Download Nephridia

Department Of Zoology
Dr. Saheel Ahmad
Class ist Year
Nephridial System In Annelida
Annelida possess segmentally repeated tubes called segmental organscoelomoducts and nephridia.
1. Coelomoducts: - They are wide mesodermal tubes that develop as
evagination (out pushing) from the coelom. They open at one end to the
exterior by a genital pore and at the other end into the coelom by a large
ciliated funnel, the coelomostome. They are confined to those segments
which bear gonads as they primarily function as an exit for the gametes.
Secondarily, however, the coelomoducts may take up excreatory role.
2. Nephridia: - Nephridia are segmentally arranged coiled tubes of ectodermal
origin that develop as invaginations from ectoderm into coelom. They
communicate with the exterior through the laterally placed small apertures
called nephridiopores and internally may end blindly (protonephridia) or
may open by small ciliated funnels, or nephrostomes, into coelom
(metanephridia). Nephridia are primarily excreatory in function but may
secondarily serve to convey the genital products to the exterior.
In Polychaete excreatory organs are either protonephridia or
metanephridia.
(a)Protonephridia: - They are primitive type of nephridia and are found in
some polychaetes e. g., Vandis they are closed internally and bear special
cell. The solenocytes (or tube cells) occur singly or in groups and resemble
the flame cells of Platyhelminthes and Rotifera. A solenocyte is a rather
rounded ciliated cell connected to the protonephridia by a thin tube, the
lumen of which encloses a flagellum. Excreatory fluid enters through the
walls of Nephridial tubules which are internally ciliated. This fluid is driven
into the lumen of nephridium by flagellum and forced to the exterior through
nephridiopore.
(b)Metanephridia: - The nephridia which open into the coelom are known
as metanephridia. They are of advanced type and are found in many
polychaetes, all oligochaetes and majority of leeches. Instead of solenocytes,
the inner end of metanephridium opens into coelom by a ciliated funnel or
nephrostome. The other end opens to the exterior through the nephridiopore.
In some oligochates the nephrostomes have been secondarily lost. This
condition is seen in integumentary nephridia. Principle nitrogenous waste in
polychates is ammonia. Excreatory waste diffuse from coelomic fluid or
blood into the lumen of nephridial tubule and discharged to outside through
nephridipore.
(i) Micro and Meganephridia. On the basis of their size and number,
nephridia may be micronephridia or meganephridia. Micronephridia or
meronephridia or smaller in size and are numerous in each segment
(Pheretima). Meganephridia or holonephridia are larger in size and generally
or pair per segment. They usually extend over two segments and their
nephrostomes open into the segments, next infront.
(ii) Exo and Enteronephridia. Nephridia are termed exonephric or
ectonephric when they directly open to the exterior through nephridiopores
(e. g, meganephridia of Neries, and integumentary micronephridia of
Pheretima). They are termed as enteronephric when they lack
nephridiopores and open into the excreatory cannals or elemantry cannal, as
septal and pharyngeal nephridia of Pheretima.
3. Nephromixia:- in some Polychaeta, coelomoducts do not remain
independent but become fused, parcially or wholly with the nephridia
forming compound segemental organs or nephromixia. They consist both of
ectoderm and mesoderm and used both as genital as well as excreatory duct.
Nephridia and the coelomoducts show various degrees of combination as
under:(a) Protonephromixium: - coelomoduct is united with a protonephridium.
It conveys both reproductory and excreatory products to the exterior e. g,
Phyllodoce.
(b)Metanephromixium: - It is formed by the union of a coelomoduct with
an open metanephridium e.g. Hesione.
(c)Myxonephrium or mixonephridium: -It is formed by an intimate
combination of coelomoduct and a nephridium, resulting in a single
composite organ. The coelomoduct forms the funnel of this organ and the
nephridium, its duct e. g. Arenicola.
(d)Ciliated organ: - In some forms coelomoducts are reduced to ciliated
organs. In Neries, they are attachged to the dorso-lateral longitudinal
muscles and are known to open externally.