Pharyngeal nephridia
They are found as paired and lie one pair in each of the 4th, 5th and 6th segment.

**Pharyngeal Nephridia in Earthworms:**
Pharyngeal nephridia are specialized excretory organs found in earthworms. These nephridia play a crucial role in maintaining the osmotic balance and eliminating waste products from the body of the earthworm. They are located in specific segments of the worm's body.

**Distribution of Pharyngeal Nephridia:**
Pharyngeal nephridia are present in the segments of the earthworm's body that are associated with the pharynx. The exact number and location of these nephridia can vary between different species of earthworms. However, in general, they can be found in the following segments:

1. **Segment 3:** The first pair of pharyngeal nephridia is located in the third segment of the earthworm's body. These nephridia are connected to the pharynx, which is a muscular tube responsible for ingesting food.

2. **Segment 4:** The second pair of pharyngeal nephridia is found in the fourth segment of the earthworm. These nephridia also have connections with the pharynx.

3. **Segment 5:** The third pair of pharyngeal nephridia is present in the fifth segment. These nephridia are also associated with the pharynx and function in excretion.

**Function of Pharyngeal Nephridia:**
Pharyngeal nephridia serve as excretory organs in earthworms. They play a vital role in removing waste materials from the body and maintaining the osmotic balance. The nephridia filter the coelomic fluid, which is the fluid present in the body cavity of the earthworm.

The process of excretion in pharyngeal nephridia involves the following steps:

1. **Filtration:** The coelomic fluid enters the nephridia through ciliated funnels located in the segments where the nephridia are present. These funnels help in the filtration of waste materials, including nitrogenous waste products like ammonia.

2. **Reabsorption:** After filtration, useful substances such as water and ions are reabsorbed back into the body, ensuring the maintenance of osmotic balance.

3. **Secretion:** Waste substances, including ammonia and other metabolic products, are actively secreted into the nephridial tubules.

4. **Excretion:** The filtered and modified waste materials, along with excess water, are then expelled from the body through the nephridiopores, which are small openings located on the body surface.

Overall, pharyngeal nephridia play a crucial role in maintaining the internal environment of earthworms by removing waste products and regulating the osmotic balance.