O cel' lus, a simple eye.
Om niv' o rous, eating or living upon food of all kinds.
O per' cu lum, a lid or cover.
O' ral, pertaining to the mouth.
Oral groove, a funnel-shaped groove in one side of some one-celled animals conducting food to the mouth.
Or' der, a term in classification used to designate a group of genera.
Or thop' ter a, the order to which locusts, grasshoppers, etc., belong.
Os' cu lum (pl. oscula), the large opening from the central cavity in sponges.
Os mo' sis, the process by which fluids of different densities become equally diffused when separated by an organic membrane or by a porous structure.
Os' ti a (sing. ostium), the inner ends of the canals in the body-walls of sponges.