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