- Excretion in Platyhelminthes through protonephridia/ flame cell:-
- Structure:-
- Mechanism of function:-
Metabolic waste
products of flat worms are excreted generally in the form of NH3 by diffusion
across the general body surface. Flatness is helpful in diffusion.
However flat worms
release excess water as well as some excretory products through flame cells.
A typical flame
cell is an uninucleated large cell.
The flame cell may
give out numerous branched protoplasmic processes in the surrounding
mesenchyme.
In the center of
the cell is a conspicuous (easily visible/ attracting attention) bulbous
cavity or cell lumen. The cavity narrows down forming a fine capillary duct.
The cytoplasm
occurs in the periphery of the cell containing a round or oval nucleus.
The broad end of
lumen encloses a tuft of long cilia or flagella. The tuft of cilia/flagella
when undulates, resembles the flickering flame of a candle. Hence the common
name is flame cell.
These cells are usually connected to the
lateral longitudinal collecting/excretory ducts.
Flame cells
function on the basis of filtration & resorption.
[
The water from the intercellular spaces are collected by the extension of the
plasmalemma. The collected water is ultrafiltered through the thin wall of
pillar like rods. The ultrafiltered fluid (excluding the protein molecules)
then moves through the neck of the cavity by the flickering movement of the
cilia/flagella.]
The continuous beating
of the cilia/flagella within the cavity of the flame cell produce sufficient
negative pressure which causes ultrafiltration.
The filtered fluid is
passed into the longitudinal duct through the capillary duct & discharged
through the nephridiopore.
During passing through the tubes ions are
selectively reabsorbed or secreted.
The protonephridia thus plays an important role in regulating ionic
& water balance in addition to the elimination of metabolic wastes.
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