Ob. Denoting against.

Obdiplostemouous. (Of stamens), in two whorls, outer opposite petals (instead of alternating with petals, the general condition), inner opposite, e.g. stitchwort.

Obligate. Being able to live only parasitically.

Occipital condyle. A single or paired bony knob at back of skull articulating with first vertebra. See Condyle Atlas. Absent in most fish, whose skull is not movable on vertebral column. Double in Amphibia and mammals single in reptiles and birds.

Occulomotor nerve. Third cranial nerve of vertebrates. Almost entirely motor, supplying four of the extrinsic eye muscles (the inferior oblique muscle, and all rectus muscles except external)

Oceanic. Inhabiting sea where it is deeper than 200 meters of Neritic.

Ocellus. Simple light-receptor occurring in amny invertebrates

Oestrogen. One of a group of female sex hormones, produced mainly by the ovaries, that promote the onset of secondary sexual characteristics (such as breast enlargement and development in women) and control thoe oestrous cycle. Oestradiol is most important. Oestrogens are secrted at particulary high levels during ovulation stimulating the uterus to prepare for pregnancy. They are used in oral contraceptives (with progestogens) and as treatment for various disorders of the female reproductive organs. Small amounts of oestrogens are produced by the adrenal glands and tests.