dactylozooid A polyp of a colonial hydroid specialized for defense or killing food.

daily torpor Daily sluggishness that some animals experience. A period of inactivity that is normally induced by cold; observed in numerous ectotherms and also endotherms; often used to describe the specific physiological state of endotherms with a circadian cycle of lowered body temperature and depressed metabolic rate.

dalton Atomic Weight Unit ; the mass of one proton or one neutron.

dark reaction that part of the photosynthetic process that, in the absence of light, utilizes the preformed high-energy molecules ATP and NADPH2 to synthesize complex organic molecules; the Calvin cycle.

Darwinism Theory of evolution emphasizing common descent of all living organisms, gradual change, multiplication of species and natural selection.

data The results in a scientific experiment, or descriptive observations, upon which a conclusion is based.

dauer juvenile Nematode juvenile in which development is arrested during unsuitable conditions and resumed when conditions improve.

daughter sporocysts In digenetic trematodes the embryonic cells that develop from sporocysts and give rise to rediae.

day length The number of hours that sunlight illuminates a given area on earth; dependent on the angle of the earth relative to the rays of the sun.

day-neutral plant A plant whose flowering is not controlled by the length of day.

deamination reaction A reaction in which an amino group, _NH2, is enzymatically removed from a compound.

debt-for-nature swap Forgiveness of international debt in exchange for nature protection in developing countries. Environmental groups and nongovernmental organizations often pay banks to write off uncollectable debts of developing countries at a steep discount in exchange for a promise by the debtor country to establish nature preserves.

decacanth Ten-hooked larva that hatches from the egg of a cestodarian tapeworm. Also called a lycophora.

decapods Crustaceans with five pairs of walking legs and a well-developed carapace. The group includes the shrimps, lobsters, hermit crabs, and crabs.

decarboxylation The removal of a single carbon atom as CO2 from an organic molecule.

deciduous Referring to plants that lose all their leaves during the cool season; as opposed to evergreen plants.

deciduous forest Forest biome in which the dominant trees are deciduous.

decline spiral A catastrophic deterioration of a species, community, or whole ecosystem; accelerates as functions are disrupted or lost in a downward cascade.

decomposer Mostly heterotrophic bacteria and fungi that obtain organic nutrients by breaking down the remains or products of other organic compounds; their activities help cycle the simple compounds back to the autotrophs.

decongestant An agent that relieves nasal or respiratory congestion.

deduction Reasoning from the general to the particular, that is, from given premises to their necessary conclusion.

deep (bottom) layer The deepest and coldest of the three layers of the ocean.

deep ecology A philosophy that calls for a profound shift in our attitudes and behavior based on voluntary simplicity; rejection of anthropocentric attitudes; intimate contact with nature; decentralization of power; support for cultural and biological diversity; a belief in the sacredness of nature; and direct personal action to protect nature, improve the environment, and bring about fundamental societal change.

deep scattering layer (DSL) A sound-reflecting layer of many types of organisms that migrates daily from the mesopelagic zone to the epipelagic zone.

deep sea The dark waters below the mesopelagic zone.

deep-sea fan A fan-like accumulation of sediment at the base of a submarine canyon.

deep-sea hot spring See hydrothermal vent.

Deferens de = away from; ferens = carry.

deficit irrigation The concept of low levels of irrigation to achieve a moderate level of productivity; even though yields are not maximized, limited water supplies are conserved.

definitive host Host in which a parasite achieves sexual maturity. If there is no sexual reproduction in the life of the parasite, the host most important to humans is the definitive host.

degeneracy The genetic code is said to be degenerate because more than one three-base sequence in DNA can code for one amino acid.

degeneration The death or functional impairment of cells connected to destroyed or severely damaged neurons.

degradation Deterioration in water quality due to contamination or pollution; makes water unsuitable for other desirable purposes.

degree of genetic determination See heritability in the broad sense.

dehiscent Mature fruit that splits open to release the seed.

dehiscent fruit Fruit that splits open at maturity, facilitating seed dispersal.

deirid Sensory papilla on each side near the anterior end of some nematodes.

Delaney Clause A controversial amendment to the Federal Food, Drug, and Cosmetic Act, added in 1958, prohibiting the addition of any known cancer-causing agent to processed foods, drugs, or cosmetics.

delayed fertilization Occurs when fertilization of an egg does not occur immediately following coitus, but may be delayed for weeks or months.

delayed implantation A pattern in the reproductive cycle of some mammals causing the blastocyst to remain dormant in the female's uterus for some time before implantation on the uterine wall.

delayed response problem An operant conditioning procedure in which the test animal sees the experimenter set up the test situation, including what will be the correct response, and then must wait until the experimenter provides the animal with an opportunity to respond.

delayed type hypersensitivity (DTH) Manifestation of cell-mediated immunity, distinguished from immediate hypersensitivity in that maximal response is reached about 24 hours or more after intradermal injection of the antigen. The lesion site is infiltrated primarily by monocytes and macrophages.

deletion A mutation resulting from the loss of a small segment of DNA.

deletion mutant A mutation in which a base pair is deleted

delta Fan-shaped sediment deposit found at the mouth of a river.

Deltoid delta [shaped like the Greek letter], triangular.

demand The amount of a product that consumers are willing and able to buy at various possible prices, assuming they are free to express their preferences.

deme A small, local subpopulation. Isolated subpopulations sometimes display genetic changes that may contribute to evolutionary change in the subpopulation.

demersal fish A bottom-dwelling fish.

demographic transition A pattern of falling death and birth rates in response to improved living conditions; could be reversed in deteriorating conditions.

demography Vital statistics about people: births, marriages, deaths, etc.; the statistical study of human populations relating to growth rate, age structure, geographic distribution, etc., and their effects on social, economic, and environmental conditions.

Demospongiae The class of poriferans whose members have monaxon or tetraaxon siliceous spicules or spongin. Leuconoid body forms are present and vary in size from a few centimeters to 1 m in height.

denaturation Disruption of bonds holding a protein in its three-dimensional form, such that its polypeptide chain(s) unfolds partially or completely. Denaturation can be caused by changes in pH, salt concentration, or environmental temperature.

denature To change the configuration of a protein molecule such that it loses specificity and no longer functions as an enzyme.

dendrite Any of nerve cell processes that conduct impulses toward the cell body.

dendrochronology The science of studying growth rings of trees to determine past conditions.

dendroclimatology Study of the annual rings of trees in order to interpret climatic changes in the past.

dengue Virus disease transmitted by mosquitoes.

denitrifying bacteria Free-living soil bacteria that convert nitrates to gaseous nitrogen and nitrous oxide.

density The weight (or more correctly the mass) of a given volume of a substance.

dental formula A notation that indicates the number of incisors, canines, premolars, and molars in the upper and lower jaw of a mammal.

denticle A small toothlike process.

denticles (denticulate) Small, toothlike projections.

deoxyribonucleic acid A polymer of deoxyribonucleotides that is in the form of a double helix; DNA is the genetic molecule of life in that it codes for the sequence of amino acids in proteins; contains nucleotide monomers with deoxyribose sugar and nitrogenous bases adenine (A), cytosine (C), guanine (G), and thymine (T).

deoxyribose A 5-carbon sugar having 1 oxygen atom less than ribose; a component of deoxyribonucleic acid (DNA).

dependency ratio The number of nonworking members compared to working members for a given population.

dependent variable What may change (e.g., aggression) in response to manipulation of an independent variable (e.g., food supply) by the experimenter.

deplasmolysis Water flowing into a plasmolized cell to correct the process that would cause cellular death from dehydration.

depolarization The loss of an electrical charge on the surface of a membrane.

depolarized The reduction in the difference in charge (potential) between the outside and inside of a membrane.

deposit feeder An animal that feeds on organic matter that settles on the bottom. Compare suspension feeder.

deposit feeding The type of feeding whereby an animal obtains its nutrients from the sediments of soft-bottom habitats (mud or sands) or terrestrial soils.

depressant Psychoactive drug that has a sedative effect on the central nervous system; actions include dulling mental awareness and inducing sleep.

deprivation experiments Experimental manipulations involving removal of particular types of stimuli (e.g., social, sensory, motor) to ascertain the effects later in development.

dermal branchiae Thin folds of the body wall of a sea star that extend between ossicles and function in gas exchange and other exchange processes.

dermal Pertaining to the skin; cutaneous.

dermal tissue Tissue that covers surfaces in plants.

dermatitis Infection or inflammation of the skin.

dermatophytes Certain imperfect fungi that cause skin diseases in humans; includes the organisms causing ringworm and athlete's foot.

dermis The layer of the skin deep to the epidermis, consisting of a dense bed of vascular connective tissue.

desalinization (or desalination)Removal of salt from water by distillation, freezing, or ultrafiltration.

desert A type of biome characterized by low moisture levels (below 25 cm per year), and infrequent and unpredictable precipitation. Daily and seasonal temperatures fluctuate widely. Deserts are characterized by cacti and desert shrubs or sagebrush. Animals include birds, rodents, reptiles, and numerous species of arthropods.

desertification A process by which fragile, semiarid ecosystems lose productivity because of loss of plant cover, soil erosion, salinization, or waterlogging. Usually associated with human misuse.

desiccation Dehydration. The process of drying.

desmosome Buttonlike plaque serving as an intercellular connection.

despot A dominant individual that controls resources.

determinate cleavage The type of cleavage, usually spiral, in which the fate of the blastomeres is determined very early in development; mosaic cleavage.

determinate growth Pertaining to a leaf or stem that stops growing after differentiating into a terminal reproductive unit.

detritivore Organisms that consume organic litter, debris, and dung.

detritus Dead organic matter and the decomposers that live on it.

Deuteromycetes A miscellaneous assemblage of fungi also termed the imperfect fungi because the sexual reproductive features are either not known, not used, or have been reduced

deuterostomes Animals in which the anus forms from, or in the region of, the blastopore; often characterized by enterocoelous coelom formation, radial cleavage, and the presence of a dipleurulalike larval stage.

Deuterostomia A group of higher phyla in which cleavage is indeterminate (regulative) and primitively radial. The endomesoderm is enterocoelous, and the mouth is derived away from the blastopore. Includes Echinodermata, Chordata, and a number of minor phyla. Compare with Protostomia.

deuterotoky Type of parthenogenesis in which all individuals are uniparental but in which both males and females occur.

deutomerite Posterior half of a cephaline gregarine protozoan.

deutonymph In the life cycle of some mesostigmatid mites, a nonfeeding stage that molts into the adult.

deutovum Incompletely developed larva that hatches from the egg of a chigger mite.

development The summation of all activities leading to changes in cells, tissues, organs, or organisms; a genetically controlled sequence.

developmental biology The study of how organisms, their

Devonian A geological period of the Paleozoic era beginning about 400 million years ago. During this period major diversification of the early land plants occurred.

dew point The temperature at which condensation occurs for a given concentration of water vapor in the air.

dextral Pertaining to the right; in gastropods, shell is dextral if opening is to right of columella when held with spire up and facing observer.

diabetes Condition characterized by a high blood glucose level and the appearance of glucose in the urine due to a deficiency of insulin or the inability of body cells to respond to insulin; diabetes mellitus.

diapause A period of arrested development in the life cycle of insects and certain other animals in which physiological activity is very low and the animal is highly resistant to unfavorable external conditions.

diapause phase A period of dormancy, common in insect species, which occurs during the more rigorous portions of the annual climatic cycle.

diaphragm The domed respiratory muscle between thoracic and abdominal compartments of mammals.

diapolar cells Ciliated somatodermal cells located between the parapolar and uropolar cells of a mesozoan.

diapsids Amniotes in which the skull bears two pairs of temporal openings; includes reptiles (except turtles) and birds.

Diastema A space, gap.

diastole Phase of the cardiac cycle during which a heart chamber wall is relaxed; also diastolic pressure.

diastolic pressure The blood pressure measurement during the interval between heartbeats; it is the second number shown in a blood pressure reading.

diatomaceous earth A powdery, soil-like material formed by the glass cell walls of dead diatoms deposited on the marine floor.

diatomaceous ooze A biogenous sediment that consists mostly of the siliceous frustules of diatoms. It is known as diatomaceous earth when found inland.

diatoms Unicellular and eukaryotic autotrophs with a siliceous frustule; mostly planktonic.

dichotomous Pertaining to the division or forking of a single axis into two branches.

dicotyledon A class of angiosperms in which the seedlings typically possess two cotyledons; commonly abbreviated to dicot.

dieback A sudden population decline; also called a population crash.

diecdysis Condition in which ecdysis processes are going on continuously and one ecdysis cycle grades rapidly into another.

differentially permeable membrane A membrane that permits the passage of certain types of particles and inhibits the passage of others; also termed selectively permeable membrane.

differentiation The chemical and physical changes associated with the developmental process of an organism or cell.

diffuse-porous wood Secondary xylem characterized by the same-sized vessels and tracheids throughout the growing season so that growth rings are difficult or impossible to detect.

diffusion The random movement of molecules from one location to another because of random thermal molecular motion; net diffusion always occurs from a region of higher concentration to a region of lower concentration.

Digastricus di = two; gaster = stomach or belly.

digestion The process by which larger molecules of food substances are broken down into smaller molecules that can be taken up by the digestive system; hydrolysis.

digestive gland An enzyme-producing gland in several groups of invertebrates where digestion and absorption take place.

digitigrade Walking on the digits with the posterior part of the foot raised; compare plantigrade.

dihybrid A hybrid whose parents differ in two distinct characters; an offspring having two different alleles at two different loci, for example, A/a B/b.

dihybrid cross A genetic cross between parents that differ for two traits.

dikaryon Mycelium of some fungi that have two separate haploid nuclei in each cell.

diminishing returns A condition in which unrestrained population growth causes the standard of living to decrease to a subsistence level where poverty, misery, vice, and starvation makes life permanently drab and miserable. This dreary prophecy has led economics to be called "the dismal science."

dimorphic Having more than one form, size, or appearance; usually referring to the difference between males and females of a species.

dimorphism Existence within a species of two distinct forms according to color, sex, size, organ structure, and so on. Occurrence of two kinds of zooids in a colonial organism.

dinoflagellates Unicellular, eukaryotic, mostly autotrophic organisms with two unequal flagella.

dioecious Characterized by having separate sexes; that is, an individual is either male or female, but never both. "Gonochoristic" means the same thing.

diphycercal A tail that tapers to a point, as in lungfishes; vertebral column extends to tip without upturning.

diphyodont Having deciduous and permanent sets of teeth successively.

diploblastic Animals whose body parts are organized into layers that are derived embryologically from two tissue layers: ectoderm and endoderm. Animals in the phyla Cnidaria and Ctenophora are diploblastic.

diploid Having the somatic (double, or 2N) number of chromosomes or twice the number characteristic of a gamete of a given species.

diploid (2n) cell A cell, such as a body cell, that contains two similar sets of chromosomes, one from each parent. Compare haploid.

Diplopoda The class of arthropods whose members are characterized by having two pairs of legs per apparent segment and a body that is round in cross section. Millipedes.

diplostomulum Strigeoid metacercaria in the family Diplostomatidae.

diporpa Larval stage in the life cycle of the monogenean Diplozoon.

direct action Civil disobedience, guerrilla street theater, picketing, protest marches, road blockades, demonstrations, and other techniques borrowed from the civil rights movement and applied to environmental protection.

direct development In arthropods refers to development in which a juvenile hatches from the egg, and the juvenile is not distinctly different from adult except in size and maturity.

direct fitness A measure of an individual's potential to contribute genes to future generations via personal reproduction.

direct flight Insect flight that is accomplished by flight muscles acting on wing bases and in which a single nerve impulse results in a single wing cycle; also called synchronous flight. See indirect (asynchronous) flight.

directional selection Natural selection that occurs when individuals at one phenotypic extreme have an advantage over individuals with more common phenotypes.

dirt Displaced soil.

disaccharide Sugar consisting of two monosaccharides; example: sucrose is composed of glucose and fructose.

discharge The amount of water that passes a fixed point in a given amount of time; usually expressed as liters or cubic feet of water per second.

disclimax community See equilibrium community.

discontinuous feeder An animal that does not feed all the time; instead it generally eats large meals sporadically and does not spend time in the continuous pursuit of prey.

discount rate The amount we discount or reduce the value of a future payment. When you borrow money from the bank at 10 percent annual interest, you are in effect saying that having the money now is worth 10 percent more to you than having the same amount one year from now.

discrete signal A form of communication that is digital, or all or none; usually given at the same intensity each time, such as an alarm call.

discrete variable A variable that can take on only certain values (usually integers), as opposed to a continuous variable.

discrimination The ability to detect differences between two or more stimuli and make choices based on those differences in a learning situation.

disease A deleterious change in the body's condition in response to destabilizing factors, such as nutrition, chemicals, or biological agents.

dispersal A more or less permanent movement of an individual from an area, such as movement of a juvenile away from its place of birth.

displacement activity An innate or stereotypic response to a stimulus that seems inappropriate or irrelevant to the situation.

display Any behavior pattern especially adapted in physical form or frequency to function as a social signal in communication.

disruptive coloration A color pattern that helps break the outline of an organism.

disruptive selection Natural selection that occurs when individuals of the most common phenotypes are at a disadvantage; produces contrasting subpopulations.

dissociation The breaking up of a molecule into ions when placed in water or other solvents.

dissolved organic matter (DOM) Organic matter that is dissolved in water rather than being in particles.

dissolved oxygen (DO) contentAmount of oxygen dissolved in a given volume of water at a given temperature and atmospheric pressure; usually expressed in parts per million (ppm).

distal Away from the point of attachment of a structure on the body (e.g., the toes are distal to the knee).

distal cytoplasm Distal cytoplasmic layer in the tegument of Monogenea, Digenea, and Cestoidea.

distillation Boiling of a liquid to evaporation and subsequent condensation of the vapors for the purposes of purification and concentration.

distilled spirit Alcoholic beverage with an alcoholic content between 80-100 proof, obtained by distillation of a beer or wine.

distome Fluke with two suckers, oral and ventral.

disulfide bond A linkage between the sulfur atoms of two different amino acids in a protein.

diurnal Pertaining to daily cycles or events.An animal with an activity period during the light portion of the daily cycle.

diurnal tide A tidal pattern with a high and a low tide each day.

dive reflex the suite of internal responses, including bradycardia and peripheral circulation shutdown, that occurs during dives by an air-breathing vertebrate

diversity (species diversity, biological diversity)The number of species present in a community (species richness), as well as the relative abundance of each species.

diving reflex The reflex certain animals have to stay underwater for prolonged periods of time.

division Taxonomic rank that includes related classes; synonymous to phylum used in animal systematics.

dizygotic Twins that arise from two different zygotes, hence, individuals no more closely related than two different-aged siblings.

DNA Deoxyribonucleic acid; the long, double-helix molecule in the nucleus of cells that contains the genetic code and directs the development and functioning of all cells.

doctrine of signatures Early concept that the Creator placed certain items on earth for humans and identified their intended use by their shape.

domestic sewage Wastewater from homes and non-industrial buildings. Compare industrial sewage.

domesticated plant A plant that has been genetically changed from the wild type due to artificial selection.

dominance hierarchy. A social ranking, formed through agonistic behavior, in which individuals are associated with each other so that some have greater access to resources than do others.

dominant A gene that masks one or more of its alleles. See recessive.

dominant plants Those plant species in a community that provide a food base for most of the community; they usually take up the most space and have the largest biomass.

dominant species A species that exerts an overriding influence in determining the characteristics of a community.

dominant (1) The most prevalent species in a plant community. (2) The allele that has its trait expressed. (3) A gene that has its phenotypic expression appear in the offspring, regardless of the nature of its allelic partner.

dominant trait A trait whose phenotype is determined by a single allele at a particular locus.

dormant Having reduced metabolic and respiratory activity.

dorsal The back of an animal; usually the upper surface; synonymous with posterior for animals that walk upright.

dorsal plate Dorsal plate on the body of a mesostigmatid mite.

double bond A covalent bond sharing two pairs of electrons.

double fertilization The fusion of egg and sperm resulting in a zygote, and the simultaneous fusion of sperm with two polar nuclei resulting in the formation of endosperm that characterizes all angiosperms.

double helix A helix composed of two molecules winding around each other, as in DNA.

dourine Disease of horses and other equids caused by Trypanosoma equiperdum.

down feathers Feathers that provide insulation for adult and immature birds.

Down syndrome. A congenital syndrome including mental retardation, caused by the cells in a person's body having an extra chromosome 21; also called trisomy 21.

drag Resistance to movement through water or any other medium.

drift net A very long fishing net that is allowed to drift for a long time before it is pulled on board.

drip irrigation Uses pipe or tubing perforated with very small holes to deliver water one drop at a time directly to the soil around each plant. This conserves water and reduces soil waterlogging and salinization.

drives Manifestations of specific motivations corresponding to specific needs.

dropsy An old term for congestive heart failure which produces edema and resulting decrease in manual dexterity, causing one to drop things.

drought An environmental condition in which precipitation is not sufficient to maximize biological productivity.

drowned river valley (or coastal plain) estuaryAn estuary that is formed when sea level rose at the end of the last glacial age.

drupe A fleshy fruit with a one-carpeled ovary and only one seed. The endocarp is hard and stony, tightly enclosing the seed; the mesocarp is fleshy, and the exocarp is soft and thin; example: cherry.

dry alkali injection Spraying dry sodium bicarbonate into flue gas to absorb and neutralize acidic sulfur compounds.

dry fruit Fruit in which the cells of the pericarp are dry (dead) at maturity.

dual-gland adhesive organ Organs in the epidermis of most turbellarians, with three cell types; viscid and releasing gland cells and anchor cells.

Duct ductus = leading.

duo-adhesive gland Platyhelminth tegument gland with two types of cells, one producing an adhesive substance and the other a releasing substance.

duodenum The first and shortest portion of the small intestine lying between the pyloric end of the stomach and the jejunum.

duplications The presence of two copies of one or more loci in a chromosome.

dura mater The outermost meninx.

dyad One of the groups of two chromosomes formed by the division of a tetrad during the first meiotic division.

dyspnea Difficult or labored breathing.

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