the nostril, or one of the nostrils, of whales, porpoises, and allied animals.-one of the external openings communicating with the air tubes or tracheae of insects, myriapods, and arachnids. they are variable in number, and are usually situated on the sides of the thorax and abdomen, a pair to a segment. these openings are usually elliptical, and capable of being closed. see illust. under coleoptera.-a tubular orifice communicating with the gill cavity of certain ganoid and all elasmobranch fishes. it is the modified first gill cleft.-any small aperture or vent for air or other fluid.