Nudibranchiate (L. nudus, naked; branchiæ, gills). An order of gasteropods, in which the gills are exposed, as the Eolis, Doris, &c.
Oxygen. A gas which is one of the constituent parts of water and of atmospheric air. It is essential to animal life.
Oviparous (L. ovum, an egg; pario, I bring forth). The animals which bring forth eggs.
Operculum (from the Latin for lid). Applied to the horny or shelly plate which closes certain univalve shells, as the Whelk, Periwinkle, &c.
Papillæ (L. papilla, a nipple). Soft prominences which resemble in form the teats of animals.
Palpi (L. palpo, I touch). The organs of touch commonly called 'feelers,' developed from the lablum and maxillæ of insects.
Pectinated (L. pecten, a comb). Toothed like a comb.
Physograde (Gr. physis, air; gradior, I advance). The acalephes that swim by means of air-bladders.
Phytophagous (Gr. phuton, a plant; phago, I eat). Plant-eating animals.
Pulmonigrade (L. pulmo, a lung; gradior, I walk). The tribe of Medusæ which swim by contraction of the respiratory disc.