T cell
A type of lymphocyte important in
cellular immune response and in regulation
of most immune responses.
T-cell receptors
Receptors borne on
surfaces of T cells. The variable region
of a T-cell receptor binds with a specific
antigen.
tactile
(L. tactilis, able to be touched,
from tangere, to touch). Pertaining to touch.
tagma
, pl. tagmata (Gr. tagma, arrangement,
order, row). A compound body section of
an arthropod resulting from embryonic
fusion of two or more segments; for
example, head, thorax, abdomen.
tagmatization, tagmosis
Organization of the
arthropod body into tagmata.
taiga
(Russ.). Habitat zone characterized
by large tracts of coniferous forests, long,
cold winters, and short summers; most
typical in Canada and Siberia.
tantulus
(Gr. tantulus, so small).
Larva of a tantulocaridan (subphylum
Crustacea).
taxis
pl. taxes (Gr. taxis,
arrangement). An orientation movement by
a (usually) simple organism in response to
an environmental stimulus.
taxon
pl. taxa (Gr. taxis,
arrangement). Any taxonomic group or entity.
taxonomy
(Gr. taxis,
arrangement, + nomos, law). Study of the
principles of scientific classification;
systematic ordering and naming of
organisms.
tectum
(L. roof). A rooflike structure,
for example, dorsal part of capitulum in
ticks and mites.
tegmen
(L. tegmen, a cover).
External epithelium of crinoids (phylum
Echinodermata).
tegument
(L. tegumentum, from tegere, to cover). An integument: specifically
external covering in cestodes and
trematodes, formerly believed to be a
cuticle.
telencephalon
(Gr. telos,
end, + encephalon, brain). The most
anterior vesicle of the brain; the anteriormost
subdivision of the prosencephalon that
becomes the cerebrum and associated
structures.
teleology
(Gr. telos, end,
+ L. logia, study of, from Gr. logos, word).
The philosophical view that natural events
are goal directed and are preordained, as
opposed to the scientific view of
mechanical determinism.
telocentric
(Gr. telos, end,
+ kentron, center). Chromosome with
centromere at the end.
telolecithal
(Gr. telos, end, + lekithos, yolk, + al). Having the yolk
concentrated at one end of an egg.
telson
(Gr. telson, extremity).
Posterior projection of the last body
segment in many crustaceans.
temnospondyls
(Gr. temno, to cut, + spondylos, vertebra). A
large lineage of amphibians that extended
from the Carboniferous to the Triassic.
template
A pattern or mold
guiding the formation of a duplicate;
often used with reference to gene
duplication.
tendon
(L. tendo, tendon). Fibrous
band connecting muscle to bone or other
movable structure.
tentaculocyst
(L. tentaculum,
feeler, + kystis, pouch). One of the sense
organs along the margin of medusae; a
rhopalium.
tergum
(L. back). Dorsal part of an
arthropod body segment.
territory
(L. territorium, from terra, earth).
A restricted area preempted by an animal
or pair of animals, usually for breeding
purposes, and guarded from other
individuals of the same species.
test
(L. testa, shell). A shell or hardened outer
covering.
tetrad
(Gr. tetras, four). Group of two
pairs of chromatids at synapsis and resulting
from the replication of paired homologous
chromosomes; the bivalent.
tetrapods
(Gr. tetras, four, + pous, podos, foot). Four-footed
vertebrates; the group includes amphibians,
reptiles, birds, and mammals.
thecodonts
(Gr. theke, box, + odontos, tooth). A large assemblage of
Triassic archosaurian diapsids of the order
Thecodontia and characterized by having
teeth set in sockets.
therapsids
(Gr. theraps, an
attendant). Extinct Mesozoic mammal-like
reptiles from which true mammals evolved.
thermocline
(Gr. therme, heat,
+ klinein, to swerve). Layer of water
separating upper warmer and lighter water
from lower colder and heavier water in a
lake or sea; a stratum of abrupt change in
water temperature.
thoracic
(L. thorax, chest).
Pertaining to the thorax or chest.
thrombin
Enzyme catalyzing fibrinogen
transformation into fibrin. Percursor is prothrombin
Tiedemann's bodies
(from
F. Tiedemann, German anatomist). Four
or five pairs of pouchlike bodies attached
to the ring canal of sea stars, apparently
functioning in production of coelomocytes.
tight junction
Region of actual fusion of cell
membranes between two adjacent cells.
tissue
(M.E. tissu, tissue). An aggregation
of cells, usually of the same kind, organized
to perform a common function.
titer
(Fr. titrer, to titrate). Concentration
of a substance in a solution as determined
by titration.
tornaria
(L. tornare, to turn). A
free-swimming larva of enteropneusts that
rotates as it swims; resembles somewhat the
bipinnaria larva of echinoderms.
torsion
(L. torquere, to twist). A twisting
phenomenon in gastropod development that
alters the position of the visceral and pallial
organs by 180 degrees.
toxicyst
(Gr. toxikon, poison,
+ kystis, bladder). Structures possessed by
predatory ciliate protozoa, which on
stimulation expel a poison to subdue the
prey.
trabecular net
(L. trabecula, a
small beam). Network of living tissue
formed by pseudopodia of amebocytes in
Hexactinellida (phylum Porifera).
trachea
(M.L. windpipe). The
windpipe. Also, any of the air tubes of
insects.
transcription
Formation of messenger RNA
from the coded DNA.
transduction
Condition in which bacterial
DNA (and the genetic characteristics it
bears) is transferred from one bacterium to
another by the agent of viral infection.
transfer RNA (tRNA)
A form of RNA of about
70 or 80 nucleotides, which are adapter
molecules in the synthesis of proteins. A
specific amino acid molecule is carried by
transfer RNA to a ribosome-messenger RNA
complex for incorporation into a
polypeptide.
transformation
Condition in which DNA in
the environment of bacteria somehow
penetrates them and is incorporated into
their genetic complement, so that their
progeny inherit the genetic characters so
acquired.
translation
(L. a transferring). The process in
which the genetic information present in
messenger RNA is used to direct the order of
specific amino acids during protein synthesis.
transporter
See permease
transverse plane
(L. transversus, across). A
plane or section that lies or passes across a
body or structure.
trichinosis
Disease caused by
infection with the nematode Trichinella spiralis.
trichocyst
(Gr. thrix, hair, + kystis,
bladder). Sac-like protrusible organelle in
the ectoplasm of ciliates, which discharges
as a threadlike weapon of defense.
triglyceride
(Gr. tria, three,
+ glykys, sweet, + ide, suffix denoting
compound). A triester of glycerol with one,
two, or three acids.
trimerous
(Gr. treis, three,
+ meros, a part). Body in three main
divisions, as in lophophorates and some
deuterostomes.
tripartite
See trimerous.
triploblastic
(Gr. triploos,
triple, + blastos, germ). Pertaining to
metazoa in which the embryo has three
primary germ layers - ectoderm, mesoderm,
and endoderm.
trisomy 21
See Down syndrome.
trochophore
(Gr. trochos, wheel,
+ pherein, to bear). A free-swimming
ciliated marine larva characteristic of most
molluscs and certain ectoprocts,
brachiopods, and marine worms; an ovoid
or pyriform body with preoral circlet of cilia
and sometimes a secondary circlet behind
the mouth.
trophallaxis
(Gr. trophe, food,
+ allaxis, barter, exchange). Exchange of
food between young and adults, especially
certain social insects.
trophi
(Gr. trophos, one who feeds).
Jaw-like structures in the mastax of rotifers.
trophic
(Gr. trophe, food). Pertaining
to feeding and nutrition.
trophoblast
(Gr. trephein, to
nourish, + blastos, germ). Outer ectodermal
nutritive layer of blastodermic vesicle; in
mammals it is part of the chorion and
attaches to the uterine wall.
trophosome
(Gr. trophe, food, + soma, body). Organ in poganophorans
bearing mutualistic bacteria, derived from
midgut.
trophozoite
(Gr. trophe, food,
+ zoon, animal). Adult stage in the life
cycle of a protozoan in which it is actively
absorbing nourishment.
tropic
(Gr. trope, to turn toward).
Related to the tropics (tropical); in endocrinology, a hormone that influences
the action of another hormone or endocrine
gland (usually pronounced tro´pic).
tropomyosin
(Gr. tropos,
turn, + mys, muscle). Low-molecular weight
protein surrounding the actin filaments of
striated muscle.
troponin
Complex of globular
proteins positioned at intervals along the
actin filament of skeletal muscle; thought to
serve as a calcium-dependent switch in
muscle contraction.
tube feet (podia)
Numerous small, muscular,
fluid-filled tubes projecting from body of
echinoderms; part of water-vascular system;
used in locomotion, clinging, food handling,
and respiration.
tubercle
(L. tuberculum, small
hump). Small protuberance, knob, or
swelling.
tubulin
(L. tubulus, small tube,
+ in, belonging to). Globular protein
forming the hollow cylinder of
microtubules.
tumor necrosis factor
A cytokine, the most
important source of which is macrophages,
that is a major mediator of inflammation.
tumor suppressor gene
A gene whose
products act as restraints on cell division by
triggering apoptosis, controlling transcription
of other genes, restraining progression in
phases of the cell cycle, or by other means.
tundra
(Russ. from Lapp, tundar,
hill). Terrestrial habitat zone, located
between taiga and polar regions;
characterized by absence of trees, short
growing season, and mostly frozen soil
during much of the year.
tunic
(L. tunica, tunic, coat). In tunicates, a
cuticular, cellulose-containing covering of
the body secreted by the underlying body
wall.
tympanic
(Gr. tympanon, drum).
Relating to the tympanum that separates the
outer and middle ear (eardrum).
type specimen
A specimen deposited in a
museum that formally defines the name of
the species that it represents.
typhlosole
(Gr. typhlos, blind,
+ solen, channel, pipe). A longitudinal fold
projecting into the intestine in certain
invertebrates such as the earthworm.
typology
(L. typus, image).
A classification of organisms in which
members of a taxon are perceived to share
intrinsic, essential properties, and variation
among organisms is regarded as
uninteresting and unimportant.
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