two external openings on the snout through which air enters the nasal cavity
As in humans, birds have paired, external openings to their respiratory systems known as nostrils (or nares). They open into passages in the skull and ultimately lead to the windpipe (trachea). Purple Martins have tiny parasites, known as nasal mites, that live and crawl around in their nostrils.
Sharks and rays have paired nostrils on the underside of the snout. Water continually flows through the nostrils, giving the shark olfactory (the sense of smell) information. Unlike humans, shark nostrils have nothing to do with breathing - they are not even connected to the mouth.