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.
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