Nephridia: Types, Function and Physiology of Excretion in Annelids

Nephridia are microscopic coiled tubes present nearly in each segment of annelids. These segmental organs in Annelids are either protonephridia or metanephridia. Nephridia function as excretory organs.

These are typically unbranched openings into coelom by a ciliated funnel called nephrostome.

They are ectodermal in origin and develop as invagination from ectoderm into the coelom. They open up to the exterior through small apertures on lateral sides called nephridiopores.

Nephridia are excretory units in Annelids.

Types of Nephridia:

Nephridia are of different types based on location, size,  number and structure ( whether open or closed type )

Types of Nephridia based on location:

They are of the following types:

Diagram of Types of Nephridia in Earthworms based on location 

1. Pharyngeal Nephridia:

 They are seen as paired tufts on either side of the pharynx and oesophagus (on the 4th, 5th and 6th segments in earthworms).

The tuft consists of numerous coiled branch tubules without nephrostome.

The terminal ducts of all the tubules join to form a single thick-walled common duct in each tuft called common pharyngeal nephridial duct which runs anteriorly parallel to the ventral nerve cord.

Pharyngeal nephridia are enteronephric

2. Integumentary Nephridia:

Integumentary nephridia are microscopic V-like in shape. 

They are scattered on the entire inner or parietal surface of the body wall in each segment, except the first two.

There are 200 - 250 nephridia in each segment which increases to more than 2000 in the segment of clitellum ( segments 14 to 16 ), constituting the "forest of nephridia".

They lack nephrostome. Their terminal ducts independently open up on the body surface through minute openings known as nephridiopores.

Integumentary nephridia are thus exonephric. 

3. Septal Nephridia:

Septal Nephridia are comparatively larger ( In Pheretima they are the largest ).

They are attached to both the faces of each intersegmental septum behind the 15th segment.

Each segment, behind the 15th segment, bears four rows of septal nephridia, two on its anterior face and two on the posterior. Each row may contain 20 to 25 nephridia so there are 80 to 100 nephridia on each Septum or in each coelomic compartment.

Nephridia remains suspended freely in each segment while the terminal ducts open into a pair of septal excretory canals, which discharge their contents dorsally into a pair of supra-intestinal excretory ducts,  extending from 15 to the last body segment. These ducts open into the intestine in each segment through narrow ductules having sphincter openings. Septal nephridia are thus entero-nephric.

Septal nephridia of earthworm diagram

Types of Nephridia based on structure:

(opening of  tube in the coelom, open or closed type): 

The nephridia are of two types. 

1. Protonephridia:

They are primitive type and terminate in the coelom as closed or blind tubes. 

Protonephridia are made up of syncytial cells which develop in larval Polychaetes, irrespective of the kind of adult nephridia.

The other part of the tube is provided with peculiar specialised excretory tube cells or solenocytes.

* These are similar to the flame cells of platyhelminthes and rotifera.

*Solenocytes may occur singly or in groups. They are rounded cells connected to the protonephridium by a thin tube with a vibrating flagellum which drives the excretory fluid from the lumen of the nephridium to the exterior through the nephridiopore.

Protonephridia are found in some adult Polychaetes such as Vanadis, Phyllodoce, Tomopteris, Glycera, Nephthys, etc.

Types of Nephridia in Annelids: Diagram

2. Metanephridia:

They are advanced and are open type, found in the majority of Polychaetes ( Neanthes ), all oligochaetes ( Lumbricus ) and leeches.

The inner end of metanephridium opens into coelom by a ciliated funnel or nephrostome ( not solenocytes) and the other end opens to the exterior by nephridiopore. 

Thus, a metanephridium is open on both ends.

Nereis illustrate a typical metanephridia. Other polychaetes alter in minor features.

Archiannelida usually possess one pair of nephridia which may be protonephridia ( e.g. Diniophilus) or metanephridia ( e.g. Polygordius, Protodrilus)

In polychaetes, the excretory waste ( ammonia) from coelomic fluid or the blood diffuses into the lumen of the nephridial tubule from where it is discharged out of the body through the nephridiopore.

Metanephridia are further divided into different types based on size and their opening status.

Types of Nephridia based on size :

Nephridia based on their size are of the following types.

Micronephridia: ( or Meronephridia)

Micronephridia are smaller ( sometimes microscopic), numerous and present on the body wall and septa as a network of fine tubes in each segment, e.g. Pheretima ( all are micronephridia).

Meganephridia: ( or Holonephridia)

They are larger and generally present as a pair in each segment.

They take up two segments and open up to the outside by nephrostome in the next segment in front,i.e. the third segment. 

Example: In Polychaetes and Hirudinea.

*Neanthes, Nereis and Hirudo - Metanephridia with internal ciliated funnels.

* Hirudinaria- No ciliated funnels or nephrostome but modified ciliated organ.

* Megascolex - Two types of nephridia in the same worm, even in the same segment.

* Serpula ( Also in some tubicolous worms) - Division of labour is seen such that the nephridia in the anterior region are larger and excretory while those in the posterior region are small and function as gonoducts.

Exonephridia: ( or metanephridia)

Nephridia are termed Exonephric ( or mesonephric) when they open directly to the exterior through nephridiopores.

Example: 

Meganephridia of Nereis, Hirudinaria and Lumbricus.

Integumentary micronephridia of Pheretima.

Enteronephridia:

Nephridia are termed enteronephric when they lack nephridiopores and open into the excretory canals or alimentary canal. Thus, nephridia without nephridiopore is called enteronephridia.

Example: 

Septal and Pharyngeal nephridia of Pheretima.

* read and compare with the diagram to understand the topic.

Function of Nephridia:

Nephridia are primarily excretory in function but secondarily they may serve as conveyors of genital products to the exterior.

Nephridia are the unit of excretion in Annelids. They remove metabolic wastes from the animal body. 

Excretion in Annelids 

In Annelids, the excretory organs are mostly paired metanephridia, but trochophore larvae and a few families of polychaetes have protonephridia with flagellated solenocytes.

The metanephridium, in annelids, occurs in almost all of the segments in oligochaetes and polychaetes but in the middle third of the body in leeches.

The ciliated funnel of each typical metanephridia opens into the coelom of one metamere, and then the nephridial tubule passes posteriorly through the septum and lies coiled within the coelom of the next segment.

Nephridia are abundantly supplied with blood vessels. Blood vessels are formed as a network around the tubule to reabsorb the useful solutes. Their gland cells extract excess water and nitrogenous wastes from the blood.

 In terrestrial and freshwater oligochaetes, the urine is lower in salts than the coelomic fluid and blood, showing that tabular reabsorption has occurred.

Each metanephridium excretes out by way of its nephridiopore, however, in some species of earthworms, the nephridiopore opens into the intestine where most of the water is reabsorbed through the intestinal wall. This makes the worm more tolerant in much dryer soils.

All the aquatic annelids excrete mainly ammonia. The terrestrial species synthesize urea as their major nitrogenous waste.  

The chloragogen tissue is an important site for the synthesis of urea and protein deamination (i.e. liberation of the ammonia made from freed amino groups), and also when waste is repleted, the chloragogen cells break free and drawn into ciliated nephrostome. 

In the case of terrestrial annelids, the cuticles are not very effective against evaporative water loss, they tolerate considerable dehydration.

Aquatic oligochaetes excrete ammonia, and terrestrial species ( earthworms) excrete urea.

However, earthworms are less ureotelic than other terrestrial animals.

In dry weather, the earthworms burrow more deeply but still may lose 70% of water content which makes them quiescent but not dormant.

In some species, estivation( dry season dormancy) is seen for up to two months, after secretion of a mucus cyst. After rain, they take up water through their skin and resume their activity.

The tunnels made by earthworms are especially beneficial for soil aeration and water absorption.

Related important Questions 

Question:

Write a short note on Nephridial system in pheretima and its adaptive significance.

Answer:

In Pheretima (earthworm) the Nephridial system performing osmoregulatory mechanism is the principle excretory system consisting several coiled tabular structures called in Nephridia, which functions similarly as kidneys invertebrates.

Types of Nephridia:

Nephridia are segmentally arranged in all segments, except the first three. They are of three types, 

Pharyngeal nephridia:

 They are located in segments 4-6 and occur in tufts on either side of pharynx, opening into buccal cavity.

Septal nephridia:

They are found on each side of septa from the 15th segment onwards and open into the intestine via supra-intestinal ducts through narrow ductules having sphinctered opening. As they open into intestine, they are enteronephric nephridia.

Integumentary nephridia:

Except the first two segments, the integumentary nephridia are scattered in entire parietal surface of body wall. They open to the exterior through nephridiopore so they are exonephtic type of nephridia.

Structure:

Each nephridia typically has a cilliated funnel called nephrostome communicating with the coelom, a coiled tubule for filteration and absorption, and a terminal duct for discharge.

Adaptive significance:

• Osmoregulation: The nephridia maintains water and salt balance in both drier and moist habitats. The enteronephric nephridia help in conserving water by channeling waste into the gut, whereas exonepheric nephridia eliminate excess water and ions.
• Moisture conservation:The enteronephric nephridia conserves water by passing waste into the gut.
• Terrestrial adaptation:Dual excretory routes (internal and external), allow survival in both wet and dry condition.
•  Functional efficiency: Segmentation ensures localized waste removal and uniform excretion.
• Metabolic efficiency:As the septal nephridia opens into intestine, presenting an advanced adaptation for soil-dwelling invertebrates showing link between excretion and digestion and promoting recycling of water and salts.
• Evolutionary adaptation:The differentiation into three types of nephridia represents progressive specialisation of the excretory system, showing the evolutionary shift from simple flatworm flame cells to complex metanephridia functioning like vertebrate kidneys.

Thus the Nephridial system is highly specialised to ensure efficient excretion and water conservation, enabling sucessful adaptation to a terrestrial mode of life.

Question: 

Write short note on Nephridial system of annelids as an example of metameric segmentation.

Answer:

Nephridial system in annelids show clear example of metameric segmentation as each segment contains its own pair of nephridia arranged in a series of repeating units or metameres.

Body of an annelid is divided by septa forming distinct segments and within almost each segment occurs a pair of nephridia which are coiled tabular organs derived from the body wall invagination.

These nephridia, open internally by nephrostome into coelom and externally by nephridiopore on the body surface.

As a characteristic representation of metameric arrangement, the nephridia of adjacent segments are similar in structure but function independently and perform excretion separately, filtering waste from the coelomic fluid of its own compartment.

   This arrangement of repetition aids to localised physiological regulation such that if one segment is damaged, others continue to work normally enhancing survival and functional redundancy.

• This system ensures continuous waste removal across all segments,
• It also gives flexibility,
•  specialisation and,
•  division of labour contributing to the success of annelids in aquatic and terrestrial ecosystem.

About the Author: This educational content on Zoology is written by Rekha Debnath, M.Sc. & M.Phil. in Zoology, with a focus on university-level academic topics. Read the full Author Credentials and Background here.