Blood vascular system of molluscs is

Options

A. Reduced
B. Open
C. Closed
D. Absent.

Answer

### Step-by-Step Solution:

1. **Understanding the Blood Vascular System**: The blood vascular system in animals can be classified into two main types: open and closed systems. In an open system, blood is not confined to blood vessels and instead flows freely within body cavities.

2. **Identifying the Type in Molluscs**: Molluscs possess an open circulatory system. This means that their blood (referred to as hemolymph) is not contained within vessels. Instead, it fills the body cavity.

3. **Function of the Heart**: The heart of molluscs pumps hemolymph directly into the body cavity, known as the hemocoel. This is a key feature of their circulatory system.

4. **Role of Hemolymph**: The hemolymph circulates through the hemocoel and bathes the internal organs and tissues, providing them with nutrients and oxygen while also removing waste products.

5. **Metabolic Rate Consideration**: Molluscs typically have a lower metabolic rate compared to organisms with a closed circulatory system. This is why an open circulatory system is suitable for them.

6. **Comparison with Other Groups**: Similar open circulatory systems are also found in arthropods, which further supports the classification of molluscs within the same category.

7. **Conclusion**: Based on the characteristics of the blood vascular system in molluscs, the correct answer to the question is that their blood vascular system is open.

### Final Answer:
The blood vascular system of molluscs is **open**.