nacre
(F. mother-of-pearl). Innermost
lustrous layer of mollusc shell, secreted by
mantle epithelium. Adj.,
nacreous
NAD
Abbreviation of nicotinamide adenine
dinucleotide, an electron acceptor or donor
in many metabolic reactions.
nares
(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.
nauplius
(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.
nekton
(Gr. neuter of nektos,
swimming). Term for actively swimming
organisms, essentially independent of wave
and current action. Compare with
plankton
nematocyst
(Gr. nema,
thread, + kystis, bladder). Stinging organelle
of cnidarians.
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 Weismann in the late
nineteenth century and, after incorporating
Mendelian genetic principles, has become
the currently favored version of Darwinian
evolutionary theory.
neopterygian
(Gr. neos,
new, + pteryx, fin). Any of a large group of
bony fishes that includes most modern
species.
neotenine
See juvenile hormone
neoteny
(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.
nephridiopore
(Gr. nephros,
kidneys, + porus, pore). An external
excretory opening in invertebrates.
nephridium
(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.
nephron
(Gr. nephros, kidney).
Functional unit of kidney structure of
vertebrates, consisting of a Bowman's
capsule, an enclosed glomerulus, and the
attached uriniferous tubule.
nephrostome
(Gr. nephros,
kidney, + stoma, mouth). Ciliated, funnelshaped
opening of a nephridium.
neritic
(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.
neurogenic
(Gr. neuron, nerve, + N.L. genic, give rise to). Originating in
nervous tissue, as does the rhythmical beat
of some arthropod hearts.
neuroglia
(Gr. neuron, nerve, + glia, glue). Tissue supporting and filling the
spaces between the nerve cells of the
central nervous system.
neurolemma
(Gr. neuron,
nerve, + lemma, skin). Delicate nucleated
outer sheath of a nerve cell; sheath of
Schwann.
neuromast
(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.
neuron
(Gr. nerve). A nerve cell.
neuropodium
(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.
neutron
A subatomic particle lacking an
electrical charge and having a mass 1839
times that of an electron and found in the
nucleus of atoms.
niche
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.
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.
notochord
(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.
notopodium
(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.
nucleoid
(L. nucleus, kernel, + oid, like). The region in a prokaryotic cell
where the chromosome is found.
nucleolus
(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.
nucleoplasm
(L. nucleus,
kernel, + Gr. plasma, mold). 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
(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.
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
(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).
nymph
(L. nympha, nymph, bride). An
immature stage (following hatching) of a
hemimetabolous insect that lacks a pupal
stage.
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