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 during gaseous exchange (breathing). Water moves in one direction only.

Concept: A bony fish breathes like this:

1. The fish opens its mouth and takes in water. The floor of the mouth cavity is lowered, and the operculum closes.
2. The mouth closes, and the mouth cavity contracts, forcing water backwards over the gills.
3. At the gills, oxygen dissolved in the water diffuses into the blood capillaries in the gill filaments. Carbon dioxide diffuses out into the water.
4. The water then pushes the operculum (gill cover) open and leaves the body through it.

Solution: Water path: mouth → gills → operculum (out)

Checking the options:

• A: mouth → operculum → gills. Wrong, because water cannot pass the operculum before the gills.
• B: mouth → gills → operculum. Correct.
• C: operculum → gills → mouth. Wrong, because this is the reverse direction.
• D: gills → operculum → mouth. Wrong, because water does not begin at the gills.

Answer: B

Exam tip: Remember "M-G-O" (Mouth, Gills, Operculum), or think "Water goes In at the Mouth, over the Gills, and Out through the Operculum." The operculum is always the exit.