These four orders are then subdivided into several subclasses which are based on the anatomy of their skulls. The 5 subclasses are determined by the openings in the skull and where they are located, and how many exist. Today s reptiles fall into only 2 of the 5 subclasses and science has determined that many more forms of reptile existed in prehistoric times that we may never discover. There are over 6,500 species of living reptiles on the Earth today. Most of the lizards on the Earth today are smaller than their predecessors, with the exception of crocodiles, which can grow as long as 30 feet, as well as some species of snakes that grow up to 30 feet, a few giant tortoises that grow up to 6 feet and the giant komodo dragon which can grow as big as 11 feet.
Reptiles are cold-blooded, scaly skinned vertebrates.
Their hearts have three or four chambers, and, like humans, they breathe through lungs. They breathe through their lungs throughout their life, unlike the amphibian, who begins life breathing through gills which may later develop into lungs. This is one of the major differences in reptiles and amphibians. Most reptiles, are considered to be tetrapods, or 4 legged animals. Though snakes are the exception to this, it is believed that the snake evolved from a reptile who had four legs.