Which is not a vestigial organ in man?

Options

A. Nictitating membrane
B. Tail vertebrae
C. Vermiform appendix
D. Nails

Explanation

To determine which organ is not a vestigial organ in humans, we need to understand the concept of vestigial organs and analyze the options provided.

### Step-by-Step Solution:

1. **Understanding Vestigial Organs**:
- Vestigial organs are remnants of structures that were functional in our ancestors but have lost their original function over time due to evolutionary changes. They are present in the body but serve little to no purpose in the current species.

2. **Identifying Examples of Vestigial Organs in Humans**:
- **Nictitating Membrane**: This is a small fold of tissue in the corner of the eye. In some animals, it helps protect and moisten the eye. In humans, it is non-functional and considered vestigial.
- **Tail Vertebrae (Coccyx)**: Humans have a coccyx, which is the remnant of a tail. It no longer serves the purpose of balance or mobility as it does in other animals, making it vestigial.
- **Vermiform Appendix**: This is a small, tube-like structure attached to the large intestine. While it may have some minor roles in gut flora, it is largely considered vestigial as it does not play a significant role in digestion.

3. **Analyzing the Options**:
- Given the examples above, all three (nictitating membrane, tail vertebrae, and vermiform appendix) are vestigial organs.
- The question asks for an organ that is **not** vestigial.

4. **Identifying the Non-Vestigial Organ**:
- **Nails**: Unlike the other options, nails are functional. They protect the tips of our fingers and toes and assist in various tasks, such as gripping and manipulating objects. Therefore, nails are not vestigial.

5. **Conclusion**:
- The organ that is **not a vestigial organ in humans** is **nails**.

### Final Answer:
Nails are not a vestigial organ in man.