Earthworms (annelids) have slightly more evolved excretory structures called nephridia.

The excretory organ in earthworm is Nephridia.

Nephridia are the excretory organs found in earthworms. They are long, tubular structures that are responsible for removing metabolic waste from the body. Each earthworm has numerous nephridia located along its body segments.

Structure of Nephridia:
Nephridia are composed of three main parts:

1. Nephrostome: It is a ciliated funnel-shaped opening located in the coelomic cavity of the earthworm. The nephrostome collects waste fluid from the surrounding tissues and coelomic fluid.

2. Nephridial Tubule: This is a long and coiled tubule that extends from the nephrostome to the external opening called the nephridiopore. The tubule is lined by specialized cells that are responsible for the selective reabsorption of essential nutrients and water.

3. Nephridiopore: This is the external opening of the nephridia through which the waste fluid is eliminated from the body.

Function of Nephridia:
The nephridia in earthworms perform the following functions:

1. Excretion: The primary function of nephridia is the removal of metabolic waste products, such as ammonia, urea, and uric acid, from the body of the earthworm. These waste products are collected by the nephrostomes and transported through the nephridial tubules to the nephridiopores for elimination.

2. Osmoregulation: Earthworms live in moist environments and face the challenge of maintaining internal water balance. Nephridia help in regulating the water content and electrolyte balance by selectively reabsorbing essential nutrients and water from the waste fluid.

3. Acid-base balance: Nephridia also play a role in maintaining the pH balance of the earthworm's body fluids. They help in the removal of excess acids or bases, thus regulating the acid-base balance in the body.

In conclusion, nephridia are the excretory organs in earthworms that perform the important functions of excretion, osmoregulation, and acid-base balance. They help in maintaining the internal environment of the earthworm and ensuring the removal of waste products from the body.