External segmentation is absent but internal segmentation is present in

Options

A. Polychaeta
B. Oligochaeta
C. Archiannelida
D. Hirudinea

Answer

To solve the question "External segmentation is absent but internal segmentation is present in," we will analyze the provided options step by step.

### Step 1: Understand the Terms
- **Segmentation** refers to the division of an organism's body into repetitive segments.
- **External Segmentation** is visible on the outside of the organism.
- **Internal Segmentation** refers to divisions within the organism that are not visible externally.

### Step 2: Analyze Each Option
1. **Polychaete**:
- Polychaetes are characterized by having external segmentation. They have visible segments on their body.
- **Conclusion**: This option is not correct.

2. **Oligochaete**:
- Oligochaetes, like earthworms, also exhibit external segmentation. They have visible segments on their body.
- **Conclusion**: This option is not correct.

3. **Archaeonelida**:
- Archaeonelida is known for having internal segmentation only. There are no visible external segments.
- **Conclusion**: This option is a strong candidate for the correct answer.

4. **Hirudinae**:
- Hirudinae, or leeches, also show external segmentation. They have visible segments on their body.
- **Conclusion**: This option is not correct.

### Step 3: Final Conclusion
Based on the analysis, the correct answer is **Archaeonelida**, as it is the only group among the options that has internal segmentation without external segmentation.

### Answer:
**C. Archaeonelida**

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