Circulatory system evolved for the first time in

Options

A. Platyhelminthes
B. Aschelminthes
C. Arthropoda
D. Annelida

Answer

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks about the first group of animals in which a circulatory system evolved. We need to analyze the options provided.

2. **Analyzing the Options**:
- **Option 1: Platyhelminthes (Flatworms)**: These organisms do not have a circulatory system. They rely on diffusion for the exchange of gases and nutrients.
- **Option 2: Aschelminthes (Roundworms)**: Similar to flatworms, roundworms also lack a true circulatory system. They exchange gases through diffusion.
- **Option 3: Arthropoda (Insects and other arthropods)**: Arthropods possess an open circulatory system where blood (hemolymph) is not always contained within blood vessels.
- **Option 4: Annelida (Earthworms)**: Annelids have a closed circulatory system with a network of blood vessels, including a dorsal and ventral blood vessel.

3. **Identifying the Correct Answer**: Based on the analysis:
- Platyhelminthes and Aschelminthes do not have a circulatory system.
- Arthropoda has an open circulatory system, but it is not the first evolution of a circulatory system.
- Annelida has a closed circulatory system, which is more advanced and represents the first true circulatory system.

4. **Conclusion**: The first true circulatory system evolved in Annelida. Therefore, the correct answer is **Option 4: Annelida**.

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