habitat
(L. habitare, to dwell). The place
where an organism normally lives or where
individuals of a population live.
habituation
A kind of learning in which
continued exposure to the same stimulus
produces diminishing responses.
halter
pl. halteres (hal-ti+rez) (Gr.
leap). In Diptera, small club-shaped
structure on each side of the metathorax
representing the hindwings; believed to be
sense organs for balancing; also called
balancer.
haplodiploidy
(Gr. haploos, single, + diploos, double, + eidos, form).
Reproduction in which haploid males are
produced parthenogenetically, and diploid
females are from fertilized eggs.
haploid
(Gr. haploos, single). The reduced, or
n, number of chromosomes, typical of
gametes, as opposed to the diploid, or 2n,
number found in somatic cells. In certain
groups, mature organisms may have a
haploid number of chromosomes.
Hardy-Weinberg equilibrium
Mathematical
demonstration that the Mendelian hereditary
process does not change the populational
frequencies of alleles or genotypes across
generations, and that change in allelic or
genotypic frequencies requires factors such
as natural selection, genetic drift in finite
populations, recurring mutation, migration
of individuals among populations, and
nonrandom mating.
hectocotylus
(Gr. hekaton,
hundred, + kotyle, cup). Specialized, and
sometimes autonomous, arm that serves as a
male copulatory organ in cephalopods.
hemal system
(Gr. haima, blood).
System of small vessels in echinoderms;
function unknown.
hemerythrin
(Gr. haima,
blood, + erythros, red). A red, ironcontaining
respiratory pigment found in the
blood of some polychaetes, sipunculids,
priapulids, and brachiopods.
hemimetabolous
(Gr. hemi, half, + metabole, change). Refers to
gradual metamorphosis during development
of insects, without a pupal stage.
hemocoel
(Gr. haima, blood, + koiloma, cavity). Major body space in
arthropods replacing the coelom, contains
the blood (hemolymph).
hemoglobin
(Gr. haima, blood, + L. globulus, globule). An iron-containing
respiratory pigment occurring in vertebrate
red blood cells and in blood plasma of
many invertebrates; a compound of an iron
porphyrin heme and globin proteins.
hemolymph
(Gr. haima, blood, + L. lympha, water). Fluid in the coelom or
hemocoel of some invertebrates that represents
the blood and lymph of vertebrates.
hemozoin
(Gr. haima, blood, + zoon, an animal). Insoluble digestion
product of malaria parasites produced from
hemoglobin.
hepatic
(Gr. hepatikos, of the liver).
Pertaining to the liver.
herbivore
(L. herba, green
crop, + vorare, to devour). Any organism
subsisting on plants. Adj., herbivorous.
heredity
(L. heres, heir). The faithful
transmission of biological traits from parents
to their offspring.
hermaphrodite
(Gr. hermaphroditos, containing both sexes;
from Greek mythology, Hermaphroditos,
son of Hermes and Aphrodite). An organism
with both male and female functional
reproductive organs.
Hermaphroditism may refer to an aberration in unisexual
animals; monoe cyimplies that this is the
normal condition for the species.
hermatypic
(Gr. herma, reef, + typos, pattern). Relating to reef-forming
corals.
heterocercal
(Gr. heteros,
different, + kerkos, tail). In some fishes, a
tail with the upper lobe larger than the
lower, and the end of the vertebral column
somewhat upturned in the upper lobe, as
in sharks.
heterochromatin
(Gr. heteros, different, + chroma, color).
Chromatin that stains intensely and appears
to represent inactive genetic areas.
heterochrony
(Gr. heteros,
different, + chronos, time). Evolutionary
change in the relative time of appearance
or rate of development of characteristics
from ancestor to descendant.
heterodont
(Gr. heteros,
different, + odous, tooth). Having teeth
differentiated into incisors, canines, and
molars for different purposes.
heterotroph
(Gr. heteros,
different, + trophos, feeder). An organism
that obtains both organic and inorganic raw
materials from the environment in order to
live; includes most animals and those
plants that do not carry on photosynthesis.
heterozygote
(Gr. heteros,
different, + zygotos, yoked). An organism in
which homologous chromosomes contain
different allelic forms (often dominant and recessive) of a locus; derived from a zygote
formed by union of gametes of dissimilar
allelic constitution.
hexamerous
(Gr. hex, six, + meros, part). Six parts, specifically,
symmetry based on six or multiples thereof.
hibernation
(L. hibernus, wintry). Condition,
especially of mammals, of passing the
winter in a torpid state in which the body
temperature drops nearly to freezing and
the metabolism drops close to zero.
hierarchical system
A scheme arranging
organisms into a series of taxa of increasing
inclusiveness, as illustrated by Linnean
classification.
histogenesis
(Gr. histos,
tissue, + genesis, descent). Formation and
development of tissue.
histone
(Gr. histos, tissue). Any of
several simple proteins found in cell nuclei
and complexed at one time or another
with DNA. Histones yield a high proportion
of basic amino acids on hydrolysis;
characteristic of eukaryotes.
holoblastic cleavage
(Gr. holo, whole, + blastos, germ). Complete and
approximately equal division of cells in
early embryo. Found in mammals,
amphioxus, and many aquatic invertebrates
that have eggs with a small amount of yolk.
holometabolous
(Gr. holo,
complete, + metabole, change). Complete
metamorphosis during development.
holophytic nutrition
(Gr. holo,
whole, + phyt, plant). Occurs in green
plants and certain protozoa and involves
synthesis of carbohydrates from carbon
dioxide and water in the presence of
light, chlorophyll, and certain enzymes.
holozoic nutrition
(Gr. holo,
whole, + zoikos, of animals). Type of
nutrition involving ingestion of liquid or
solid organic food particles.
home range
The area over which an animal
ranges in its activities. Unlike territories,
home ranges are not defended.
homeobox
(Gr. homoios, like,
resembling, + L. buxus, boxtree [used in
the sense of enclosed, contained]). A highly
conserved 180-base pair sequence found in
regulatory sequences of protein-coding
genes that regulate development.
homeostasis
(Gr. homeo,
alike, + stasis, state or standing).
Maintenance of an internal steady state by
means of self-regulation.
homeothermic
(Gr. homeo, alike, + therme, heat). Having a
nearly uniform body temperature, regulated
independent of the environmental
temperature; “warm blooded.”
homeotic genes
(Gr. homoios,
like, resembling). Genes, identified through
mutations, that give developmental identity
to specific body segments.
hominid
(L. homo, hominis,
man). A member of the family Hominidae, now represented by one living species,
Homo sapiens.
hominoid
Relating to the
Hominoidea, a superfamily of primates to
which the great apes and humans are
assigned.
homocercal
(Gr. homos,
same, common, + kerkos, tail). A tail with
the upper and lower lobes symmetrical and
the vertebral column ending near the
middle of the base, as in most telost fishes.
homodont
(Gr. homos, same, + odous, tooth). Having all teeth similar in
form.
homograft
See allograft
homology
(Gr. homologos, agreeing).
Similarity of parts or organs of different
organisms caused by evolutionary derivation
from a corresponding part or organ in a
remote ancestor, and usually having a similar
embryonic origin. May also refer to a matching
pair of chromosomes. Serial homology is
the correspondence in the same individual of
repeated structures having the same origin
and development, such as the appendages of
arthropods. Adj.,
homologous
homoplasy
Phenotypic
similarity among characteristics of different
species or populations (including molecular,
morphological, behavioral or other features)
that does not accurately represent patterns
of common evolutionary descent
(= nonhomologous similarity); it is
produced by evolutionary parallelism,
convergence and/or reversal, and is revealed
by incongruence among different characters
on a cladogram or phylogenetic tree.
homozygote
(Gr. homos, same, + zygotos, yoked). An organism having
identical alleles at one or more genetic loci.
Adj., homozygous.
humoral
(L. humor, a fluid).
Pertaining to an endocrine secretion.
humoral immune response
Immune
response involving production of antibodies,
specifically the TH2 arm of the immune
response. Contrast cell-mediated immune
response
hyaline
(Gr. hyalos, glass). Adj.,
glassy, translucent. Noun, a clear, glassy,
structureless material occurring, for
example, in cartilage, vitreous body, mucin,
and glycogen.
hybridoma
(contraction of
hybrid + myeloma). Fused product of a
normal and a myeloma (cancer) cell, which
has some of the characteristics of the
normal cell.
hydatid cyst
(Gr. hydatis, watery
vesicle). A type of cyst formed by juveniles
of certain tapeworms (Echinococcus) in
their vertebrate hosts.
hydranth
(Gr. hydor, water, + anthos, flower). Nutritive zooid of
hydroid colony.
hydrocaulus
(Gr. hydor, water, + kaulos, stem of a plant). Stalks or “stems” of a hydroid colony, the parts between the
hydrorhiza and the hydranths.
hydrocoel
(Gr. hydor, water, + koilos, hollow). Second or middle
coelomic compartment in echinoderms; left
hydrocoel gives rise to water vascular system.
hydrocorals
Members of phylum Cnidaria,
class Hydrozoa, with massive calcareous
skeletons.
hydrogen bond
A relatively weak chemical
bond resulting from unequal charge
distribution within molecules, in which a
hydrogen atom covalently bonded to
another atom is attracted to the
electronegative portion of another molecule.
hydroid
The polyp form of a cnidarian as
distinguished from the medusa form. Any
cnidarian of the class Hydrozoa, order
Hydroida.
hydrolysis
(Gr. hydor, water, + lysis, a
loosening). The decomposition of a
chemical compound by the addition of
water; the splitting of a molecule into its
groupings so that the split products acquire
hydrogen and hydroxyl groups.
hydrorhiza
(Gr. hydor, water, + rhiza, a root). Rootlike stolon that attaches
a hydroid to its substrate.
hydrosphere
(Gr. hydor, water, + sphaira,
ball, sphere). Aqueous envelope of the
earth.
hydrostatic pressure
The pressure exerted by
a fluid (gas or liquid), defined as force per
unit area. For example, the hydrostatic
pressure of one atmosphere (1 atm) is
14.7 lb/in2.
hydrostatic skeleton
A mass of fluid or
plastic parenchyma enclosed within a
muscular wall to provide the support
necessary for antagonistic muscle action; for
example, parenchyma in acoelomates and
perivisceral fluids in pseudocoelomates
serve as hydrostatic skeletons.
hydrothermal vent
A submarine hot spring;
seawater seeping through the sea bottom
is heated by magma and expelled back
into the sea through the hydrothermal
vent.
hydroxyl
(hydrogen + oxygen, + yl).
Containing an OH− group, a negatively
charged ion formed by alkalies in water.
hyomandibular
(Gr. hyoeides [shaped like the Gr. letter upsilon
Y, + eidos, form], + L. mandere, to chew).
Bone derived from the hyoid gill arch,
forming part of articulation of the lower jaw
of fishes, and forming the stapes of the ear
of amniotic vertebrates.
hyperosmotic
(Gr. hyper, over, + osmos,
impulse). Refers to a solution whose
osmotic pressure is greater than that of
another solution to which it is compared;
contains a greater concentration of dissolved
particles and gains water through a
selectively permeable membrane from a
solution containing fewer particles; contrasts
with hypoosmotic
hyperparasitism
(Gr.
hyper, over, + para, beside, + sitos, food).
Parasitism of a parasite by another parasite.
hypertrophy
(Gr. hyper, over, + trophe, nourishment). Abnormal increase
in size of a part or organ.
hypodermis
(Gr. hypo, under, + L. dermis, skin). The cellular layer lying
beneath and secreting the cuticle of
annelids, arthropods, and certain other
invertebrates.
hypoosmotic
(Gr. hypo, under, + osmos,
impulse). Refers to a solution whose osmotic
pressure is less than that of another solution
with which it is compared or taken as a
standard, contains a lesser concentration of
dissolved particles and loses water during
osmosis; contrasts with hyperosmotic.
hypophysis
(Gr. hypo, under, + physis, growth). Pituitary body.
hypostome
(Gr. hypo, under, + stoma, mouth). Name applied to structure
in various invertebrates (such as mites and
ticks), located at posterior or ventral area of
mouth.
hypothalamus
(Gr. hypo,
under, + thalamos, inner chamber). A
ventral part of the forebrain beneath the
thalamus; one of the centers of the
autonomic nervous system.
hypothesis
(Gr. hypothesis, foundation,
supposition). A statement or proposition
that can be tested by experiment.
hypothetico-deductive (Gr. hypotithenai, to
suppose, + L. deducere, to lead). Scientific
process of making a conjecture and then
seeking empirical tests that potentially lead
to its rejection.
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