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. Water moves in one direction only, entering at one end and leaving at the other.

Concept: Fish extract dissolved oxygen from water using gills. The flow works like this:

1. Mouth: The fish opens its mouth and takes in water. The floor of the mouth cavity lowers, and the operculum (gill cover) stays closed.
2. Gills: The mouth closes and the mouth floor rises, pushing water over the gills. The gill filaments and lamellae are rich in blood capillaries. Oxygen diffuses from the water into the blood, and carbon dioxide diffuses out of the blood into the water.
3. Operculum: The operculum opens and the water, now low in oxygen, exits through it.

Step-by-step check of the options:

• A. operculum → gills → mouth: This is the reverse direction, so it is wrong.
• B. gills → operculum → mouth: Water cannot enter at the gills, so this is wrong.
• C. mouth → operculum → gills: The water would skip the gills before leaving, so this is wrong.
• D. mouth → gills → operculum: This matches the actual flow, so it is correct.

Answer: D. mouth → gills → operculum

Tip: Remember the flow as "Mouth-Gills-Out." Water goes in at the mouth, passes over the gills where exchange happens, and leaves through the operculum. The gills are always in the middle because they are the site of exchange.