Setae help in locomotion in earthworm but not uniformly present in all the segments. Select among the following that represents setae

Options

A. `1^(st)` segment
B. Last segment
C. Clitellar segment
D. `20^(th) - 22^(th)` segment

Answer

To solve the question regarding the presence of setae in earthworms, we will analyze each option provided and determine which segment(s) contain setae.

### Step-by-Step Solution:

1. **Understanding Setae**:
- Setae are small bristle-like structures found on the body of earthworms. They play a crucial role in locomotion by anchoring the worm to the soil as it moves.

2. **Identifying the Segments**:
- Earthworms have a segmented body, and setae are not uniformly distributed across all segments. Some segments contain setae, while others do not.

3. **Evaluating Each Option**:
- **Option 1: First Segment**:
- The first segment of an earthworm is known as the mouth segment. This segment does not have setae, so this option is incorrect.

- **Option 2: Last Segment**:
- The last segment is called the periproct, which also does not contain setae. Therefore, this option is incorrect as well.

- **Option 3: Clitellar Segment**:
- The clitellar segments (14th, 15th, and 16th segments) are involved in reproduction and cocoon formation. They are not primarily for locomotion and do not have setae, making this option incorrect.

- **Option 4: 20th to 22nd Segment**:
- These segments are known to contain setae. The presence of setae in these segments aids in locomotion, making this option the correct answer.

4. **Conclusion**:
- Based on the evaluation, the correct answer is **Option 4: 20th to 22nd Segment**, as these segments contain setae that assist in locomotion.

### Final Answer:
**Option 4: 20th to 22nd Segment**