NICTITATING MEMBRANE.--A semi-transparent membrane, which can be drawn
across the eye in birds and reptiles, either to moderate the effects of
a strong light or to sweep particles of dust, etc., from the surface of
the eye.

OCELLI.--The simple eyes or stemmata of insects, usually situated on the
crown of the head between the great compound eyes.

OESOPHAGUS.--The gullet.

OOLITIC.--A great series of secondary rocks, so called from the texture
of some of its members, which appear to be made up of a mass of small
EGG-LIKE calcareous bodies.

OPERCULUM.--A calcareous plate employed by many Molluscae to close the
aperture of their shell. The OPERCULAR VALVES of Cirripedes are those
which close the aperture of the shell.

ORBIT.--The bony cavity for the reception of the eye.

ORGANISM.--An organised being, whether plant or animal.

ORTHOSPERMOUS.--A term applied to those fruits of the Umbelliferae which
have the seed straight.

OSCULANT.--Forms or groups apparently intermediate between and
connecting other groups are said to be osculant.

OVA.--Eggs.

OVARIUM or OVARY (in plants).--The lower part of the pistil or female
organ of the flower, containing the ovules or incipient seeds; by growth
after the other organs of the flower have fallen, it usually becomes
converted into the fruit.

OVIGEROUS.--Egg-bearing.

OVULES (of plants).--The seeds in the earliest condition.

PACHYDERMS.--A group of Mammalia, so called from their thick skins, and
including the elephant, rhinoceros, hippopotamus, etc.

PALAEOZOIC.--The oldest system of fossiliferous rocks.

PALPI.--Jointed appendages to some of the organs of the mouth in insects
and Crustacea.

PAPILIONACEAE.--An order of plants (see LEGUMINOSAE), The flowers of
these plants are called PAPILIONACEOUS, or butterfly-like, from the
fancied resemblance of the expanded superior petals to the wings of a
butterfly.

PARASITE.--An animal or plant living upon or in, and at the expense of,
another organism.

PARTHENOGENESIS.--The production of living organisms from unimpregnated
eggs or seeds.

PEDUNCULATED.--Supported upon a stem or stalk. The pedunculated oak has
its acorns borne upon a footstool.

PELORIA or PELORISM.--The appearance of regularity of structure in the
flowers of plants which normally bear irregular flowers.

PELVIS.--The bony arch to which the hind limbs of vertebrate animals are
articulated.

PETALS.--The leaves of the corolla, or second ci