Most of vertebrates can maintain a constant internal osmolarity different from the surrounding medium, except

Options

A. Myxine
B. Sharks
C. Bony fishes
D. Both (1) & (2)

Answer

To solve the question regarding which vertebrates cannot maintain a constant internal osmolarity different from the surrounding medium, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Osmolarity**:
- Osmolarity refers to the concentration of solutes in a solution, which affects the movement of water across cell membranes. Vertebrates typically regulate their internal osmolarity to maintain homeostasis.

2. **Identifying the Options**:
- The options provided are:
- Option 1: Myxine
- Option 2: Sharks
- Option 3: Bony fishes
- Option 4: Both 1 & 2

3. **Analyzing Myxine**:
- Myxine, also known as hagfish, are considered to have a body fluid osmolarity that is similar to that of seawater. They do not actively regulate their internal osmolarity and instead allow it to change with the surrounding environment.

4. **Analyzing Sharks**:
- Sharks are known to have a unique adaptation where they retain urea in their bodies, which helps them maintain osmotic balance with seawater. However, they do not maintain a constant internal osmolarity that is different from their surrounding medium.

5. **Analyzing Bony Fishes**:
- Bony fishes, unlike myxine and sharks, are osmoregulatory and can maintain a constant internal osmolarity that is different from their surrounding water, whether they are in freshwater or saltwater.

6. **Conclusion**:
- Based on the analysis, both myxine and sharks cannot maintain a constant internal osmolarity different from their surrounding medium. Therefore, the correct answer is **Option 4: Both 1 & 2**.

### Final Answer:
**Both 1 & 2 (Myxine and Sharks)**