Q.Differentiate protonephridia from metanephridia

What animals excrete, and why it varies

Ammonia produced from breaking down amino acids is toxic and must be removed from the body, but how an animal removes it depends on how much water it can afford to spend.

• Ammonotelism - excreting nitrogen mainly as ammonia. Ammonia is very soluble and needs a large volume of water to flush safely, so it suits animals surrounded by water: most bony fish, aquatic amphibians and aquatic insects diffuse it directly across gills or body surface.
• Ureotelism - excreting nitrogen mainly as urea, made in the liver by the ornithine (urea) cycle. Urea is far less toxic and less soluble than ammonia, so it can be stored safely and voided with a moderate amount of water. Mammals and terrestrial amphibians are ureotelic.
• Uricotelism - excreting nitrogen mainly as uric acid, which is nearly insoluble and can be excreted as a semi-solid paste with almost no water loss. Birds, most reptiles, land snails and insects are uricotelic - a key adaptation for dry habitats and shelled eggs.

Some animals switch: earthworms are ureotelic on land but ammonotelic in water.

Excretory organs across the animal kingdom …