ocellus
(L. dim. of oculus, eye). A
simple eye or eyespot in many types of
invertebrates.
octomerous
(Gr. oct, eight, + meros, part). Eight parts, specifically,
symmetry based on eight.
odontophore
(Gr. odous,
tooth, + pherein, to carry). Tooth-bearing
organ in molluscs, including the radula,
radular sac, muscles, and cartilages.
olfactory
(L. olor, smell, + factus, to bring about). Pertaining to the
sense of smell.
omasum
(L. paunch). The third
compartment of the stomach of a ruminant
mammal.
ommatidium
(Gr. omma, eye, + idium, small). One of the optical units of
the compound eye of arthropods.
omnivore
(L. omnis, all, + vorare,
to devour). An animal that uses a variety of
animal and plant material in its diet.
oncogene
(Gr. onkos,
protuberance, tumor, + genos, descent).
Any of a number of genes that are
associated with neoplastic growth (cancer).
The gene in its benign state, either
inactivated or carrying on its normal role, is
a
proto-oncogene
oncomiracidium
(Gr. onkos, barb, hook, + meirakidion, youthful person). A ciliated larva of a monogenetic
trematode.
oncosphere
(Gr. onkinos, a hook, + sphaira, ball). Rounded larva common to
all cestodes, bears hooks.
ontogeny
(Gr. ontos, being, + geneia, act of being born, from genes,
born). The course of development of an
individual from egg to senescence.
oocyst
(Gr. oion, egg, + kystis,
bladder). Cyst formed around zygote of
malaria and related organisms.
oocyte
(Gr. oion, egg, + kytos,
hollow). Stage in formation of ovum, just
preceding first meiotic division (primary
oocyte) or just following first meiotic
division (secondary oocyte).
ooecium
(Gr. oion, egg, + oikos,
house, + L. ium, from). Brood pouch;
compartment for developing embryos in
ectoprocts.
oogenesis
(Gr. oion, egg, + genesis, descent). Formation,
development, and maturation of a female
gamete or ovum.
oogonium
(Gr. oion, egg, + gonos, offspring). A cell that, by
continued division, gives rise to oocytes; an
ovum in a primary follicle immediately
before the beginning of maturation.
ookinete
(Gr. oion, egg, + kinein,
to move). The motile zygote of malarial
parasites.
ootid
(Gr. oion, egg, + idion, dim.).
Stage of formation of ovum after second
meiotic division following expulsion of
second polar body.
ootype
(Gr. oion, egg, + typos,
mold). Part of oviduct in flatworms that
receives ducts from vitelline glands and
Mehlis' gland.
operculum
(L. cover). The gill
cover in bony fishes; horny plate in some
snails.
operon
A genetic unit consisting of
a cluster of genes under the control of other
genes, found in prokaryotes.
ophthalmic
(Gr. ophthalamos, an
eye). Pertaining to the eye.
opisthaptor
(Gr. opisthen,
behind, + haptein, to fasten). Posterior
attachment organ of a monogenetic
trematode.
opisthosoma
(Gr. opisthe,
behind, + soma, body). Posterior body
region in arachnids and pogonophorans.
opsonization
(Gr. opsonein, to buy victuals, to cater). The
facilitation of phagocytosis of foreign
particles by phagocytes in the blood or
tissues, mediated by antibody bound to the
particles.
organelle
(Gr. organon, tool, organ, + L. ella,
dim.). Specialized part of a cell; literally, a
small organ that performs functions
analogous to organs of multicellular
animals.
organizer
(Gr. organos,
fashioning). Area of an embryo that directs
subsequent development of other parts.
orthogenesis
A
unidirectional trend in the evolutionary
history of a lineage as revealed by the fossil
record; also, a now discredited, anti-
Darwinian evolutionary theory, popular
around 1900, postulating that genetic
momentum forced lineages to evolve in a
predestined linear direction that was
independent of external factors and often
led to decline and extinction.
osculum
(L. osculum, a little
mouth). Excurrent opening in a sponge.
osmole
Molecular weight of a solute, in grams,
divided by the number of ions or particles
into which it dissociates in solution. Adj., osmolar
osmoregulation
Maintenance of proper
internal salt and water concentrations in a
cell or in the body of a living organism,
active regulation of internal osmotic
pressure.
osmosis
(Gr. osmos, act of
pushing, impulse). The flow of solvent
(usually water) through a semipermeable
membrane.
osmotic potential
Osmotic pressure.
osmotroph
(Gr. osmos, a
thrusting, impulse, + trophe, to eat). A
heterotrophic organism that absorbs
dissolved nutrients.
osphradium
(Gr. osphradion,
small bouquet, dim. of osphra, smell). A
sense organ in aquatic snails and bivalves
that tests incoming water.
ossicles
(L. ossiculum, small bone). Small
separate pieces of echinoderm
endoskeleton. Also, tiny bones of the
middle ear of vertebrates.
osteoblast
(Gr. osteon, bone, + blastos, bud). A bone-forming cell.
osteoclast
(Gr. osteon, bone, + klan, to break). A large, multinucleate
cell that functions in bone dissolution.
osteocyte
(Gr. osteon, bone, + kytos, hollow). A bone cell that is
characteristic of adult bone, has developed
from an osteoblast, and is isolated in a
lacuna of the bone substance.
osteoderm
(Gr. osteon, bone, + derma, skin). A bony, dermal plate located
under and supporting an epidermal scale.
osteon
(Gr. bone). Unit of bone
structure; Haversian system.
osteostracans
(Gr. osteon,
bone, + ostrakon, shell). A group of
Paleozoic (Upper Silurian to Upper
Devonian) agnathans belonging to the order
Cephalaspidiformes.
ostium
(L. door). Opening.
otolith
(Gr. ous, otos, ear, + lithos,
stone). Calcareous concretions in the
membranous labyrinth of the inner ear of
lower vertebrates, or in the auditory organ
of certain invertebrates.
outgroup
In phylogenetic systematic studies, a
species or group of species closely related
to but not included within a taxon whose
phylogeny is being studied, and used to
polarize variation of characters and to root
the phylogenetic tree.
oviger
(L. ovum, egg, + gerere, to
bear). Leg that carries eggs in pycnogonids.
oviparity
(L. ovum, egg, + parere, to bring forth). Reproduction in
which eggs are released by the female;
development of offspring occurs outside the
maternal body. Adj.,
oviparous
ovipositor
(L. ovum, egg, + positor, builder, placer, + or, suffix
denoting agent or doer). In many female
insects a structure at the posterior end of
the abdomen for laying eggs.
ovoviviparity
(L. ovum,
egg, + vivere, to live, + parere, to bring
forth). Reproduction in which eggs develop
within the maternal body without additional
nourishment from the parent and hatch
within the parent, or immediately after laying.
Adj.,
ovoviviparous
ovum
(L. ovum, egg). Mature female germ cell
(egg).
oxidation
(Fr. oxider, to
oxidize, from Gr. oxys, sharp, + ation). The
loss of an electron by an atom or molecule;
sometimes addition of oxygen chemically to
a substance. Opposite of reduction, in
which an electron is accepted by an atom
or molecule.
oxidative phosphorylation
The conversion of
inorganic phosphate to energy-rich
phosphate of ATP, involving electron
transport through a respiratory chain to
molecular oxygen.
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