Reptilia
Most reptiles can be classified into three large groups: the turtles (order Chelonia), the snakes and lizards (order Squamata), and the alligators and crocodiles (order Crocodilia). Most reptiles share a number of general morphological features. In general, reptiles are lung-breathing vertebrates with two pairs of limbs and a horny, scaly skin. Reptiles are amniotes, which means that their large, yolky eggs have a protective layer called an amnion which prevents them from drying out on land. Rather than laying eggs, some snakes and lizards bear their young live.
Nonavian reptiles are poikilothermic (cold-blooded) creatures, which means that they derive their body heat from external sources (in contrast to homothermic animals that maintain a constant body temperature through internal mechanisms). Contrary to popular belief, the "cold-bloodedness"of reptiles does not mean that they maintain low body temperatures. Reptiles control their body temperature through a process called thermoregulation, and their internal temperature can fluctuate greatly according to their surroundings. Researchers have found that many reptiles exert precise control over body temperature by moving around to different areas within their surrounding habitat.
Chelonia
Turtles are classified into two sister groups, Pleurodira and Cryptodira. Pleurodires (side-necked turtles) bring their head and neck against their body by bending the neck to the side.
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