O

oceanic zone O-shee-AN-ik zone The open sea. 883

ocelli o-SELL-ee Eyespots; visual organs in flatworms. 644

octet rule OC-tet ROOL The tendency of an atom to fill its outermost shell. 35

olfactory epithelium ol-FAK-tore-ee ep-e-THEEL-e-um Neurons specialized to detect odors, in a small patch of tissue high in the nostrils. 646

oligochaetes OL-eh-go-keets A class of annelids that includes the earthworms. 521

oligotrophic OL-ah-go-TRO-fik A lake with few nutrients, usually very blue. 880

ommatidia o-mah-TID-ee-ah The visual units of a compound eye. 644

oncogene ON-ko-jean A gene that normally controls cell division but when overexpressed leads to cancer. 183

oocyte OO-site The female sex cell before it is fertilized. 196

oogenesis oo-GEN-eh-sis The differentiation of an egg cell from a diploid oogonium, to a primary oocyte, to two haploid secondary oocytes, to ootids, and finally, after fertilization, to a mature ovum. 204

oogonium oo-GO-nee-um The diploid cell where egg formation begins. 204, 464

open circulatory system A circulatory system in which blood is not always in vessels. 708

operant conditioning Trial-and-error learning, in which an animal voluntarily repeats any behavior that brings success. 816

operculum o-PER-cu-lum A flap of tissue that protects gills. 734

operon OP-er-on A series of genes with related functions and their controls. 327

optic nerve Nerve fibers that connect the retina to the visual cortex. 649

oral end The mouth. 511

organ A structure of two or more tissues that functions as an integrated unit. 5

organelles OR-gan-NELLZ Specialized structures in eukaryotic cells that carry out specific functions. 5, 60

organogenesis or-GAN-o-gen-eh-sis Development of organs in an embryo. 226

organotroph or-GAN-o-trofe Organism that obtains hydrogen or electrons from organic compounds. 445

orgasm OR-gazz-m A pleasurable sensation associated with sexual activity. 197

origin The end of a muscle on an immobile bone. 700

osculum OS-kew-lum A large hole at the top of a sponge that expels water and wastes. 515

osmoconformer OZ-mo-con-FORM-er An invertebrate whose ion concentrations match those in the ocean. 776

osmoregulation OZ-mo-reg-u-LAY-shun An organism's control of water and ion balance. 776

osmoregulator OZ-mo-REG-u-lay-tor An animal that controls its body fluid composition. 776

osmosis oz-MO-sis Passive diffusion of water. 88

ossicles OSS-i-kulls Calcium-containing structures in echinoderms' coverings. 523

osteoblast OSS-tee-o-blast A bone cell that secretes matrix. 595

osteoclast OSS-tee-o-clast A bone cell that degrades matrix. 595

osteocyte OSS-tee-o-site A mature bone cell in a lacuna. 595

osteon OSS-tee-on Concentric circles of osteocytes in bone. 688

osteonic canal oss-tee-ON-ik ca-NAL Portals that house blood vessels in bone. 688

osteoprogenitor cell OSS-tee-o-pro-GEN-i-tor SEL A cell lining a bone passageway that can differentiate into an osteoblast in the event of injury or growth. 595

ostia OSS-tee-a Surface pores on a sponge. 515

otolith O-toe-lith Calcium carbonate granules in the vestibule of the inner ear whose movements provide information on changes in velocity. 655

oval window A membrane between the middle ear and the inner ear. 653

ovary O-var-ee One of the paired female gonads that house developing oocytes. In a flowering plant, the carpels and the ovules they enclose. 197, 563

ovulation OV-u-LAY-shun The release of an oocyte from the largest ovarian follicle just after luteinizing hormone peaks in the blood in the middle of a menstrual cycle. 674

ovule OV-yul In flowering plants, a megasporangium that contains a megaspore mother cell. 563

oxidation OX-e-DAY-shun A chemical reaction that loses electrons. 117

oxidative phosphorylation ox-i-DAY-tiv fos-for-e-LAY-shun Phosphorylation of ADP to ATP coupled to protons moving down their concentration gradient across the inner mitochondrial membrane. 129

oxyhemoglobin ox-ee-HEEM-o-GLO-bin Bright red hemoglobin that has just picked up oxygen in the lungs. 715

oxytocin ox-ee-TOE-sin A hormone made in the hypothalamus and released from the posterior pituitary that stimulates muscle contraction in the mammary glands and the uterus. 664

ozone O-zone A bluish gas that forms high in the atmosphere when oxygen reacts with UV radiation. Consists of three oxygen atoms. 894

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