Excretion in Non-Chordates
Excretion as described under Introduction of this unit refers to removal of the waste products of metabolism - carbon dioxide (CO2) and water released by the oxidation of energy rich compounds and the nitrogenous wastes released through the metabolism of proteins and nucleic acids. Though, we will here limit ourselves to elimination of nitrogenous waste. In small ones, particularly aquatic ones, simple diffusion from body surface plays a significant role in elimination of nitrogenous wastes. Even in larger aquatic animals some diffusion occurs from body surface, but they have evolved specialised organs for excretion, which play the main role in elimination of nitrogenous waste material. However, it has to be remembered, that many of these organs serve basically osmoregulatory function rather than excretory. Well described excretory organs are seen from the pseudocoelomate animals onwards. Several groups of metazoans have nephridia as excretory organs. A nephridium develops from the ectoderm centripetally. The lumen of the nephridium is formed by using the hollowing out of nephridial cells. Thus nephridia are intracellular. In primitive animals this lumen is closed internally but subsequently attains an opening into the coelom. The opening is termed as nephridial funnel or nephrostome. Its opening to the outside is by nephridiopore.