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Amphibians have aquatic, gill-breathing larvae and lung-breathing
adults; their eggs are laid in the water. Reptiles all have a type
of shell-covered egg that must be laid on land. Reptiles were the dominant
land animals of the Mesozoic Era; their descendants include the birds
and mammals.
Class Amphibia: Eggs are laid in contact with fresh water, then fertilized externally. Larvae ("tadpoles") breathe with gills, then undergo metamorphosis into an adult, usually with lungs and legs. Living species always have slippery, moist skin. Examples: salamanders, newts, frogs, toads, and extinct labyrinthodonts. Origin of amphibians: Amphibians evolved from a group of bony fishes (Osteichthyes) called Crossopterygians who already had lungs and internal nostrils. The critical change transformed the fleshy fins into walking legs. Class Reptilia (Sauropsida): Reptiles differ sharply from amphibians in laying a shell-covered type of egg (amniote or cleidoic egg). These eggs must be laid on land, or else hatched inside the females body. The shell, secreted by the female, prevents passage of liquids. Since the sperm must swim in a liquid medium, it must enter the egg first; this requires internal fertilization. Reptiles also have a dry, tough (leathery or scaly) skin. Legs are generally stronger than in amphibians; most toes have claws. Body temperature changes to match surroundings. Examples: turtles, snakes, lizards, dinosaurs, and crocodiles. Embryonic membranes: Reptiles, birds, and mammals all have amniote eggs within which several specialized membranes develop from the embryo:
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