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

Analyze the question: It asks for the order in which water flows through a fish for gaseous exchange (a one-way flow).

Concept: Fish extract dissolved oxygen from water using gills. Water flows in one direction only:

1. Mouth: The fish opens its mouth, the floor of the buccal cavity lowers, and water is drawn in.
2. Gills: The mouth closes and the buccal cavity contracts, forcing water over the gill filaments (lamellae). Here oxygen diffuses into the blood capillaries and carbon dioxide diffuses out into the water.
3. Operculum: The water then pushes out under the operculum (gill cover), which opens to let it exit.

Step-by-step check of the options:

• A. gills → operculum → mouth: water would exit through the mouth, which is wrong.
• B. mouth → gills → operculum: water enters at the mouth, passes over the gills, and leaves via the operculum. This is correct.
• C. mouth → operculum → gills: water would pass the gill cover before reaching the gills, which is impossible, since the operculum covers the gills.
• D. operculum → mouth → gills: water would enter at the operculum, which is the exit.

**Answer: B.

Tip: Remember "M-G-O" (Mouth, Gills, Out through the Operculum). Water goes in at the front and leaves at the side of the head, passing the gills in between.