Osmolarity
Solute concentration expressed as molarity.
Osmoregulation
Adaptations to control the water balance in organisms living in hypertonic, hypotonic, or
terrestrial environments.
Osmoregulator
An animal whose body fluids have a different osmolarity than the environment, and that
must either discharge excess water if it lives in a hypotonic environment or take in water if it
inhabits a hypertonic environment.
Osmosis
The diffusion of water across a selectively permeable membrane.
Osmotic potential
The tendency of water to move across a selectively permeable membrane into a solution; it
is determined by measuring the pressure required to stop the osmotic movement of water
into the solution.
Osmotic pressure
A measure of the tendency of a solution to take up water when separated from pure water
by a selectively permeable membrane.
Ossicle
A small bone, especially one of the three bones of the middle ear.
Osphradium
The olfactory organ of some mollusca. It is connected with the organ of respiration.
Osteichthyes
The vertebrate class of bony fishes, characterized by a skeleton reinforced by calcium
phosphate; the most abundant and diverse vertebrates.
Ostia
The water moves into the spongocoel in porifera through ostia
Ostracoderm
An extinct agnathan; a fishlike creature encased in an armor of bony plates.
Outgroup
A species or group of species that is closely related to the group of species being studied,
but clearly not as closely related as any study-group members are to each other.
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