H

habitat imprinting The tendency of an animal to prefer a place to live based on early experience in that habitat.

habitat restoration The recovery of stressed or destroyed habitats.

habitat selection The choice of an animal's place to live. Habitat selection involves the interaction of physiological and psychological factors.

habituation The relatively persistent waning of a response that results from repeated presentations not followed by any form of reinforcement.

hadal zone The bottom below 6,000 m (20,000 ft).

Hadley cells Circulation patterns of atmospheric convection currents as they sink and rise in several intermediate bands.

hair A long slender filament. Applied especially to such filamentous appendages of the skin.

hair cell An epithelial cell that bears modified cilia on one end; when the cilia are deflected, they change the receptor cell's potential.

half-life (of radioisotopes)The length of time required for half the nuclei in a sample to change into another isotope.

Haller's organ Depression on the first tarsi of ticks; functions as an olfactory and humidity receptor.

hallucinogen Psychoactive drug capable of altering moods and perceptions of time and space.

hallux L. Digit number one of the foot.

halophilic Pertaining to microorganisms that tolerate high concentrations of saline media.

halophyte A salt-tolerant terrestrial plant.

halter 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.

haltere Vestigial wing on the metathorax of a fly of the order Diptera; necessary for balance during flight.

halzoun Disease resulting from blockage of the nasopharynx by a parasite. Also called marrara.

hamuli Large hooks on the opisthaptor of a monogenetic trematode; referred to as anchors by American authors.

hamulus hamul = little hook.

handicap hypothesis The hypothesis that apparently deleterious sexual "ornaments" possessed by males in many species are attractive to females because they indicate that the males have such vigor (good genes) that they can survive even with the handicap.

haplodiploidy The mode of sex determination by which males develop from haploid (unfertilized) eggs and females from diploid (fertilized) eggs; characteristic of the insect order Hymenoptera.

haploid 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.

haploid cell a cell containing only one of each type of chromosome characteristic of its species

haptens Molecules of small molecular weight (usually) that are immunogenic only when attached to carrier molecules, usually proteins.

haptera Short, sturdy rootlike structures that form the holdfast of seaweeds.

hardwood A term often used in reference to all woody dicots; more accurately, wood having a high specific gravity.

Hardy-Weinberg equilibrium. Mathematical demonstration that the Mendelian hereditary process does not change the populational frequencies of alleles or genotypes across generations, and that changes 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.

Hardy-Weinberg formula The application of the binomial expansion to determine the frequency of two alleles in the absence of any evolutionary forces.

harem A large group of females herded by a large male pinniped for the purpose of mating; harems also occur in some fishes.

hazardous Describes chemicals that are dangerous, including flammables, explosives, irritants, sensitizers, acids, and caustics; may be relatively harmless in diluted concentrations.

hazardous waste Any discarded material containing substances known to be toxic, mutagenic, carcinogenic, or teratogenic to humans or other life-forms; ignitable, corrosive, explosive, or highly reactive alone or with other materials.

HDL (high density lipoprotein) Transport molecule that removes excess cholesterol from the body's tissues to the liver for degradation and elimination.

head Many individual flowers tightly compressed into the shape of a single large flower.

head-foot The body region of a mollusc that contains the head and is responsible for locomotion as well as retracting the visceral mass into the shell.325

health A state of physical and emotional well-being; the absence of disease or ailment.

heap-leach extraction A technique for separating gold from extremely low-grade ores. Crushed ore is piled in huge heaps and sprayed with a dilute alkaline-cyanide solution, which percolates through the pile to extract the gold, which is separated from the effluent in a processing plant. This process has a high potential for water pollution.

heart urchins Burrowing sea urchins with a flattened test and short spines.

heartwood The wood found in the center of a tree trunk; often a darker color due to the accumulation of resins, oils, gums, and other metabolic by-products, which prevent water movement through this tissue.

heartworm disease A parasitic infection in dogs caused by the nematode Dirofilaria immitis.

heat capacity The amount of heat energy that must be added or subtracted to change the temperature of a body; water has a high heat capacity.

heat of vaporization The amount of heat energy required to convert water from a liquid to a gas.

heat A form of energy transferred from one body to another because of a difference in temperatures.

heavy metals A group of toxic metals: mercury, lead, and others.

hectocotylus Specialized, and sometimes autonomous, arm that serves as a male copulatory organ in cephalopods.

helpers-at-the-nest Individuals other than the parents that forego reproduction to assist rearing the young of others.

hemal system Strands of tissue found in echinoderms. The hemal system is of uncertain function. It may aid in the transport of large molecules or coelomocytes, which engulf and transport waste particles within the body.

Hematopoiesis hema = blood; poie = produce. The formation and development of blood cells.

hemelytra The front wing of an insect of the order Hemiptera.

hemerythrin A red, iron-containing respiratory pigment found in the blood of some polychaetes, sipunculids, priapulids, and brachiopods.

hemicellulose Polysaccharide in plant cell walls that cross-link cellulose fibrils.

Hemichordata The phylum of marine, wormlike animals whose members have an epidermal nervous system and pharyngeal gill slits. Acorn worms and pterobranchs.

hemichordates Invertebrates with a dorsal nerve cord and gill slits. Includes the acorn worms, or enteropneusts.

hemimetabolous Refers to gradual metamorphosis during development of insects, without a pupal stage.

hemimetabolous metamorphosis A type of insect metamorphosis in which immature insects are different in form and habitats from the adult. It is different from holometabolous metamorphosis in that there is a gradual series of changes in form during the transition from immature to adult.

hemizygous An individual having one member of a pair of genes.

hemocoel Main body cavity of arthropods, the embryonic development of which differs from that of a true coelom but that includes a vestige of a true coelom.

hemocyanin A nonheme, blue respiratory pigment that is found in the plasma of many molluscs and arthropods and is composed of monomers, each of which contains two atoms of copper and can bind one molecule of oxygen.

hemocyte Any blood corpuscle or formed element of the blood.

hemoglobin 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.

hemoglobinuria Bloody urine.

hemolymph Fluid in the coelom or hemocoel of some invertebrates that represents the blood and lymph of vertebrates.

hemozoin Insoluble digestion product of malaria parasites produced from hemoglobin.

hemp Cannabis sativa; source for fibers used in making hemp rope.

hepatic Pertaining to the liver.

hepatosplenomegaly Swollen liver and spleen.

herb Nonwoody plant; aromatic plants whose leaves are used in seasoning.

herbaceous Without woody tissues; typical of most annuals and biennials; herbaceous perennials die back to the soil level each year.

herbal A text that describes plants that are useful medicinally and in other ways.

herbalism The use of plants for treating ailments.

herbarium A permanent collection of dried and pressed plants that provides information on the location and identification of the local flora.

herbicide A plant growth regulator that inhibits growth or kills a plant when applied at relatively low concentrations.

herbivore Animals that feed only on plants.

herbivory The process of existing by eating macroscopic plants.

heredity The faithful transmission of biological traits from parents to their offspring.

heritability (in the narrow sense)A measure of the extent to which genes inherited from parents determine phenotypes; defined as the ratio of the additive genetic variance to total phenotypic variance; also called realized heritability.

heritability (in the broad sense)The amount of the variance in a trait in a population that is due to genetic factors; defined as the ratio of the variance due to genetic factors to the sum of variance due to both genetic and environmental factors; also called the degree of genetic determination.

heritable Capable of being inherited, such as a physical trait.

hermaphrodite An organism with both male and female functional reproductive organs. Hermaphroditism may refer to an aberration in unisexual animals; monoecy implies that this is the normal condition for the species.

hermaphrodite An organism that has both male and female gonads.

hermaphroditism Possession of gonads of both sexes by a single (monoecious) individual.

hermaphroditism A state characterized by the presence of both male and female reproductive organs in the same animal.

hermatypic Relating to reef-forming corals.

hermatypic coral A reef-building coral.

hesperidium A berry with a thick leathery peel (exocarp and mesocarp) and a fleshy endocarp arranged in sections.

heterocercal 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.

heterochromatic regions Having inactive genes. Inactive regions of chromosomes are said to be heterochromatic.

heterochromatin Chromatin that stains intensely and appears to represent inactive genetic areas.

heterochrony Evolutionary change in the relative time of appearance or rate of development of characteristics from ancestor to descendant.

heterocyst A specialized cell of certain cyanobacteria; usually larger and thick-walled, this cell excludes oxygen and allows for anaerobic nitrogenase activity.

heterodont Having teeth differentiated into incisors, canines, and molars for different purposes.

heterogonic life cycle Life cycle involving alternation of parasitic and free-living generations.

heterophile reaction Antigen-antibody reaction in which the antibody was not specifically elicited by the antigen to which it binds.

heterosporous Producing two distinct types of spores.

heterotherm An animal whose body temperature fluctuates markedly; "cold-blooded."

heterotroph An organism that is incapable of synthesizing its own food and, therefore, must feed upon organic compounds produced by other organisms.

heterotrophic An organism that obtains its food from other organisms.

heterotrophic theory A theory of the origin of life which proposes that the first organisms obtained nutrition from the spontaneous formation of organic molecules derived from primordial gases.

heterotrophs Organisms that obtain both inorganic and organic raw materials from the environment in order to live; animals, fungi, many protists, and most bacteria are heterotrophs.

heteroxenous Describes a parasite that lives within more than one host during its life cycle.

heterozygote 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.

heterozygous A genotype for a given phenotypic expression containing a dominant and a recessive allele for that trait.

hexacanth Oncosphere; a six-hooked larva hatching from the egg of a eucestode.

Hexactinellida The class of sponges whose members are characterized by triaxon siliceous spicules, which are sometimes formed into an intricate lattice. Cup or vase shaped. Scyconoid body form. Glass sponges.

hexamerous Six parts, specifically, symmetry based on six or multiples thereof.

Hexapoda The class of mandibulate arthropods whose members are characterized by having three pairs of legs. Commonly called insects. Hexapods often have wings and a body divided into head, thorax, and abdomen. Insecta has been used as an alternate class name.

hexose A 6-carbon sugar.

hibernation 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.

high tide The highest level reached by the rising tide.

high-level waste repository A place where intensely radioactive wastes can be buried deep in the ground and remain unexposed to groundwater and earthquakes for tens of thousands of years. The first such site chosen in the United States is Yucca Mountain, Nevada, although it is not certain that a waste repository will ever be built there.

high-quality energy Intense, concentrated, and high-temperature energy that is considered high-quality because of its usefulness in carrying out work.

Hill reaction The splitting of a molecule of water during the light reactions of photosynthesis; the photolysis of water.

hilum Scar on a seed indicating where it was attached to the ovary.

hindbrain Includes the medulla oblongata, cerebellum, and pons.

Hirudinea The class of annelids whose members are characterized by bodies with 34 segments, each of which is subdivided into annuli. Anterior and posterior suckers present. Leeches.

histogenesis Formation and development of tissue.

histology hist = web; ology = study [of tissues].

histone 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.

histozoic Dwelling within the tissues of a host.

holdfast An organ at the base of macroalgae that attaches the stalk to a rocky surface.

holistic medicine The treatment of the whole entity, the human body, spirit, and mind_especially accepted in Eastern and primitive cultures.

holoblastic Division of a zygote that results in separate blastomeres.

holoblastic cleavage 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 metamorphosis A type of insect metamorphosis in which immatures, called larvae, are different in form and habitats from the adult; the last larval molt results in the formation of a pupa; radical cellular changes in the pupal stage end in adult emergence.

holophytic nutrition Formation of carbohydrates by chloroplasts.

holophytic nutrition 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.

holoplankton species of zooplankton that remain in a planktonic stage throughout their lives

Holothuroidea The class of echinoderms whose members are elongate along the oral-aboral axis, have microscopic ossicles embedded in a muscular body wall, and have circumoral tentacles. Lack rays. Sea cucumbers.

holozoic nutrition 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 A highly conserved 180-base pair sequence found in regulatory sequences of protein-coding genes that regulate development.

homeostasis Tendency of living organisms to maintain a steady state in their internal environmental conditions, including body temperature, blood sugar level, and metabolic rate.

homeotherm An animal, such as a bird or mammal, that maintains precisely controlled internal body temperatures using its own heating and cooling mechanisms

homeothermic Having a nearly uniform body temperature, regulated independent of the environmental temperature; "warm blooded."

homeotic genes Genes, identified through mutations, that give developmental identity to specific body segments.

Homestead Act Legislation passed in 1862 allowing any citizen or applicant for citizenship over twenty-one years old and head of a family to acquire 160 acres of public land by living on it and cultivating it for five years.

homing The ability of an animal to return to its home site after being displaced.

hominid 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 A tail with the upper and lower lobes symmetrical and the vertebral column ending near the middle of the base, as in most teleost fishes.

homodont Having a series of similar, unspecialized teeth.

homodont Having all teeth similar in form.

homogeneous Consistent and similar in composition.

homogonic life cycle Life cycle in which all generations are parasitic or all are free living. There is no (or little) alternation of the two.

homologous Structures that have a common evolutionary origin; the wing of a bat and the arm of a human are homologous; each can be traced back to a common ancestral appendage.

homologous chromosomes Chromosomes that exist in pairs; each homologue possesses the same genes or loci, but the homologues may have different alleles at the same locus; one member of each pair comes from each parent.

homologous pairs Two chromosomes that are morphologically and structurally identical and that pair during prophase of meiosis.

homologue One chromosome morphologically and structurally identical to another one in a somatic cell nucleus.

homology 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.

homosporous Producing only one kind of spore.

homothetogenic fission Mitotic fission across the rows of cilia of a protozoan.

homozygote An organism in which the pair of alleles for a trait is composed of the same genes (either dominant or recessive but not both). Adj., homozygous.

homozygous Having two identical alleles for a given trait.

hood Dorsal wall of the camerostome that extends over the capitulum.

horizons Soil zones from the surface downward that reveal visible horizontal layers of soil.

hormone 1. An organic molecule synthesized by a plant that exerts, even in low concentrations, profound regulation of growth and/or development. 2. Chemical products of ductless glands that are carried by the circulatory system and that influence various physiological processes in the body.

horn The paired growths on the head of certain ungulate animals; the median growth of hair on the snout of the rhinoceros.

horsehair worms Pseudocoelomate animals that belong to the phylum Nematomorpha. Also known as Gordian worms or hairworms (Gordius is the name for an ancient king who tied an intricate knot).

horseshoe crabs Arthropods with a large horseshoe-shaped carapace.

horticultural therapy The activities of propagating, growing, and planting plants is therapeutic for a variety of people including the elderly and mentally challenged.

host one member of the host-symbiont pairing characteristic of all symbiotic relationships

host organism An organism that provides lodging for a parasite.

host specificity Degree to which a parasite is able to mature in more than one host species.

host An animal or protist that harbors or nourishes another organism (parasite).

hot desert Deserts of the American Southwest and Mexico; characterized by extreme summer heat and cacti.

human ecology The study of the interactions of humans with the environment.

human resources Human wisdom, experience, skill, labor, and enterprise.

humerus humer = shoulder.

humidity Water vapor in the atmosphere.

humoral Pertaining to an endocrine secretion.

humoral immune response Binding of antigen with soluble antibody in blood serum. Also, the entire process by which the body responds to an antigen by producing antibody to that antigen.

humoral immunity The type of immunity that results from the presence of antibodies that are soluble in blood and lymph.

humus Sticky, brown, insoluble residue from the bodies of dead plants and animals; gives soil its structure, coating mineral particles and holding them together; serves as a major source of plant nutrients.

hunter-gatherers Human social group secures food sources from wild resources such as hunting animal prey or collecting edible plants from the wild.

hyaline Adj., glassy, translucent. Noun, a clear, glassy, structureless material occurring, for example, in cartilage, vitreous body, mucin, and glycogen.

hybrid Offspring of a cross between two species or between alternate homozygous conditions.

hybridoma Fused product of a normal and a myeloma (cancer) cell, which has some of the characteristics of the normal cell.

hydatid cyst A type of cyst formed by juveniles of certain tapeworms (Echinococcus) in their vertebrate hosts.

hydatid sand Sediment in a hydatid cyst formed by free protoscolices.

hydranth Nutritive zooid of hydroid colony.

hydraulic skeleton The use of body fluids in open circulatory systems to give support and facilitate movement; muscles contracting in one part of the body force body fluids into some distant tissue space, thus causing a part of the body to extend or become turgid. See hydrostatic skeleton.

hydrocarbon An organic molecule that contains only carbon and hydrogen and has its carbons bonded in a linear fashion.

hydrocaulus Stalks or "stems" of a hydroid colony, the parts between the hydrorhiza and the hydranths.

hydrocoel Second or middle coelomic compartment in echinoderms; left hydrocoel gives rise to water vascular system.

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.

hydrogen potential Same as pH; refers to the number of charged hydrogen atoms or protons present, acidity of the soil.

hydrogen sulfide (H₂S) The gas that is produced in anoxic sediments.

hydrogen An element that is one of the constituents of water, organic matter, and many other chemicals. Hydrogen gas (H₂) is composed of two hydrogen atoms.

hydrogenation Addition of one or more hydrogens to monunsaturated and polyunsaturated fatty acid chains.

hydrogenosomes Small organelles in certain anaerobic protozoa that produce molecular hydrogen as an end product of energy metabolism.

hydroid. The polyp form of a cnidarian as distinguished from the medusa form. Any cnidarian of the class Hydrozoa, order Hydroida.

hydrological cycle The cycling of water between reservoirs in oceans, lakes, and groundwater, and the atmosphere.

hydrolysis 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.

hydrophilic The property of a substance that has a tendency to attract water.

hydrophobic The property of a substance that has a tendency to repel water.

hydroponics A system of growing plants in a liquid nutrient solution without soil.

hydrorhiza Rootlike stolon that attaches a hydroid to its substrate.

hydrosphere 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 vents Deep, oceanic regions where the tectonic plates of the earth's crust are moving apart. They are characterized by occasional lava flows and hot water springs.

hydroxyl Containing an OH_ group, a negatively charged ion formed by alkalies in water.

Hydrozoa The class of cnidarians whose members have epidermally derived gametes, mesoglea without wandering amoeboid cells, and gastrodermis without nematocysts. Medusae, when present, with a velum. Hydra, Obelia, Physalia.

hydrozoans Cnidarians whose life cycle typically includes a polyp, which is often colonial, and a medusa.

hygroreceptor A receptor found in insects that detects the water content of air.

hymenium The layer of fertile cells that produces spores in a fungal fruiting body.

Hyoid Y-shaped.

hyomandibular 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.

hyperapolysis Detachment of a tapeworm proglottid while still immature, before eggs are formed.

hyperendemic Condition in which a disease or infection has high, usually seasonal, transmission in a certain geographic area.

hyperinfection Condition in Strongyloides infections in which filariform juveniles repenetrate mucosa of the small intestine and proceed with migration.

hypermetamorphosis Type of metamorphic development in which different larval instars have markedly dissimilar body forms.

hyperosmotic 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 Parasitism of a parasite by another parasite.

hyperphagia A condition in which an animal does not stop eating when it normally would.

hyperpolarized A description for a membrane whose polarity is greater than its typical resting potential.

hypertonic A solution having a greater number of solute particles than another solution to which it is compared.

hypertrophy Abnormal increase in size of a part or organ.

hypha Microscopic threads that make up the body of most fungi.

hypnozoite Dormant exoerythrocytic form found in certain Plasmodium species.

hypoblast An inner layer of cells that results from the proliferation and movement of cells in the blastoderm of an avian or reptilian embryo.

hypocotyl hook The hooked portion of a hypocotyl formed at germination that assists a dicot shoot in pulling itself through the soil crust.

hypocotyl The shoot portion of a seedling below the cotyledonary node.

hypodermis The cellular layer lying beneath and secreting the cuticle of annelids, arthropods, and certain other invertebrates.

hypogynous Floral whorls (sepals, petals, stamens) inserted below the ovary of a flower.

hypoosmotic 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.

hypophagia A condition in which an animal does not eat as much as it normally would.

hypopharynx Tonguelike lobe arising from the floor of the mouth in insects; variously modified for feeding in many groups.

hypophysis Pituitary body.

hypostome Name applied to structure in various invertebrates (such as mites and ticks), located at posterior or ventral area of mouth.

hypothalamus A ventral part of the forebrain beneath the thalamus; one of the centers of the autonomic nervous system.

hypotheca the smaller portion of a diatom frustule

hypothesis A tentative explanation of a question; an explanation based on careful observations.

hypothesis A statement or proposition that can be tested by experiment.

hypothetico-deductive Scientific process of making a conjecture and then seeking empirical tests that potentially lead to its rejection.

hypothyroidism Listlessness and other metabolic symptoms caused by low thyroid hormone levels.

hypotonic A solution having a lesser number of solute particles than another solution to which it is compared.

hysterosoma Combination of the metapodosoma and opisthosoma of the body of a tick or mite.