CO₂ is the most important gaseous waste and is eliminated through the lungs.

This nitrogenous waste product is found in spiders only.

The process of excretion of nitrogenous waste in the form of ammonia is called ammonotelism.

In marine teleost fishes, trimethylamine oxide occurs as a waste product in large quantities.

Excretory system maintains the structure, pH and osmotic pressure of the body fluids.

Earthworm is both ammonotelic and ureotelic.

Ammonia is highly soluble in water and a very large volume of water is needed by the animal to dissolve ammonia. 1 gm of ammonia needs about 300 - 500 ml of water. But this is not a problem for animals living in an aqueous habitat which are generally found to be ammonotelic.

The semisolid whitish material excreted out of the body of birds contains concentrated uric acid.

Animals which excrete uric acid as nitrogenous waste are called uricotelic animals.

Cockroach is uricotelic.

Rennet and cob cells are present in free living nematodes and are excretory in nature.

Amoeba possesses a contractile vacuole for excretion and osmoregulation. Undigested food is excreted by exocytosis. Some wastes and CO₂ are excreted through the plasma membrane by diffusion.

Porifers and coelenterates have a specific canal system in tissues from which the waste matter diffuses to external medium.

Excretion in platyhelminthes takes place through flame cells.

Annelids have nephridia as excretory organs.