Lack embryonic membranes-amnion, chorion, allantois. Lack outer protective shell. Eggs can desiccate, but are far more self contained, do not require water for hydration and removal of waste products to the extent that amphibian eggs do. C. Excretory System 1. Amphibians a. Kidney tubules in adults that are capable of reabsorbing water and solutes, but water reabsorption is inefficient.
Nitrogenous wastes are urea and ammonia.
Both are toxic and require large amounts of water for dilution during excretion. In general, kidney is better at ridding amphibian of excess water rather than in conserving water. More compact kidney, therefore more tubules for filtration and reabsorption. May excrete nitrogenous wastes as ammonia, urea, or uric acid. Less toxic than ammonia and urea, forms insoluble crystals that can be removed with little concomitant water loss.