Nephridia and Nephromixia of annelida

Nephridia and Nephromixia:

Some Polychaeta have nephridia of closed tubes, their blind ends project into the coelom, this is a primitive condition. The blind end is fringed with solenocytes lying separately or in groups. The solenocytes are round ciliated cells connected to the nephridium by a thin tube each, in the lumen of the tube is a long vibratile flagellum; solenocytes resemble flame cells of Platyhelminthes.

Such nephridia are known as protonephridia, e.g., Phyllodoce, Vanadis. But in many Polychaeta and Oligochaeta the nephridia are of the open type, each having a ciliated nephrostome opening into the coelom, they are called metanephridia, e.g., Neanthes, Lumbricus. Some Polychaeta have compound excretory organs formed by the union of nephridia and coelomoducts, they are called nephromixia.

In these the functions of excretory organs and genital ducts become combined.

They are of three types:

(a) Protonephromixium, the coelomoduct has combined with a closed protonephridium, e.g., Aliciopidae, Phyllodoce.

(b) Metanephromixium, coelomoduct is attached to an open metanephridium, e.g., Hesione.

(c) Mixonephrium or Nephromixium, coelomoduct is attached to a nephridium in such a way that they form a single organ, the funnel being coelomoduct and the duct is nephridial, e.g., Capitellidae, Arenicola. There is no hard-and-fast boundary between metanephromixium and mixonephrium.

In some Polychaeta, e.g., Neanthes, a part of the coelomoduct separates from the metanephromixium and becomes attached to the dorsolateral muscles as a dorsal ciliated organ, this keeps the coelomic fluid in circulation.

In some tubicolous worms, e.g., Serpula there is a division of labour, the nephridia of the anterior region are large and carry on excretion, and those of the posterior region are small and act as genital ducts exclusively.

In Oligochaeta and Hirudinea nephridia and coelomoducts are separate. There is generally one pair of metanephridia in each segment, but coelomoducts are restricted only to a few reproductive segments. Their nephridia may open to the outside of the body and are called exonephric nephridia (e.g., Lumbricus) or they may open into the gut and are known as enteronephric nephridia (e.g. Pheretima).

In a majority of earthworms there is a pair of original large-sized metanephridia in each segment, they are called holonephridia or meganephridia, e.g., Lumbricus. But in Pheretima there is large number of small-sized nephridia in each segment, they are called meronephridia or micronephridia. It is assumed that the original pair of holonephridia has broken up to form a large number of meronephridia.