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  Section: Zoological Terms used in General Zoology
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N - Zoological Terms

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(F. mother-of-pearl). Innermost lustrous layer of mollusc shell, secreted by mantle epithelium. Adj., nacreous


Abbreviation of nicotinamide adenine dinucleotide, an electron acceptor or donor in many metabolic reactions.


(na´rez), sing. naris (L. nostrils). Openings into the nasal cavity, both internally and externally, in the head of a vertebrate.

natural killer cells

Lymphocyte-like cells that can kill virus-infected cells and tumor cells in the absence of antibody.

natural selection

A nonrandom reproduction of varying organisms in a population that results in the survival of those best adapted to their environment and elimination of those less well adapted; leads to evolutionary change if the variation is heritable.


(L. a kind of shellfish). A free-swimming microscopic larval stage of certain crustaceans, with three pairs of appendages (antennules, antennae, and mandibles) and median eye. Characteristic of ostracods, copepods, barnacles, and some others.


(Gr. neuter of nektos, swimming). Term for actively swimming organisms, essentially independent of wave and current action. Compare with plankton


(Gr. nema, thread, + kystis, bladder). Stinging organelle of cnidarians.


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 Weismann in the late nineteenth century and, after incorporating Mendelian genetic principles, has become the currently favored version of Darwinian evolutionary theory.


(Gr. neos, new, + pteryx, fin). Any of a large group of bony fishes that includes most modern species.


See juvenile hormone


(Gr. neos, new, + teinein, to extend). 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.


(Gr. nephros, kidneys, + porus, pore). An external excretory opening in invertebrates.


(Gr. nephridios, of the kidney). 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.


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


(Gr. nephros, kidney, + stoma, mouth). Ciliated, funnelshaped opening of a nephridium.


(Gr. nerites, a mussel). Portion of the sea overlying the continental shelf, specifically from the subtidal zone to a depth of 200 m.

nested hierarchy

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


(Gr. neuron, nerve, + N.L. genic, give rise to). Originating in nervous tissue, as does the rhythmical beat of some arthropod hearts.


(Gr. neuron, nerve, + glia, glue). Tissue supporting and filling the spaces between the nerve cells of the central nervous system.


(Gr. neuron, nerve, + lemma, skin). Delicate nucleated outer sheath of a nerve cell; sheath of Schwann.


(Gr. neuron, sinew, nerve, + mastos, knoll). Cluster of sense cells on or near the surface of a fish or amphibian that is sensitive to vibratory stimuli and water.


(Gr. nerve). A nerve cell.


(Gr. neuron, nerve, + pous, podos, foot). Lobe of parapodium nearer the ventral side in polychaete annelids.

neurosecretory cell

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


A subatomic particle lacking an electrical charge and having a mass 1839 times that of an electron and found in the nucleus of atoms.


The role of an organism in an ecological community; its unique way of life and its relationship to other biotic and abiotic factors.

nictitating membrane

(L. nicto, to wink). Third eyelid, a transparent membrane of birds and many reptiles and mammals, that can be pulled across the eye.

nitrogen fixation

(Gr. nitron, soda, + gen, producing). 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.


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.


(Gr. notos, back, + chorda, cord). An elongated cellular cord, enclosed in a sheath, which forms the primitive axial skeleton of chordate embryos and adult cephalochordates.


(Gr. notos, back, + pous, podos, foot). Lobe of parapodium nearer the dorsal side in polychaete annelids.

nucleic acid

(L. nucleus, kernel). 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.


(L. nucleus, kernel, + oid, like). The region in a prokaryotic cell where the chromosome is found.


(dim. of L. nucleus, kernel). 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.


(L. nucleus, kernel, + Gr. plasma, mold). Protoplasm of nucleus, as distinguished from cytoplasm.


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


(L. nucleus, kernel, + soma, body). A repeating subunit of chromatin in which one and threequarter turns of the double-helical DNA are wound around eight molecules of histones.


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.


(L. nucleus, a little nut, the kernel). The organelle in eukaryotes that contains the chromatin and which is bounded by a double membrane (nuclear envelope).

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).


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

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