Which one of the following statements is incorrect ?

Options

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

Answer

To determine which statement is incorrect, let's analyze each statement one by one:

1. **Statement 1**: "The principle of counter current flow facilitates efficient respiration in gills of fishes."
- **Analysis**: This statement is correct. The counter current mechanism in fish gills allows for more efficient gas exchange by maintaining a gradient for oxygen diffusion.

2. **Statement 2**: "The residual air in lungs slightly decreases the efficiency of respiration in mammals."
- **Analysis**: This statement is also correct. The residual air (the air that remains in the lungs after exhalation) does reduce the efficiency of gas exchange since it mixes with fresh air, thereby lowering the overall oxygen concentration available for exchange.

3. **Statement 3**: "The presence of non-respiratory air sacs increases the efficiency of respiration in birds."
- **Analysis**: This statement is correct. Birds have air sacs that help in maintaining a continuous flow of air through the lungs, which enhances their respiratory efficiency.

4. **Statement 4**: "Circulating body fluids serve to distribute oxygen to tissues in insects."
- **Analysis**: This statement is incorrect. Insects do not have hemoglobin or similar pigments in their body fluids to transport oxygen. Instead, they rely on a tracheal system that directly delivers oxygen to tissues.

**Conclusion**: The incorrect statement is the fourth one regarding the distribution of oxygen in insects.