Which one of the following statement is incorrect ?

Options

A. The principle of countercurrent flow facilitates efficient respiration in the gills of fishes .
B. The residual air in lungs slightly decreases the efficiency of respiration in mammlas.
C. The presence of non- respiratory air sacs increase the efficiency of respiration in birds.
D. In insects, circulating body fluids serves to distribute oxygen to tissues.

Answer

To determine which statement is incorrect, let's analyze each option step by step.

### Step 1: Analyze the First Statement
**Statement:** The principle of counter current flow facilitates efficient respiration in the gills of fishes.

**Explanation:** The counter current flow mechanism allows blood to flow in the opposite direction to the water passing over the gills. This arrangement maximizes the oxygen absorption from water into the blood, making this statement **correct**.

### Step 2: Analyze the Second Statement
**Statement:** Residual air in lungs slightly decreases the efficiency of respiration in mammals.

**Explanation:** Residual air is the air that remains in the lungs after a forceful exhalation. This air does not participate in gas exchange and does not decrease the efficiency of respiration. Therefore, this statement is **incorrect**.

### Step 3: Analyze the Third Statement
**Statement:** The presence of non-respiratory air sacs increases the efficiency of respiration in birds.

**Explanation:** Birds have air sacs that help in the continuous flow of air through the lungs, enhancing oxygen exchange. This statement is **correct**.

### Step 4: Analyze the Fourth Statement
**Statement:** In insects, circulating body fluid serves to distribute oxygen to tissues.

**Explanation:** Insects use hemolymph (their circulating fluid) to distribute oxygen, although they primarily rely on a tracheal system for gas exchange. This statement is **correct**.

### Conclusion
After analyzing all statements, the **incorrect statement** is:
- **Residual air in lungs slightly decreases the efficiency of respiration in mammals.**