Nephridia in Annelida

• Describe the structure of septal nephridia in Pheretima.

The septal nephridia may be considered typical of all the nephridia of Pheretima.

• Structure:-

Each septal nephridium consists of nephrostome , neck, body of nephridium & terminal duct.

1. Nephrostome :-

 Nephrostome is the ciliated funnel communicating with the coelom.

It consists of an elliptical pore bounded by so called upper & lower lips. Upper lip is formed of a large central cell & 8 or 9 marginal cells.

The lower lip consists of 4 to 5 compact cells. All the cells are ciliated.

2. Neck:-

Nephrostome leads into the main body of nephridium through a short, narrow & ciliated, tube like neck.

3. Body of nephridium:-

Body consists of 2 parts - a short straight lobe & a long twisted loop with a narrow apical part.

Twisted loop consists of a proximal limb & a distal limb which are spirally twisted upon each other. Proximal limb is jointed to the neck.

4. Terminal duct:-

Distal limb of the body of nephridium ends in a short & narrow duct called terminal duct.

• Nephridial tubule:-

Nephridium consists of a connective tissue matrix traversed by a coiled tubule. It has 4 ciliated tracts in its course 1 in neck, 2 in body & 1 in terminal duct.

There are 4 parallel tubules in the straight lobe.

Each limb of twisted loop contains 3 parallel tubule in basal part & 2 in the apical part.

A single tubule is present in each of the neck & terminal duct.

• Nephridia in Annelida:-

Nephridia are segmentally arranged coiled tubules formed by the invagination of ectoderm into coelom.

Internally they may end blindly into the coelom (protonephridia) or may open into the coelom by a ciliated funnels or nephrostomes (metanephridia).

Externally they opens through small apertures called nephridiopores.

Nephrostome may open into coelom either in the same segment in which it lies or in the segment just in front.

Nephridia are primarily excretory in function but may secondarily serve to carry genital products to the exterior.

• Protonephridia:-

The 'closed' or protonephridium is the primitive type. It terminates in the coelom as a blind tube.

Their blind ends are fringed with solenocytes. The solenocytes are round ciliated cells connected to the nephridium by a narrow tube. The lumen of the tube encloses a long vibrating flagellum.

Excretory fluid enters through the walls of solenocytes. The fluid is then driven into the lumen of the nephridium by the flagellum & forced to the exterior through nephridiopore.

Protonephridia always develop in the larval polychaetes. They are also found in some adult polychaetes like Vanadis, Phyllodoce, Glycera etc.

• Metanephridia :-

The 'opened' or metanephridia are more advanced. They are found in the majority of polychaetes (Neanthes), all the oligochaetes (Lumbricus) & leeches.

A metanephridium is opened at both ends. It opens into the coelom by a ciliated funnel or nephrostome. The other end of it opens to the exterior through the nephridiopore.

Principal nitrogenous product in annelids is NH3. The excretory products diffuse from coelomic fluid or blood into the lumen of nephridial tubule. It is discharged to the outside through nephridiopore.

1. Micro & Meganephridia:-

On the basis of their size & no. nephridia are divided into 2 groups-

Micronephridia:-

These are smaller in size, sometimes microscopic, & are numerous in each segment. They are networks of fine tubes lying on the body wall & septa in each segment.

All the nephridia of Pheretima are micronephridia.

Meganephridia/ Holonephridia :-

 These are larger in size & generally one pair per segment. They usually extend over 2 segments & their nephrostomes open into the segments next in front.

They are found in Polychaeta & Hirudinea.

2. Exo & enteronephridia:-

Nephridia are termed as exonephric or ectonephric when they directly open to the exterior through nephridiopores.

Such as the meganephridia of Nereis, Hirudinaria, Lumbricus & integumentary micronephridia of  Pheretima.

Nephridia are termed as enteronephric when they lack nephridiopores & open into the excretory or alimentary canal, as septal & pharyngeal nephridia of Pheretima.