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  Section: Zoological Terms used in General Zoology
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O - Zoological Terms

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(L. dim. of oculus, eye). A simple eye or eyespot in many types of invertebrates.


(Gr. oct, eight, + meros, part). Eight parts, specifically, symmetry based on eight.


(Gr. odous, tooth, + pherein, to carry). Tooth-bearing organ in molluscs, including the radula, radular sac, muscles, and cartilages.


(L. olor, smell, + factus, to bring about). Pertaining to the sense of smell.


(L. paunch). The third compartment of the stomach of a ruminant mammal.


(Gr. omma, eye, + idium, small). One of the optical units of the compound eye of arthropods.


(L. omnis, all, + vorare, to devour). An animal that uses a variety of animal and plant material in its diet.


(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


(Gr. onkos, barb, hook, + meirakidion, youthful person). A ciliated larva of a monogenetic trematode.


(Gr. onkinos, a hook, + sphaira, ball). Rounded larva common to all cestodes, bears hooks.


(Gr. ontos, being, + geneia, act of being born, from genes, born). The course of development of an individual from egg to senescence.


(Gr. oion, egg, + kystis, bladder). Cyst formed around zygote of malaria and related organisms.


(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).


(Gr. oion, egg, + oikos, house, + L. ium, from). Brood pouch; compartment for developing embryos in ectoprocts.


(Gr. oion, egg, + genesis, descent). Formation, development, and maturation of a female gamete or ovum.


(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.


(Gr. oion, egg, + kinein, to move). The motile zygote of malarial parasites.


(Gr. oion, egg, + idion, dim.). Stage of formation of ovum after second meiotic division following expulsion of second polar body.


(Gr. oion, egg, + typos, mold). Part of oviduct in flatworms that receives ducts from vitelline glands and Mehlis' gland.


(L. cover). The gill cover in bony fishes; horny plate in some snails.


A genetic unit consisting of a cluster of genes under the control of other genes, found in prokaryotes.


(Gr. ophthalamos, an eye). Pertaining to the eye.


(Gr. opisthen, behind, + haptein, to fasten). Posterior attachment organ of a monogenetic trematode.


(Gr. opisthe, behind, + soma, body). Posterior body region in arachnids and pogonophorans.


(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.


(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.


(Gr. organos, fashioning). Area of an embryo that directs subsequent development of other parts.


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.


(L. osculum, a little mouth). Excurrent opening in a sponge.


Molecular weight of a solute, in grams, divided by the number of ions or particles into which it dissociates in solution. Adj., osmolar


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.


(Gr. osmos, act of pushing, impulse). The flow of solvent (usually water) through a semipermeable membrane.

osmotic potential

Osmotic pressure.


(Gr. osmos, a thrusting, impulse, + trophe, to eat). A heterotrophic organism that absorbs dissolved nutrients.


(Gr. osphradion, small bouquet, dim. of osphra, smell). A sense organ in aquatic snails and bivalves that tests incoming water.


(L. ossiculum, small bone). Small separate pieces of echinoderm endoskeleton. Also, tiny bones of the middle ear of vertebrates.


(Gr. osteon, bone, + blastos, bud). A bone-forming cell.


(Gr. osteon, bone, + klan, to break). A large, multinucleate cell that functions in bone dissolution.


(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.


(Gr. osteon, bone, + derma, skin). A bony, dermal plate located under and supporting an epidermal scale.


(Gr. bone). Unit of bone structure; Haversian system.


(Gr. osteon, bone, + ostrakon, shell). A group of Paleozoic (Upper Silurian to Upper Devonian) agnathans belonging to the order Cephalaspidiformes.


(L. door). Opening.


(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.


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.


(L. ovum, egg, + gerere, to bear). Leg that carries eggs in pycnogonids.


(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.,



(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.


(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.,



(L. ovum, egg). Mature female germ cell (egg).


(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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