Introduction
The excretory system of earthworm is one of the most important and frequently tested topics under Phylum Annelida. Unlike higher animals that rely on kidneys, earthworms remove nitrogenous waste through small tubular structures called nephridia, distributed throughout the body in three distinct forms.
Overview of the Excretory System in Earthworm
In earthworms, the organ responsible for excretion is the metanephridium. Each metanephridium has two key openings that allow it to function effectively.
- Nephrostome — It is the internal and ciliated opening that collects fluid from the body cavity
- Nephridiopore — It is the external opening through which waste is released from the body
Earthworms possess three distinct sets of excretory organs working together:
- Pharyngeal nephridia
- Integumentary nephridia
- Septal nephridia
Types of Nephridia in Erthworm
Pharyngeal Nephridia
The pharyngeal nephridia are located on either side of the pharynx.
- They open directly into the gut and are classified as enteronephric nephridia
- Structurally, they are metanephridia
- They occur as 3 pairs of bunches, found in the 4th, 5th, and 6th segments
- Each bunch is made of numerous tubules whose ends join a common duct
- The common ducts of the 4th & 5th segments open into the pharynx
- The common duct of the 6th segment opens into the buccal chamber
Integumentary Nephridia
Each integumentary nephridium is a tubular V-shaped metanephridium with one blind end and an external opening called a nephridiopore.
The integumentary nephridia line the inner surface of the body wall.
- They release waste outside the body and are therefore called exonephric nephridia.
- Present in all segments except the first two
- 200–250 nephridia per normal segment
- 2000–2500 nephridia in clitellar segments called as "forests of nephridia"
- Each nephridium is V-shaped, with one end closed and the other opening via a nephridiopore
Septal Nephridia
The nephrostome is funnel-shaped and its mouth is ciliated.
The septal nephridia are attached to the septa dividing the body into segments.
- They open into the gut and are classified as enteronephric nephridia
- About 50 nephridia on each side of every septum
- Arranged in 4 rows
Structure of a Septal Nephridium
Each septal nephridium is a syncytial glandular mass made of:
- Nephrostome
- Neck
- Main body
- Terminal nephridial duct
The main body is divided into:
- Straight lobe
- Twisted lobe
The twisted lobe is formed from a proximal limb and distal limb.
- Ciliated tracts: neck (1), main body (2), terminal duct (1)
- Tubules: 4 in straight lobe, 3 at limb bases, 2 near limb apex, 1 each in neck and terminal duct
- Terminal ducts drain into a septal excretory canal
- Canals open into supra-intestinal excretory ducts
- Ducts extend from the 15th segment to the posterior end
Physiology of Excretion
Earthworms are ammonotelic animals, meaning ammonia is their primary nitrogenous waste product.
- Urine also contains urea, uric acid, and amino acids
- The nephridium is highly vascular, enabling efficient removal of nitrogenous wastes from blood and coelomic fluid
- The septal nephridium collects wastes from the coelomic fluid through the nephrostome
- Waste is released through nephridiopores or into the gut via enteronephric nephridia
Conclusion
The excretory system of earthworm demonstrates efficient waste regulation through pharyngeal nephridia, integumentary nephridia, and septal nephridia. Understanding the difference between enteronephric and exonephric nephridia, along with the structure of the septal nephridium.
Referrence
- Kotpal RL - Modern Textbook of Zoology Invertebrates
- Jordan and Verma - Invertebrate Zoology
- Barnes - Invertebrate Zoology
Suggested Reading
- Digestive system of earthworm
- Circulatory system of earthworm