In earthworm the dorsal surface of the body is marked by a dark median mid-dorsal line called-

Answer

**Step-by-Step Solution:**

1. **Identify the Structure**: The question asks about a specific feature on the dorsal surface of the earthworm's body.

2. **Understand the Orientation**: The dorsal surface refers to the upper side of the earthworm, which is typically darker in color compared to the ventral side.

3. **Recognize the Line**: There is a notable feature on the dorsal surface, described as a dark median mid-dorsal line.

4. **Name of the Line**: This dark median mid-dorsal line is known as the **dorsal blood vessel**.

5. **Function and Importance**: The dorsal blood vessel is significant as it is the largest blood vessel in the vascular system of the earthworm and runs along the length of the body, contributing to the circulatory system.

6. **Conclusion**: Therefore, the correct answer to the question is **dorsal blood vessel**.

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