Which of the following statements is incorrect regarding circulatory system of fishes?

Options

A. Fishes have two chambered heart with an atrium and a ventricle
B. Only deoxygenated blood is pumped through their heart
C. Mixing of oxygenated and deoxygenated blood occurs in ventricle
D. Oxygenation of blood occurs at gills

Answer

To determine which statement is incorrect regarding the circulatory system of fishes, we will analyze each statement based on the known characteristics of fish physiology.

### Step-by-Step Solution:

1. **Understanding the Circulatory System of Fishes**:
- Fishes possess a two-chambered heart, consisting of one atrium and one ventricle. This is the most primitive type of circulatory system among vertebrates.

2. **Evaluating the Statements**:
- **Statement 1**: "Fishes have a two-chambered heart with an atrium and ventricle."
- This statement is **true**. Fishes indeed have a two-chambered heart.

- **Statement 2**: "Only deoxygenated blood is pumped through the heart."
- This statement is also **true**. In fishes, the heart pumps deoxygenated blood from the body to the gills for oxygenation.

- **Statement 3**: "Mixing of oxygenated and deoxygenated blood occurs in ventricles."
- This statement is **false**. In fishes, there is no mixing of oxygenated and deoxygenated blood in the ventricle because there is only one ventricle. Mixing occurs in the hearts of amphibians, which have three chambers (two atria and one ventricle).

- **Statement 4**: "Oxygenation of blood occurs in gills."
- This statement is **true**. The gills are responsible for oxygenating the deoxygenated blood that comes from the heart.

3. **Conclusion**:
- The incorrect statement regarding the circulatory system of fishes is **Statement 3**: "Mixing of oxygenated and deoxygenated blood occurs in ventricles."

### Final Answer:
The incorrect statement is: **Mixing of oxygenated and deoxygenated blood occurs in ventricles.**

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