During retrogressive metamorphosis of the ascidian tadpole, which one of the following does not regress?

Options

A. Caudal myotomes
B. Gill slits
C. Nerve cord
D. Notochord

Answer

To answer the question regarding the retrogressive metamorphosis of the ascidian tadpole and which structure does not regress, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Retrogressive Metamorphosis**: Retrogressive metamorphosis is a process where an organism undergoes significant changes as it develops from a larval stage to an adult form. In the case of ascidians (sea squirts), the tadpole larva exhibits a notochord, a nerve cord, and other chordate features.

2. **Identify Structures in Ascidian Tadpole**: The ascidian tadpole has several key structures, including:
- Notochord
- Nerve cord
- Pharyngeal slits
- Tail

3. **Determine Which Structures Regress**: During metamorphosis, many of these structures regress as the organism transitions to its adult form. Typically, the notochord, tail, and nerve cord undergo regression.

4. **Identify the Structure That Does Not Regress**: The structure that does not regress during the metamorphosis of the ascidian tadpole is the **pharyngeal slits**. These structures persist and play a crucial role in the adult form as they are involved in filter feeding.

5. **Conclusion**: Therefore, the answer to the question is that the **pharyngeal slits** do not regress during the retrogressive metamorphosis of the ascidian tadpole.

### Final Answer:
The structure that does not regress during the retrogressive metamorphosis of the ascidian tadpole is the **pharyngeal slits**.