Introduction
The excretory system in earthworm is one of the most important and frequently tested topics under Phylum Annelida. Earthworms remove nitrogenous wastes through specialized tubular excretory organs called nephridia. The three main types of nephridia in earthworm are pharyngeal, septal, and integumentary nephridia, which together help in excretion and osmoregulation.
Overview of the Excretory System in Earthworm
In earthworm, excretion is carried out by specialized tubular organs called nephridia. Depending on their type, nephridia may collect fluid from the coelom through a nephrostome or may have a closed inner end. The final waste may be discharged outside through a nephridiopore or into the alimentary canal.
- 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 Earthworm
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 a closed inner end and an external opening called 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
Comparison Between Pharyngeal, Septal & Integumentary Nephridia in Earthworm
|
Type |
Location |
Opens into |
Nature |
|
Pharyngeal |
4th-6th segments |
Gut |
Enteronephric |
|
Septal |
Septa |
Gut |
Enteronephric |
|
Integumentary |
Body wall |
Outside |
Exonephric |
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 in earthworm demonstrates efficient waste regulation through pharyngeal nephridia, integumentary nephridia, and septal nephridia. Understanding the differences between enteronephric and exonephric nephridia, along with the structure and physiology of the septal nephridium, is essential for understanding excretion in earthworm.
The excretory system in earthworm plays a vital role in osmoregulation and nitrogenous waste removal, helping maintain internal homeostasis.
Reference
- Kotpal RL - Modern Textbook of Zoology Invertebrates
- Jordan and Verma - Invertebrate Zoology
- Barnes - Invertebrate Zoology