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.