External segmentation is absent but internal segmentation is present in

Options

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

Answer

To answer the question "External segmentation is absent but internal segmentation is present in," we will analyze the characteristics of the different groups within the phylum Annelida.

### Step-by-Step Solution:

1. **Understanding Annelida**: Annelida is a phylum that includes segmented worms. Organisms in this phylum can be categorized based on their segmentation.

2. **Types of Annelids**: The main groups within Annelida include:
- Polychaeta (polykeets)
- Oligochaeta (oligo ket)
- Hirudinea (hyrodenia)
- Archiannelida (archi-analids)

3. **Defining External and Internal Segmentation**:
- **External Segmentation**: This refers to the visible segmentation on the outside of the organism's body.
- **Internal Segmentation**: This refers to the segmentation that is present within the body structure but not visible externally.

4. **Analyzing Each Group**:
- **Polychaeta**: These organisms have both external and internal segmentation. They are characterized by numerous bristles (setae) and are mostly marine.
- **Oligochaeta**: This group includes earthworms, which also exhibit both external and internal segmentation.
- **Hirudinea**: Leeches, which belong to this group, have external segmentation but may not show it distinctly due to their smooth body.
- **Archiannelida**: This group is characterized by the absence of external segmentation while still having internal segmentation.

5. **Conclusion**: Based on the analysis, the group that has external segmentation absent but internal segmentation present is **Archiannelida**.

### Final Answer:
**External segmentation is absent but internal segmentation is present in Archiannelida.**

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