In earthworms, the excretory organs are in the form of segmentally arranged coiled tubules, which are called nephridia. They remove the wastes through the pores of the body wall.
Nephridia are tube-like structures that function as excretory organs.
Except for the first three and the last segment, each segment of the earthworm’s body has a pair of nephridia. These nephridia filter waste products from the coelomic fluid, the fluid that fills the worm’s body cavity.
There are three main types of nephridia:
- Pharyngeal nephridia
- integumentary nephridia
- septal nephridia
Pharyngeal Nephridia
They are located in 4, 5 and 6 sections. They open into the anterior part of the alimentary canal, which is the oral cavity and pharynx. These are without nephrostome and are enteronephric type.
Integumentary Nephridia
They are distributed on the body wall. These are the smallest, V-shaped, without nephrostome, and are exonephric type.
They form forests of nephridia in the clitoral segment.
Septal Nephridia
These are the largest, attached in front of each intersegmental septum behind the 15th segment.
Banded nephridia are the only nephridia that contain nephrostomes or funnels. The terminal duct opens into the septal excretory canal. This canal opens into the supra-intestinal excretory canal.
The stipular nephridia are enteronephric, ultimately excretory products are discharged into the bowel. The enteronephric position is an adaptation for the conservation of water or osmotic regulation.
- An earthworm has about 200-250 nephridia in each segment of its body.
- The nephridia are arranged in pairs, with one nephridium on each side of the body.
- Septal nephridia are more numerous than integumentary nephridia.
- The excretory system of earthworms is very efficient in removing waste products from the body.
Nephridia
Each nephridium consists of the following main parts:
Nephrostome
Nephrostome is a pore located in the body wall of each segment of the earthworm. It collects waste material and extra fluid from the body cavity. The cilia lining the nephrostome create a current that moves fluid into the nephridium.
Tubules
After entering the nephridium through the nephrostome, the fluid moves through a series of tubules. These tubules are covered with specialized cells that are involved in the processes of filtration, reabsorption, and secretion.
Tubules have a coiled structure, providing a large surface area for the exchange of substances.
Collecting Tubule
The terminal part of the nephridium is the collecting duct. It receives filtered fluid from the ducts and carries it to the bladder, or vesicle, which temporarily stores excreta.
Urinary Bladder
It is a sac-like structure at the end of the duct. It collects filtered waste products.
Excretory Pore
It is a small opening in the body wall through which waste products are excreted.
Excretory Products of Earthworm
urea (about 50%), ammonia (about 40%), and creatinine. Earthworms are mainly ureotelic. That is, they emit urea.
The main excretory product of earthworms is urea. Some amount of nitrogenous waste is also released as ammonia gas. The excretory system of earthworms helps to maintain the body’s homeostasis by removing waste products and regulating the water balance.
Here are some more details about the excretory products of earthworms:
Urea is a relatively non-toxic nitrogenous waste product that is easily dissolved in water. This makes it easier for earthworms to excrete urea, as they do not have to produce a lot of urine to remove it from the body.
Ammonia is a more toxic nitrogenous waste product. Earthworms can convert some of the ammonia they produce into urea, which is less toxic. However, some ammonia is still released as a gas.
The excretory system of earthworms is very efficient at removing waste products from the body. This is important for earthworms, as they live in a moist environment where bacteria can easily grow. By removing waste products from the body, earthworms help to prevent the growth of harmful bacteria.
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