Nephridia in Earthworms

 An earthworm has a segmented body. Each segment has a pair of tubes called nephridia. These are the excretory organs of the earthworm. Each nephridium has a ciliated funnel, the nephrostome, at one end. This opens into the body cavity of the segment in front. The nephrostome leads into a long coiled tube made up of the following parts:

 • Narrow ciliated tube, •

• Middle ciliated tube,

• Wide, non ciliated tube, and

 • Muscular tube which opens to the exterior via an excretory pore.

     The nephridium thus opens at both ends, unlike the flame cells which are blind tubules opening only at one end to the exterior.

    Each nephridium is surrounded by s capillary network. The waste products, mainly urea are extracted from the blood in the capillaries surrounding the nephridia. Wastes are also removed from the fluid in the body cavity and wastes moves through the long tubes of the nephridia. Along the way, salts and other reabsorbed through the walls of the tubes. This composition and concentration of the body fluid constant.

     The unabsorbed substances, including water collect in the muscular tube as hypotonic urine. The sphincter (ring of muscle) guarding the excretory pore relaxes to allow the urine to escape to the exterior.

     Although the earthworm is terrestrial, it Lives in a moist environment since it needs a moist skin surface for gaseous exchange. Carbon dioxide is excreted by diffusion through the skin. The earthworm cannot survive in a dry environment. Within limits, it can tolerate dry conditions and conserve water by producing very little urine which is hypertonic.