Retrogressive metamorphosis is the feature of
(a) Amphioxus
(b) Salpa
(c) Balanoglossus
(d) Myxine

Options

A. Amphioxus
B. Salpa
C. Balanoglossus
D. Myxine

Answer

To solve the question regarding which organism exhibits retrogressive metamorphosis, we will analyze the options provided and understand the concept of retrogressive metamorphosis.

### Step-by-Step Solution:

1. **Understanding Retrogressive Metamorphosis**:
- Retrogressive metamorphosis refers to a process where an organism undergoes degenerative changes during its development, particularly from an active larval stage to a sedentary adult form. In this process, features that are present in the larval stage may be reduced or lost in the adult stage.

2. **Identifying the Organisms**:
- **Amphioxus**: This organism does not exhibit retrogressive metamorphosis. It retains its notochord and other features throughout its life cycle.
- **Salpa**: Salpa is known to undergo retrogressive metamorphosis. The larval form is free-swimming and active, while the adult form is sedentary and attached to a substrate.
- **Balanoglossus**: This organism does not exhibit retrogressive metamorphosis; it has a more complex life cycle without the degenerative changes typical of retrogressive metamorphosis.
- **Myxine**: Also known as hagfish, myxine does not demonstrate retrogressive metamorphosis.

3. **Conclusion**:
- Based on the analysis, the organism that exhibits retrogressive metamorphosis is **Salpa** (option B).

### Final Answer:
**(b) Salpa**