The document discusses how different animals excrete nitrogenous wastes like ammonia while balancing water levels. Small animals allow ammonia to diffuse through their skin, while larger ones have excretory systems. Mammals convert ammonia to urea, while birds and reptiles produce uric acid. Aquatic animals face challenges with water balance due to osmosis, while land animals conserve water through specialized structures and waste forms like uric acid pastes.

1. Lesson Overview 27.4 Excretion