Modes of Excretion

Every animal cell must get rid of ammonia, the toxic by-product of breaking down amino acids for energy. How an animal does this depends heavily on how much water it has to spare. Ammonia itself is highly soluble and easy to release, but it needs a large volume of water to wash it away safely, so animals that excrete mostly ammonia (ammonotelic animals, such as most bony fish and aquatic amphibians and insects) simply diffuse it out across gills or body surfaces into the surrounding water. Animals that cannot afford that much water loss convert ammonia into a less toxic, more water-economical form. Terrestrial mammals and amphibians convert it into urea (ureotelic excretion) - urea is far less toxic than ammonia and can be safely stored in blood and tissues before being voided in a moderate volume of water. Birds, reptiles, land snails and insects go a step further and convert it into uric acid (uricotelic excretion), which is nearly insoluble and can be excreted as a semi-solid paste with almost no water loss at all - a crucial adaptation for animals that lay shelled eggs or live in dry habitats. Interestingly, some animals switch strategies with their environment: earthworms excrete urea while in soil but switch to ammonia excretion when submerged in water. Excretory organs also range enormously in complexity. Simple invertebrates use flame cells (in flatworms such as tapeworms) or solenocytes (in Amphioxus) as protonephridia, or the more advanced tubular metanephridia of earthworms and molluscs; insects use Malpighian tubules that drain directly into the gut, and crustaceans such as prawns use antennal (green) glands. Vertebrates instead evolved a single complex organ, the kidney, whose filtering unit is the nephron. Even among vertebrates the nephron itself varies with habitat: reptiles have a reduced or absent glomerulus and no loop of Henle, so the …