A lizard is definitively not an amphibian; it belongs to the Class Reptilia, along with snakes, turtles, and crocodiles. Their biological adaptations reflect two distinct paths for life on land. Understanding the differences between the classes Amphibia and Reptilia clarifies why the lizard is classified as a reptile.
Lizards Belong to the Class Reptilia
Lizards are members of the Class Reptilia. Their skin is covered in scales composed of beta-keratin, a tough protein. This dry, scaly skin acts as a waterproof barrier, preventing water loss and enabling reptiles to thrive in arid environments.
Reptiles are also defined by their reproductive strategy, which involves the amniotic egg. This egg possesses a protective shell—either leathery or hard—and specialized internal membranes that encase the developing embryo in a fluid-filled sac. This allows the embryo to develop entirely on land, free from the need for an external water source for reproduction. Respiration in lizards and other reptiles relies solely on lungs.
Defining Features of Amphibians
The Class Amphibia is named for its “double life,” reflecting a dependency on both aquatic and terrestrial habitats. Amphibian skin is moist, smooth, and highly permeable, lacking the protective keratinized scales found on lizards. This thin, porous skin allows for cutaneous respiration, supplementing lung or gill breathing.
Amphibians exhibit a biphasic life cycle. The life typically begins as an aquatic larva, such as a tadpole, which breathes using gills. The larva then transforms into a terrestrial adult with lungs, though many adults still require a moist environment. Reproduction almost always requires water, as their eggs are gelatinous, lack a shell, and would quickly desiccate on dry land.
Key Biological Differences Separating the Classes
The most significant distinction between reptiles and amphibians lies in their adaptations for water conservation and reproduction. Reptiles possess dry, scaly skin that minimizes water loss, allowing them to be fully independent of water for daily survival. In contrast, the moist, permeable skin of amphibians makes them highly susceptible to dehydration, forcing them to remain near water or in humid conditions.
Reproductive differences solidify the separation. Reptiles reproduce via internal fertilization and lay amniotic eggs on land, which provide protection for development. Amphibians generally use external fertilization, laying soft, jelly-coated eggs directly in water, and their young must undergo metamorphosis. Furthermore, reptiles rely exclusively on lungs for respiration, while amphibians utilize a combination of lungs, gills (in the larval stage), and their highly functional skin for gas exchange.