Reptiles have been on earth for 340 million years and its largest species, the dinosaurs, of course are now extinct. There are four orders of reptiles: crocodylia, chelonian (or testudines), squamata and sphenodontia. Crocodylia includes 23 species of crocodiles and alligators. Chelonians consist of 300 types of turtles and tortoises. The squamata order has more than 7,000 species of snakes and lizards. The smallest group is sphenodontia, which has two species of tuataras (large lizards) found in New Zealand.
Amphibians are animals who can live on land and in water. The three orders of amphibians are anura, salamanders (or caudata) and caecilians (or gymnophiona or apoda). The anura order contains 5,000 species of tailless frogs and toads. The salamander order contains 500 species of amphibians with tails. Caecilians are limbless, simple-structured amphibians that are similar to worms and snakes.
Reptiles, with the exception of snakes, generally have four legs, tails and are covered with protective, dry scales. They are thought to be the first animals with backbones to live on land. Reptiles are distinguished from amphibians by their well-developed lungs and their hard, protective shells. They lay their hard-shelled eggs on land.
Amphibians are smooth-skinned creatures that require frequent emersion in water to survive. They are active during the cool periods of the day and thrive in humid climates or near bodies of water. When in water they breathe through their skin. When on land, they use their lungs and gills to breathe but can still draw up to 50 percent of their oxygen and exhale 75 percent of their carbon dioxide through their skin. Amphibians lay their soft-shelled eggs in water.
Amphibians and reptiles have several traits in common. They are both cold-blooded animals that are unable to maintain their own body temperatures. They depend on external temperatures in order to keep their bodies cool or warm. The other common trait they have is that they all have backbones, and thus are both species of vertebrates. Both also have effective self-defense mechanisms in the form of strong biting instincts and toxic skin secretions or poisons injected through biting.