Blood vascular system in first eumetazoan is

Options

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

Answer

**Step-by-Step Solution:**

1. **Understanding Eumetazoans**: Eumetazoans are a clade of animals that include all major groups of animals except for sponges and placozoans. They are characterized by having true tissues and a more complex body structure.

2. **Types of Blood Vascular Systems**: There are different types of blood vascular systems in animals:
- **Open Circulatory System**: Blood is not confined to vessels; it flows freely through cavities and directly bathes the organs. This system is common in invertebrates like mollusks and insects.
- **Closed Circulatory System**: Blood is contained within vessels (arteries and veins) and is pumped by a heart. This system is found in more complex organisms, such as earthworms and vertebrates.
- **Absence of Circulatory System**: Some primitive eumetazoans do not have a circulatory system at all.
- **Reduced Circulatory System**: This is a less efficient system where circulation is diminished and not well developed, often found in certain invertebrates.

3. **Identifying the Blood Vascular System in the First Eumetazoans**: The question asks about the blood vascular system in the first eumetazoans. According to the information provided, the earliest eumetazoans did not possess a true circulatory system, indicating that the correct answer is that the blood vascular system was absent.

4. **Conclusion**: Therefore, the blood vascular system in the first eumetazoans is best described as "absent."

**Final Answer**: The blood vascular system in the first eumetazoans is absent.

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