vacuole
(L. vacuus, empty, + Fr. ole, dim.). A membrane-bound, fluid-filled
space in a cell.
valence
(L. valere, to have power).
Degree of combining power of an element
as expressed by the number of atoms of
hydrogen (or its equivalent) that the
element can hold (if negative) or displace in
a reaction (if positive). The oxidation state
of an element in a compound. The number
of electrons gained, shared, or lost by an
atom when forming a bond with one or
more other atoms.
valve
(L. valva, leaf of a double door). One of
the two shells of a typical bivalve mollusc
or brachiopod.
variation
(L. varius, various). Differences
among individuals of a group or species
that cannot be ascribed to age, sex, or
position in the life cycle.
vector
(L. a bearer, carrier, from vehere, vectum, to carry). Any agent that carries and
transmits pathogenic microorganisms from
one host to another host. Also, in molecular
biology, an agent such as bacteriophage or
plasmid that carries recombinant DNA.
veins
(vanz) (L. vena, a vein). Blood vessels
that carry blood toward the heart; in insects,
fine extensions of the tracheal system that
support the wings.
velarium
(L. velum, veil,
covering). Shelf-like extension of thesubumbrella edge in cubozoans (phylum
Cnidaria).
veliger
(L. velum, veil,
covering). Larval form of certain molluscs;
develops from the trochophore and has the
beginning of a foot, mantle, shell, and so on.
velum
(L. veil, covering). A
membrane on the subumbrella surface of
jellyfish of class Hydrozoa. Also, a ciliated
swimming organ of the veliger larva.
ventral
(L. venter, belly). Situated on
the lower or abdominal surface.
venule
(L. venula, dim. of vena,
vein). Small vessel conducting blood from
capillaries to vein; small vein of insect wing.
vermiform
(L. vermis, worm, + forma, shape). Adjective to describe any
wormlike organism; an adult (nematogen)
rhombozoan (phylum Mesozoa).
vestige
(L. vestigium, footprint). A
rudimentary organ that may have been well
developed in some ancestor or in the
embryo.
vibrissa
, pl. vibrissae (L. nostrilhair).
Stiff hairs that grow from the nostrils
or other parts of the face of many mammals
and that serve as tactile organs; "whiskers".
vicariance
(L. vicarius, a
substitute). Geographical separation of
populations, especially as imposed by
discontinuities in the physical environment
that fragmented populations that were
formerly geographically continuous.
villus
, pl. villi (L. tuft of hair). A small
fingerlike, vascular process on the wall of
the small intestine. Also one of the
branching, vascular processes on the
embryonic portion of the placenta.
virus
(L. slimy liquid, poison). A
submicroscopic noncellular particle
composed of a nucleoprotein core and a
protein shell; parasitic; will grow and
reproduce in a host cell.
viscera
(L. pl. of viscus, internal
organ). Internal organs in the body cavity.
visceral
Pertaining to viscera.
vitalism
(L. vita, life). The discredited
viewpoint that natural processes are
controlled by supernatural forces and
cannot be explained through the laws of
physics and chemistry alone, as opposed to
mechanism.
vitamin
(L. vita, life, + amine, from former
supposed chemical origin). An organic
substance required in small amounts for
normal metabolic function; must be
supplied in the diet or by intestinal flora
because the organism cannot synthesize it.
vitellaria
(L. vitellus, yolk of an
egg). Structures in many flatworms that
produce vitelline cells, that is, cells that
provide eggshell material and nutrient for
the embryo.
vitelline gland
See vitellaria
vitelline membrane
(L. vitellus, yolk of an egg). The noncellular
membrane that encloses the egg cell.
viviparity
(L. vivus, alive,
+ parere, to bring forth). Reproduction in
which eggs develop within the female
body, with nutritional aid of maternal parent
as in therian mammals, many reptiles, and
some fishes; offspring are born as juveniles.
Adj., viviparous
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