Nostrils are paired nasal openings located in front of the eyes and not communicating with the pharynx. In most fish are divided by a septum into anterior and posterior nostril. Location, shape and size of the nasal openings depend on the lifestyle of the fish.
At the anterior end of the terminal brain in fish are the olfactory bulbs, which receive signals from olfactory receptors. Each fish has nostrils leading into blindly closed olfactory chambers, the walls of which are covered with these receptors. Unlike other vertebrates, the nostrils of fish do not connect to the pharynx (the exception is the double-breathing fish). Many fish, especially predators, species with crepuscular activity, species living in the dark or in murky water, have a very fine sense of smell.