Reptiles are scaly ectothermic creatures with backbones and a lung system, according to the National Wildlife Federation. More than 280 reptile species are found in the United States, and approximately 8,700 species have been identified across the globe.
The most common sub-classes of Reptilia include turtles, tortoises, crocodiles, alligators, snakes and lizards. As ectotherms, reptiles cannot regulate their own body heat and must derive warmth from environmental sources, Scholastic states. This characteristic is often described as being "cold-blooded," because a reptile's body temperature can fluctuate drastically in differing climates. Reptiles are diverse in size, ranging from tiny dwarf geckos to pythons as long as 33 meters.
All reptiles have vertebrae and can breathe air. Equipped with tough, dry scales, reptilian creatures are able to hold moisture in their skins and prevent dehydration in dry habitats, according to the Saint Louis Zoo. This thick layer of outer skin is shed multiple times during a reptile's life cycle, whether all at once or in flakes. Turtles and tortoises stand out from other reptilian species because of their hard protective shells made of strong bony structures.
In all reptile species, fertilization occurs internally, and developing offspring are protected by an amniotic egg that can survive on land, according to Scholastic. The embryo is surrounded by a resilient outer shell, an internal double-walled sac and amniotic fluid. Unlike many amphibians, reptiles are fully formed at birth and do not progress through any larval stages.