In which of the following animal the notochord is replaced by bony vertebral column in the adult ?

Options

A. Ascidia
B. Branchiostoma
C. Petromyzon
D. Labeo

Answer

To answer the question "In which of the following animals is the notochord replaced by a bony vertebral column in the adult?" we will analyze the given options: Brachiostoma, Petromyza, and Labio.

### Step-by-Step Solution:

1. **Understanding Notochord and Vertebral Column**:
- The notochord is a flexible rod-like structure found in the embryos of all chordates. In many chordates, it is replaced by a bony vertebral column (spine) during development.

2. **Identifying the Groups**:
- Chordates are divided into three subphyla:
- Urochordata (e.g., Ascidia)
- Cephalochordata (e.g., Brachiostoma)
- Vertebrata (e.g., Labio and Petromyza)

3. **Analyzing Each Option**:
- **Brachiostoma**: This organism belongs to the subphylum Cephalochordata. In this group, the notochord persists throughout life and is not replaced by a vertebral column.
- **Petromyza (Lamprey)**: This is a member of the subphylum Vertebrata. However, in adult lampreys, the notochord is retained and does not develop into a bony vertebral column.
- **Labio (Rohu Fish)**: This fish is also part of the subphylum Vertebrata. In rohu fish, the notochord is replaced by a bony vertebral column during development.

4. **Conclusion**:
- The correct answer is **Labio (Rohu Fish)**, as it is the only option among the three where the notochord is replaced by a bony vertebral column in the adult stage.

### Final Answer:
**Labio (Rohu Fish)**

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