In vertebrates, metamerism is

Options

A. Both external and internal
B. External only
C. Internal only
D. External in certain regions and internal in others.

Answer

To answer the question "In vertebrates, metamerism is," we need to analyze the concept of metamerism and its occurrence in vertebrates, particularly mammals.

### Step-by-Step Solution:

1. **Understanding Metamerism**:
- Metamerism refers to the segmentation of the body into repeated units or segments. These segments are similar in structure and can be observed both externally and internally in some organisms.

2. **Occurrence in Invertebrates**:
- Metamerism is prominently observed in lower invertebrates, especially in the phylum Annelida (e.g., earthworms), where segmentation is clearly visible both on the outside (external) and the inside (internal) of the body.

3. **Metamerism in Vertebrates**:
- Vertebrates, including mammals, belong to the phylum Chordata. While they exhibit some form of metamerism, it is important to distinguish how it manifests in these organisms.

4. **External vs. Internal Metamerism in Mammals**:
- In mammals, including humans, external metamerism (visible segmentation on the body surface) is largely absent. However, internal metamerism can be observed in the arrangement of muscles, nerves, and other internal structures.
- For example, the muscles are segmented, and the nervous system is organized into segments (e.g., spinal nerves, cranial nerves).

5. **Conclusion**:
- Based on the analysis, in vertebrates, metamerism is primarily internal. Therefore, the correct answer to the question is "internal only."

### Final Answer:
In vertebrates, metamerism is **internal only**.

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