visible to human eye.

nectar A sweet, syrupy exudate produced by some flowers as an attractant for pollinators.

nectary A gland that secretes nectar.

nekton Term for actively swimming organisms, essentially independent of wave and current action. Compare with plankton.

nematocyst An organelle characteristic of the Cnidaria that is used in defense, food gathering, and attachment.The stinging structure of cnidarians.

Nematoda The phylum of aschelminths that contains members commonly called either roundworms or nematodes. Tripoblastic, bilateral, vermiform, unsegmented, and pseudocoelomate.

nematodes Invertebrates with a cylindrical body, a conspicuous body cavity, and a complete digestive tract.

nematogen State in the life cycle of a dicyemid mesozoan.

Nematomorpha The phylum of aschelminths commonly called horsehair worms.

Nemertea The phylum that has members commonly called the proboscis worms; elongate, flattened worms found in marine mud and sand; triploblastic; complete digestive tract with anus; closed circulatory system.

nemerteans See ribbon worms.

neo-Darwinism A modified version of Darwin's evolutionary theory that eliminates elements of the Lamarckian inheritance of acquired characteristics and pangenesis that were present in Darwin's formulation; this theory originated with August Weissmann in the late nineteenth century and, after incorporating Mendelian genetic principles, has become the currently favored version of Darwinian evolutionary theory.

neo-Malthusian A belief that the world is characterized by scarcity and competition in which too many people fight for too few resources. Named for Thomas Malthus, who predicted a dismal cycle of misery, vice, and starvation as a result of human overpopulation.

Neolithic The Stone Age period following the advent of agriculture.

neopterygian Any of a large group of bony fishes that includes most modern species.

neotenine. See juvenile hormone.

neoteny An evolutionary process by which organismal development is retarded relative to sexual maturation; produces a descendant that reaches sexual maturity while retaining a morphology characteristic of the preadult or larval stage of an ancestor.

Nephric nephros = kidney.

nephridiopore An external excretory opening in invertebrates.

nephridium One of the segmentally arranged, paired excretory tubules of many invertebrates, notably the annelids. In a broad sense, any tubule specialized for excretion and/or osmoregulation; with an external opening and with or without an internal opening.

nephron Functional unit of kidney structure of vertebrates, consisting of a Bowman's capsule, an enclosed glomerulus, and the attached uriniferous tubule.

nephrostome Ciliated, funnel-shaped opening of a nephridium.

neritic Portion of the sea overlying the continental shelf, specifically from the subtidal zone to a depth of 200 m.

neritic ecosystems The marine ecosystem that consists of relatively shallow water extending from the littoral zone to the edge of the continental shelves.

nerve A bundle of neurons or nerve cells outside the central nervous system.

nerve cell A cell specialized to originate or transmit nerve impulses.

nerve cell body The largest part of a neuron that typically contains the nucleus.

nerve cord A long, compact bundle of nerve cells that is part of the central nervous system.

nerve net A diffuse, two-dimensional plexus of bi- or multipolar neurons; found in cnidarians.

nerve tract A bundle of neurons, particularly axons, held together by connective tissue.

nervous tissue The type of tissue composed of individual cells called neurons and supporting neuroglial cells.

nested hierarchy. A pattern in which species are ordered into a series of increasingly more inclusive clades according to the taxonomic distribution of synapomorphies.

net energy yield Total useful energy produced during the lifetime of an entire energy system minus the energy used, lost, or wasted in making useful energy available.

net plankton Plankton that is caught in a standard plankton net.

net primary production The total energy converted into biomass by autotrophs. Net primary production is equal to gross primary production less energy lost in maintenance functions of autotrophs.

net venation The netlike pattern of branching of veins on a leaf blade. Also known as reticulate venation; characteristic of most dicot leaves.

netted venation A type of vein arangement in leaves in which the profusely branching veins form an interconnecting network; also termed reticulate venation.

neuroendocrine system The combination of the nervous and endocrine systems.

neurofibril node Regular gaps in a myelin sheath around a nerve fiber; formerly called node of Ranvier.

neurogenic Originating in nervous tissue, as does the rhythmical beat of some arthropod hearts.

neuroglia Tissue supporting and filling the spaces between the nerve cells of the central nervous system.

neurohormone A chemical transmitter produced by nervous tissue. Uses the bloodstream or other body fluids for distribution to its target site.

neurolemma Delicate nucleated outer sheath of a nerve cell; sheath of Schwann.

neurolemmocyte The cell that surrounds a fiber of a peripheral nerve and forms the neurolemmal sheath and myelin; formerly called Schwann cell.

neuromast Cluster of sense cells on or near the surface of a fish or amphibian that is sensitive to vibratory stimuli and water.

neuromuscular junction The junction between nerve and muscle; myoneural junction or neuromuscular cleft.

neuron A nerve cell that consists of a cell body and its processes.

neuropeptide A hormone produced by secretory nervous tissue.

neuropodium Lobe of parapodium nearer the ventral side in polychaete annelids.

neurosecretions Hormonelike chemical substances that are produced by specialized neurons that affect various physiological processes in the body.

neurosecretory cell Any cell (neuron) of the nervous system that produces a hormone.

neurotoxins Toxic substances, such as lead or mercury, that specifically poison nerve cells.

neurotransmitter Chemical substance secreted by the terminal end of an axon that stimulates a muscle fiber contraction or an impulse in another neuron.

neurulation External changes along the upper surface of a chordate embryo that result in the formation of the neural tube.

neuston Organisms that live on the surface of the sea.

neutral selection See genetic drift.

neutral Soil pH of 7, neither acid nor alkaline.

neutron Uncharged atomic particles of essentially the same mass as protons. In the most common stable atoms the number of neutrons is equivalent to the number of protons.

neutrophil Most abundant of polymorphonuclear leukocytes; an important phagocyte; so called because it stains with both acidic and basic stains.

new towns Experimental urban environments that seek to combine the best features of the rural village and the modern city.

niche See ecological niche.

nicotine adenine dinucleotide A local electron carrier that transfers hydrogen atoms and electrons within metabolic pathways; a free-moving carrier, not membrane bound in a transport system; NAD+.

nictitating membrane Third eyelid, a transparent membrane of birds and many reptiles and mammals, that can be pulled across the eye.

nidus Specific locality of a given disease; result of a unique combination of ecological factors that favors the maintenance and transmission of the disease organism.

nihilists Those who believe the world has no meaning or purpose other than a dark, cruel, unceasing struggle for power and existence.

NIMBY Not In My Backyard: the rallying cry of those opposed to LULUs.

nitrate (NO3_2)An important nutrient in the ocean.

nitrate-forming bacteria Bacteria that convert nitrites into compounds that can be used by green plants to build proteins.

nitrogen (N₂)A colorless and tasteless gas that is an essential constituent of proteins.

nitrogen base The basic component of nucleic acids; composed of a nitrogen-containing molecule of purine or pyrimidine.

nitrogen cycle The circulation and reutilization of nitrogen in both inorganic and organic phases.

nitrogen fixation Reduction of molecular nitrogen to ammonia by some bacteria and cyanobacteria, often followed by nitrification, the oxidation of ammonia to nitrites and nitrates by other bacteria.

nitrogen oxides Highly reactive gases formed when nitrogen in fuel or combustion air is heated to over 650C (1200 F) in the presence of oxygen or when bacteria in soil or water oxidize nitrogen-containing compounds.

nitrogen-fixing bacteria Bacteria that convert nitrogen from the atmosphere or soil solution into ammonia that can then be converted to plant nutrients by nitrite- and nitrate-forming bacteria.

nitrogenase The enzyme involved in the conversion of atmospheric nitrogen gas into ammonia.

nitrous oxide NO₂, an atmospheric gas that traps infrared radiation and also can form nitric acid in rain droplets, resulting in acid rain.

nociceptor A sensory receptor responding to potentially harmful stimuli; produces a sensation of pain.

nocturnal Animals whose primary activity occurs during the dark portion of the daily cycle.

node of Ranvier A constriction of mylelinated nerve fibers at regular intervals at which the myelin sheath is absent and the axon is enclosed only by sheath cell processes; also known as a neurofibril node.

node The point on a stem at which leaves and buds are attached.

nodule A tumorlike growth on the roots of certain higher plants that encloses a population of nitrogen-fixing bacteria.

nomenclature The study of the naming of organisms in the fashion that reflects their evolutionary relationships.

nonamniote lineage The vertebrate lineage leading to modern amphibians.

nonbiodegradable A chemical that cannot be broken down by bacteria or other organisms. It is said to be persistent.

noncriteria pollutants See unconventional pollutants.

noncyclic photophosphorylation The formation of ATP utilizing the Z-scheme of photosynthesis.

nondisjunction. Failure of a pair of homologous chromosomes to separate during meiosis, leading to one gamete with n + 1 chromosomes (see trisomy) and another gamete with n _ 1 chromosomes.

nongovernmental organizations (NGOs) A term referring collectively to pressure and research groups, advisory agencies, political parties, professional societies, and other groups concerned about environmental quality, resource use, and many other issues.

noninterference A principle that requires us to refrain from interfering with the lives or destinies of other moral agents or moral subjects except to satisfy essential needs.

nonmalfeasance Doing no harm to any being that has goods (values, interests, moral values) of their own.

nonpoint sources Scattered, diffuse sources of pollutants, such as runoff from farm fields, golf courses, construction sites, etc.

nonpolar covalent bond The type of bond that is formed when electrons spend as much time orbiting one nucleus as the other; thus, the distribution of charges is symmetrical.

nonrenewable resources Materials or services from the environment that are not replaced or replenished by natural processes at a rate comparable to our use of the resource; a resource depleted or exhausted by use.

nonseptate Lacking cross walls; commonly used in reference to certain fungal hyphae. Also termed aseptate.

nonseptate hypha Fungal hyphae that lack septa or cross walls.

nonshattering Seeds and/or fruits do not split off and scatter from the fruiting head; a trait associated with domesticated plants.

nonshivering thermogenesis The hormonal triggering of heat production.

norepinephrine A neurotransmitter released from the axon ends of some nerve fibers; noradrenaline.

North/South division A description of the fact that most of the world's wealthier countries tend to be in North America, Europe, and Japan while the poorer countries tend to be located closer to the equator.

northern coniferous forest The biome characterized by cool summers, cold winters, and short growing seasons. Characteristic animals include insects, snowshoe hares, lynx, wolves, caribou, and moose.

notochord An elongated cellular cord, enclosed in a sheath, which forms the primitive axial skeleton of chordate embryos and adult cephalochordates.

notopodium Lobe of parapodium nearer the dorsal side in polychaete annelids.

nucellus Tissue in the ovule within which the embryo sac develops; integuments surround the nucellus.

nuchal ligament L. The back of the head or neck; ligare = to bind.

nuclear envelope Double membrane forming the surface boundary of a eukaryotic nucleus; consists of outer and inner membranes perforated by nuclear pores.

nuclear fission The radioactive decay process in which isotopes split apart to create two smaller atoms.

nuclear fusion A process in which two smaller atomic nuclei fuse into one larger nucleus and release energy; the source of power in a hydrogen bomb.

nuclear membrane The membrane surrounding the nucleus of eucaryotic cells.

nuclear pores Pitted regions in the nuclear envelope through which processed mRNA migrates to the ribosomes.

nucleic acid One of a class of molecules composed of joined nucleotides; chief types are deoxyribonucleic acid (DNA), found in cell nuclei (chromosomes) and mitochondria, and ribonucleic acid (RNA), found both in cell nuclei (chromosomes and nucleoli) and in cytoplasmic ribosomes.

nucleoid The region in a prokaryotic cell where the chromosome is found.

nucleolar organizer An area on certain chromosomes associated with the formation of the nucleolus.

nucleolus A deeply staining body within the nucleus of a cell and containing RNA; nucleoli are specialized portions of certain chromosomes that carry multiple copies of the information to synthesize ribosomal RNA.

nucleon The nucleus of an atom is built up of protons and neutrons, collectively called a nucleon.

nucleoplasm Protoplasm of nucleus, as distinguished from cytoplasm.

nucleoprotein. A molecule composed of nucleic acid and protein; occurs in the nucleus and cytoplasm of all cells.

nucleosome A repeating subunit of chromatin in which one and three-quarter turns of the double-helical DNA are wound around eight molecules of histones.

nucleotide A molecule consisting of phosphate, 5-carbon sugar (ribose or deoxyribose), and a purine or a pyrimidine; the purines are adenine and guanine, and the pyrimidines are cytosine, thymine, and uracil.

nucleus Cell nucleus; a spheroid body within a cell, contained in a double membrane, the nuclear envelope, and containing chromosomes and one or more nucleoli. The genetic control center of a eukaryotic cell. The cell bodies of nerves within the central nervous system.

Nuda The class of ctenophorans whose members lack tentacles and have a flattened body with a highly branched gastrovascular cavity.

nudibranchs (or sea slugs)Gastropods that lack a shell and have exposed gills.

nuées ardentes Deadly, denser-than-air mixtures of hot gases and ash ejected from volcanoes.

null hypothesis In a statistical sense, to the statement that there will not be any difference between effects of experimental treatments.

numbers pyramid A diagram showing the relative population sizes at each trophic level in an ecosystem; usually corresponds to the biomass pyramid.

numerical taxonomy A system of classification in which there is no attempt to distinguish true and false similarities.

nuptial flight The mating flight of insects, especially that of the queen with male or males.

nurse cells. Single cells or layers of cells surrounding or adjacent to other cells or structures for which the nurse cells provide nutrient or other molecules (for example, for insect oocytes or Trichinella spp. juveniles).

nut A dry, indehiscent one-seeded fruit with a hard pericarp (the shell).

nutrient regeneration The release of nutrients from organic matter by decomposers.

nutrient A raw material other than carbon dioxide and water that is needed by an autotroph to produce organic matter. Examples are nitrate and phosphate.

nutrition The study of the sources, actions, and interactions of nutrients.

nyctinasty The "sleep movements" of leaves in response to change in turgor pressure of cells at the base of the petiole.

nymph An immature stage (following hatching) of a hemimetabolous insect that lacks a pupal stage.

nymphochrysalis Nonfeeding, prenymph stage in the life cycle of a chigger mite.

nymphs Juvenile instars in insects with hemimetabolous metamorphosis. Also, juvenile instars of mites and ticks with a full complement of legs.