Operculum.
(1) (in zoology) A lid or flap of skin covering an aperture, such as the gill slit cover of fish and larval amphibians and the horny calcareous operculum secreted by many gastropod molluscs, which closes the opening of the shell when the animal is side.
(2) (in botany) the cone-shaped lid of the capsule of mosses, which is forcibly detached to release the spores.

Opiate. A narcotic substance derived from opium.

Optic. Of or relating to the eye or to vision.

Oral. Of mouth.

Orbit. Cavity or depression in the skull of vertebrates eyeball.

Order. One of the kinds of group used in classifying organisms. Consists of number of similar families (sometimes of only one family). Similar orders are grouped into a class. ‘Natural orders’ of flowering plants are equivalent to Families.

Organism. Any living animal or plant.

Organizer.
(1) Dorsal lip of blastopore of amphibian embryo, consisting of material which will become notochord and sosmites; and corresponding region of other vertebrate embryos. When transplanted to blastula of same or relate species, performs a complex of inductions as a result of which the transplanted organizer and the surrounding host tissues develop into a complete embryonic axis. The best known amongst inductions is the evocation of neural plate.
(2) Any part of an embryo which performs an induction on the other part.

Organothroph. An organism that uses organic compounds as electron donors.