Test: Modes of Excretion (NCERT) - NEET MCQ

Urea, uric acid and ammonia are known as nitrogenous wastes as they are produced during protein metabolism. Ammonia needs very large amount of water for excretion, second in order comes urea and then uric acid. Ammonia is highly toxic, then comes urea and then uric acid. Ammonia and urea are produced in the liver and uric acid is formed mostly in the liver and to some extent in kidneys.

Aquatic animals are ammonotelic as they excrete ammonia. Frog, amphibians and humans are ureotelic as they excrete urea. Pigeon, lizards and cockroach are uricotelic as they excrete uric acid.

In reotelic organisms, ammonia is produced by metabolism which then gets converted into urea. This urea is then excreted out by the kidneys and thus these animals are ureotelic.

Birds and insects excrete nitrogenous wastes as uric acid in the form of a pellet or paste with a minimum loss of water and are thus called uricotelic animals.

Reptiles, birds land snails and insects excrete nitrogenous wastes as uric acid in the form of white paste and thus they are called uricotelic animals. 

The principal nitrogenous excretory compound is humans is ammonia which is converted into such lies toxic urea in the liver but this urea is eliminated by the kidneys via urine.

Ammonia is the end product of protein metabolism. Ammonia diffuses through the cell membrane extremely fast because of its high water solubility and small molecular size.

Humans are ureotelic i.e., they excrete urea. Birds are uricotelic, bony fishes are ammonotelic, cartilaginous fishes and frogs are ureotelic.

Ammonia is the primary excretory material of many aquatic freshwater forms. It is most toxic waste material which requires large amount of water for its excretion.

• In earthworms, carbon dioxide and nitrogenous wastes are the main waste materials. The carbon dioxide is excreted out from the body through its moist skin by the process of diffusion. The nitrogenous wastes are excreted out of the body by special excretory organs called nephridia.
• In centipede, the excretory system consists of a pair of Malpighian tubules that run from the midgut to the hindgut. Waste exits the body through the anus at the posterior end.
• Excretory organs of prawns are known as green glands or antennal glands. They are called green glands for their colour and antennal gland for the location at the base of the second antennae.
• Invertebrates such as flatworms use a nephridium as their excretory organ. At the end of each blind tubule of the nephridium is a ciliated flame cell. As the fluid passes down the tubule, solutes are reabsorbed and returned to the body fluids.

So, the correct option is '(i) Nephridia (ii) Malpighian tubules (iii) Green gland (iv) Flame cell'.