The sequence of the one-way gaseous exchange mechanism in a fish is

The sequence of the one-way gaseous exchange mechanism in a fish is: mouth → gills → operculum. Fish breathe by taking in water through their mouth and passing it over their gills. The gills are the organs responsible for extracting oxygen from the water and releasing carbon dioxide. Once the water passes over the gills, it is expelled through an opening on the side of the fish's body called the operculum. The operculum helps to regulate the flow of water over the gills and also protects them from damage. Therefore, the correct sequence of the one-way gaseous exchange mechanism in a fish is mouth → gills → operculum.