Which one of the following statements is incorrect

Options

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

Answer

To determine which statement is incorrect, we will analyze each of the four statements provided.

1. **Statement 1**: "The principles of counter current flow facilitate efficient respiration in gills of fishes."
- This statement is **true**. The counter current exchange mechanism in fish gills allows for efficient oxygen extraction from water, as water flows in one direction while blood flows in the opposite direction, maximizing oxygen uptake.

2. **Statement 2**: "In insects, circulatory body fluid serves to distribute oxygen to tissues."
- This statement is also **true**. Insects have a tracheal system that delivers oxygen directly to tissues, and the circulatory fluid (hemolymph) helps in the distribution of nutrients and waste products, but it is not primarily responsible for oxygen transport.

3. **Statement 3**: "The presence of non-respiratory air sacs increases the efficacy of respiration in worms."
- This statement is **true**. Non-respiratory air sacs in some organisms can help in gas exchange and improve the efficiency of respiration.

4. **Statement 4**: "The residual air in lungs slightly decreases the efficacy of respiration in mammals."
- This statement is **incorrect**. Residual air (the air that remains in the lungs after exhalation) actually helps to keep the alveoli open and prevents them from collapsing. It does not decrease the efficacy of respiration; rather, it aids in maintaining a surface area for gas exchange.

Based on the analysis, the incorrect statement is **Statement 4**.

### Summary of the Solution:
- Statement 1: True
- Statement 2: True
- Statement 3: True
- Statement 4: **Incorrect** (Residual air aids in maintaining lung function)