Nitrogenous wastes and excretory organs

Animals build up wastes they must remove: ammonia, urea and uric acid (from breaking down proteins), and also CO₂, water and extra ions. The three nitrogen wastes differ in how poisonous they are and how much water is needed to flush them out.

• Ammonia: the most toxic; needs a lot of water to remove.
• Urea: less toxic; needs less water.
• Uric acid: the least toxic; needs the least water and can be removed almost dry.

So an animal's waste depends on how much water it can afford to lose:

• Ammonotelic animals excrete ammonia: many bony fishes, aquatic amphibians and aquatic insects. Living in water, they can let ammonia simply diffuse out through the body surface or gills as ammonium ions; the kidneys play little part. (That is why a fish pond or aquarium can smell of ammonia.)
• Ureotelic animals excrete urea, made from ammonia in the liver: mammals, many land amphibians and marine fishes. Some urea is kept in the kidney to help it work.
• Uricotelic animals excrete uric acid as a pellet or paste, saving water: reptiles, birds, land snails and insects.

Different animals have different excretory organs:

• Protonephridia (flame cells): in flatworms like Planaria, rotifers, some annelids and Amphioxus. Their main job is to balance water and salts (osmoregulation).
• Nephridia: earthworms and other annelids.
• Malpighian tubules: most insects, including cockroaches.
• Antennal glands (green glands): crustaceans like prawns.
• Kidneys: all vertebrates.