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 circle of organs in a flower. They are usually of delicate texture and brightly coloured.
PHYLLODINEOUS.--Having flattened, leaf-like twigs or leafstalks instead of true leaves.
PIGMENT.--The colouring material produced generally in the superficial parts of animals. The cells secreting it are called PIGMENT-CELLS.
PINNATE.--Bearing leaflets on each side of a central stalk.
PISTILS.--The female organs of a flower, which occupy a position in the centre of the other floral organs. The pistil is generally divisible into the ovary or germen, the style and the stigma.
PLACENTALIA, PLACENTATA.--or PLACENTAL MAMMALS, See MAMMALIA.
PLANTIGRADES.--Quadrupeds which walk upon the whole sole of the foot, like the bears.
PLASTIC.--Readily capable of change.
PLEISTOCENE PERIOD.--The latest portion of the Tertiary epoch.
PLUMULE (in plants).--The minute bud between the seed-leaves of newly-germinated plants.
PLUTONIC ROCKS.--Rocks supposed to have been produced by igneous action in the depths of the earth.
POLLEN.--The male element in flowering plants; usually a fine dust produced by the anthers, which, by contact with the stigma effects the fecundation of the seeds. This impregnation is brought about by means of tubes (POLLEN-TUBES) which issue from the pollen-grains adhering to the stigma, and penetrate through the tissues until they reach the ovary.
POLYANDROUS (flowers).--Flowers having many stamens.
POLYGAMOUS PLANTS.--Plants in which some flowers are unisexual and others hermaphrodite. The unisexual (male and female) flowers, may be on the same or on different plants.
POLYMORPHIC.--Presenting many forms.
POLYZOARY.--The common structure formed by the cells of the Polyzoa, such as the well-known seamats.
PREHENSILE.--Capable of grasping.
PREPOTENT.--Having a superiority of power.
PRIMARIES.--The feathers forming the tip of the wing of a bird, and inserted upon that part which represents the hand of man.
PROCESSES.--Projecting portions of bones, usually for the attachment of muscles, ligaments, etc.
PROPOLIS.--A resinous material collected by the hivebees from the opening buds of various trees.
PROTEAN.--Exceedingly variable.
PROTOZOA.--The lowest great division of the animal kingdom. These animals are composed of a gelatinous material, and show scarcely any trace of distinct organs. The Infusoria, Foraminifera, and sponges, with some other forms, belong to this division.
PUPA (pl. PUPAE).--The second stage in the development of an insect, from which it emerges in the perfect (winged) reproductive form. In most insects the PUPAL STAGE is passed in perfect repose. The CHRYSALIS is the pupal state of butterflies.