MYSIS-STAGE.—A stage in the development of certain crustaceans (prawns), in which they closely resemble the adults of a genus (Mysis) belonging to a slightly lower group.
NATATORY.—Adapted for the purpose of swimming.
NAUPLIUS-FORM.—The earliest stage in the development of many Crustacea, especially belonging to the lower groups. In this stage the animal has a short body, with indistinct indications of a division into segments, and three pairs of fringed limbs. This form of the common fresh-water Cyclops was described as a distinct genus under the name of Nauplius.
NEURATION.—The arrangement of the veins or nervures in the wings of insects.
NEUTERS.—Imperfectly developed females of certain social insects (such as ants and bees), which perform all the labours of the community. Hence, they are also called workers.
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, &c., 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.
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 Molluscæ 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 Umbelliferæ which have the seed straight.
OSCULANT.—Forms or groups apparently intermediate between and connecting other groups are said to be osculant.
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.
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, &c.
PALÆOZOIC.—The oldest system of fossiliferous rocks.
PALPI.—Jointed appendages to some of the organs of the mouth in insects and Crustacea.