Which one of the following statements is incorrect ?

Options

A. The principal of countercurrent flow facilitates efficient respiration in gills of fishes.
B. The residual air in lungs slightly decreases the efficiency of repiration in mammals.
C. The presence of non-respiratory air sacs, increases the efficiency of respiration in birds.
D. in insects, circulating body fluids serve to distribute oxygen to tissues.

Explanation

To determine which statement is incorrect, we will analyze each statement provided in the question step by step.

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

- **Explanation:** The counter current flow mechanism in fish gills allows for efficient gas exchange. In this process, water flows over the gills in one direction while blood flows in the opposite direction. This arrangement maximizes the oxygen uptake from the water into the blood.
- **Conclusion:** This statement is **correct**.

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

- **Explanation:** Residual air refers to the air that remains in the lungs after a forceful exhalation. This air does not participate in gas exchange and does not affect the efficiency of respiration. Therefore, it does not decrease the efficiency of respiration.
- **Conclusion:** 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 allow for a unidirectional flow of air through their lungs, enhancing the efficiency of gas exchange. These air sacs are essential for maintaining a constant supply of fresh air in the lungs.
- **Conclusion:** This statement is **correct**.

### Step 4: Analyze the Fourth Statement
**Statement:** In insects, circulating blood fluids serve to distribute oxygen to the tissues.

- **Explanation:** Insects have a circulatory fluid known as hemolymph, which helps in transporting oxygen to tissues. This is part of their unique respiratory system, which does not rely solely on blood for oxygen transport.
- **Conclusion:** This statement is **correct**.

### Final Conclusion
After analyzing all the statements, we find that the **incorrect statement** is:
- The residual air in the lungs slightly decreases the efficiency of respiration in mammals.