In earthworm, which type of structure regulate the volume and composition of the body fluid.

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks about the structures in earthworms that regulate the volume and composition of body fluids. This pertains to the excretory system of the earthworm.

2. **Identifying the Excretory Organ**: In earthworms, the primary excretory structures responsible for regulating body fluids are called **nephridia**.

3. **Types of Nephridia**: There are three types of nephridia in earthworms:
- **Septal Nephridia**: These are located on both sides of the intersegmental septa, extending from the 15th segment to the last segment, and they open into the intestine.
- **Integumentary Nephridia**: These are attached to the lining of the body wall and are present from the 3rd segment to the last segment of the earthworm.
- **Pharyngeal Nephridia**: These are found in pairs in segments 4, 5, and 6.

4. **Function of Nephridia**: The nephridia help in the excretion of waste products and play a crucial role in osmoregulation, which is the regulation of water and electrolyte balance in the body.

5. **Conclusion**: Therefore, the structures that regulate the volume and composition of body fluids in earthworms are the **nephridia**.