dactylozooid
(Gr. dakos, bite, sting,+ tylos, knob, + zoon, animal). A polyp of a colonial hydroid specialized for defense or killing food.
Darwinism
Theory of evolution emphasizing common descent of all living organisms, gradual change, multiplication of species and natural selection.
data
sing. datum (Gr. dateomai, to divide, cut in pieces). The results in a scientific experiment, or descriptive observations, upon which a conclusion is based.
deciduous
(L. decidere, to fall off). Shed or falling off at end of a growing period.
deduction
(L. deductus, led apart, split, separated). Reasoning from the general to the particular, that is, from given premises
to their necessary conclusion.
definitive host
The host in which sexual reproduction of a symbiont takes place; if no sexual reproduction, then the host in which the symbiont becomes mature and reproduces; contrast intermediate host.
delayed type hypersensitivity
Inflammatory reaction based primarily on cell-mediated immunity.
deme
(dem) (Gr.populace). A local population of closely related animals.
demography
(Gr. demos, people,+ graphy). The properties of the rate of growth and the age structure of populations.
dendrite
(Gr. dendron, tree). Any of nerve cell processes that conduct impulses toward the cell body.
deoxyribonucleic acid (DNA
) The genetic material of all organisms, characteristically organized into linear sequences of genes.
deoxyribose
(L. deoxy, loss of oxygen, + ribose, a pentose sugar having 1 oxygen atom less than ribose; a component of deoxyribonucleic acid (DNA).
dermal
(Gr. derma, skin). Pertaining to the skin; cutaneous.
dermis
The inner, sensitive mesodermal layer of skin; corium.
desmosome
(Gr. desmos, bond, + soma, body). Buttonlike plaque serving as an intercellular connection.
determinate cleavage
The type of cleavage, usually spiral, in which the fate of the blastomeres is determined very early in development; mosaic cleavage.
detritus
(L. that which is rubbed or worn away). Any fine particulate debris of organic or inorganic origin.
Deuterostomia
(Gr. deuteros, second, secondary, + stoma, mouth. A group of higher phyla in which cleavage is indeterminate (regulative) and primitively radial. The endomesoderm is enterocoelouth is derived away from the blastopore. Includes Echinodermata, Chordata, and a number of minor phyla. Compare with Protostomia.
dextral
(L. dexter, right-handed). Pertaining to the right; in gastropods, shell is dextral if opening is to right of columella when held with spire up and facing observer.
diapause
(Gr. diapausis,pause). 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.
diapsids
(Gr. di, two,+ apsis, arch). Amniotes in which the skull bears two pairs of temporal openings; includes reptiles (except turtles) and birds.
diastole
(Gr. diastole, dilation). Passive relaxation and expansion of the heart during which the chambers are filled with blood.
diffusion
(L. diffusus, dispersion). The movement of particles or molecules from area of high concentration of the particles or molecules to area of lower concentration.
digitigrade
(L. digitus, finger, toe,+ gradus, step, degree). Walking on the digits with the posterior part of the foot raised; compare plantigrade.
dihybrid
(Gr. dis, twice, + L. hibrida, mixed offspring). 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.
dimorphism
(Gr. di, two, + morphe, form). 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.
dioecious
(Gr. di, two, + oikos, house). Having male and female organs in
separate individuals..
diphycercal
(Gr. diphyes, twofold, + kerkos, tail). A tail that tapers to a point, as in lungfishes; vertebral column extends to tip without upturning.
diphyodont
(Gr. diphyes, twofold,+ odous, tooth). Having deciduous and permanent sets of teeth successively.
diploblastic
(Gr. diploos,double, + blastos, bud). Organism with two
germ layers, endoderm and ectoderm.
diploid
(Gr. diploos, double, + eidos, form). Having the somatic (double,
or 2n) number of chromosomes or twice the number characteristic of a gamete of a given species.
disaccharides
(Gr. dis, twice, + L. saccharum, A class of sugars (such as lactose, maltose, and sucrose) that yield two monosaccharides on hydrolysis.
distal
Farther from the center of the body than a reference point.
DNA
See deoxyribonucleic acid.
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
An allele that is expressed regardless of the nature of the corresponding allele on the homologous chromosome.
dorsal
(L. dorsum, back). Toward the back, or upper surface, of an animal.
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.
dual-gland adhesive organ
Organs in the epidermis of most turbellarians, with three
cell types; viscid and releasing gland cells
and anchor cells.
duodenum
(L. duodeni, twelve each, fr. its length, about 12 fingers' width). The first and shortest portion of the small intestine lying between the pyloric end of the stomach and the jejunum.
dyad
(Gr. dyas, two). One of the groups of two chromosomes formed by the division of a tetrad during the first meiotic division.
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