The muscles associated with each setal/ setigerous sac respectively is 

Options

A. one pair of protractor muscles and a retractor muscle
B. one pair of protractor muscles and one pair of retractor muscles
C. one protractor muscle and one pair of retractor muscles
D. one protractor muscle and one retractor muscle

Answer

To answer the question regarding the muscles associated with each setal/setigerous sac in earthworms, we can break down the information step by step.

### Step-by-Step Solution:

1. **Understanding the Earthworm Structure**:
- Earthworms belong to the phylum Annelida, characterized by their segmented bodies. Each segment contains muscles and bristles known as setae.

2. **Identifying the Components of Setae**:
- Each seta consists of three parts:
- The neck (upper part)
- The modulus (middle swollen part)
- The root or body (inner part), which attaches to the setigerous sac.

3. **Muscle Types Associated with Setae**:
- The movement of the setae is controlled by specific muscles. There are two types of muscles associated with each setigerous sac:
- **Protractor Muscles**: These muscles extend the setae outwards. There are two protractor muscles (one pair) for each setigerous sac.
- **Retractor Muscle**: This muscle retracts the setae back into the body. There is one retractor muscle for each setigerous sac.

4. **Conclusion**:
- Therefore, for each setigerous sac, there is one pair of protractor muscles and one retractor muscle.

### Final Answer:
The muscles associated with each setal/setigerous sac respectively are: **One pair of protractor muscles and one retractor muscle.**

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