A structure peculiar to Nautilus is

Options

A. Siphuncle/living tube
B. External shell
C. Spirally coiled shell
D. Nacturnal nature

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks for a structure that is unique or peculiar to the Nautilus, which is a marine mollusk known for its distinctive shell.

2. **Identifying the Options**: The options provided are:
- Sphinacle
- Living tube
- External shell
- Spirally coiled shell
- Nocturnal nature

3. **Analyzing Each Option**:
- **Sphinacle**: This term is not commonly associated with Nautilus specifically.
- **Living tube**: This is a specialized structure found in Nautilus that runs through its spiraled shell and is used for expelling water and gases.
- **External shell**: While Nautilus has an external shell, this feature is not unique to it as other mollusks also possess external shells.
- **Spirally coiled shell**: This is a characteristic of Nautilus, but again, other mollusks may also have spirally coiled shells.
- **Nocturnal nature**: This refers to the behavior of being active at night, which is not a structural feature.

4. **Conclusion**: The structure that is peculiar to Nautilus is the **living tube**. This tube is essential for the Nautilus as it aids in buoyancy and movement by allowing the organism to expel water and gases, thereby adjusting its density.

5. **Final Answer**: The structure peculiar to Nautilus is the **living tube**.