Dorsal surface of earthworm is recognised by

Options

A. Genital apertures
B. Setae
C. Dark mid - dorsal line
D. Cuticle

Answer

To determine how the dorsal surface of an earthworm is recognized, we will analyze the provided options step by step.

### Step-by-Step Solution:

1. **Understanding the Options**:
- The options given are:
a) Genital apertures
b) CT (setae)
c) Dark mid dorsal line
d) Cuticle

2. **Evaluating Option A: Genital Apertures**:
- Genital apertures are openings that are located on the ventral (belly) side of the earthworm, specifically on the ventral lateral side. Since they are not found on the dorsal side, this option cannot be correct.

3. **Evaluating Option B: CT (Setae)**:
- CT, or setae, are small bristle-like structures that aid in locomotion. They protrude from the body during movement but are not a distinguishing feature of the dorsal surface. Therefore, this option is also not correct.

4. **Evaluating Option C: Dark Mid Dorsal Line**:
- The dark mid dorsal line is a prominent feature on the dorsal side of the earthworm. It runs along the length of the body, excluding the first segment (where the mouth is located) and the last segment (the anal segment). This line is a clear indicator of the dorsal surface and is used to identify it.

5. **Evaluating Option D: Cuticle**:
- The cuticle is the outer layer of the earthworm's body, serving as an exoskeleton. While it covers the entire body, it does not specifically identify the dorsal surface. Thus, this option is not the correct answer.

6. **Conclusion**:
- After evaluating all options, the correct answer is the **dark mid dorsal line**, as it is the feature that specifically helps in recognizing the dorsal surface of the earthworm.

### Final Answer:
The dorsal surface of the earthworm is recognized by the **dark mid dorsal line**.