Amphibians are primarily characterized by their dual life stages, which include both aquatic and terrestrial environments. Most amphibians, such as frogs, toads, and salamanders, begin their life cycle as eggs laid in water, where they hatch into larvae (tadpoles) that breathe using gills. As they mature, they undergo metamorphosis, developing lungs and limbs, allowing them to transition to land. This unique adaptability to both water and land distinguishes amphibians from other vertebrate groups. Additionally, their skin is highly permeable, which plays a critical role in respiration and moisture absorption, making them sensitive indicators of environmental health.