40.Reptiles
Reptiles include snakes, lizards, crocodilians, and turtles. Reptiles are cold-blooded. (They depend upon their environment to provide warmth.) Most reptiles are egg-layers, although some produce live young. In cold regions, reptiles hibernate. In extremely hot or dry climates, some reptiles will estivate, or go into a torpor. Because they lack intenal heating mechanisms, reptiles depend on external heat sources. Thus, you will often find them lying in the sun on rocks and logs. Most reptiles have a three-chambered heart.
Many reptiles besides Milk Snakes look different in different areas and have many subspecies. Other examples are the Common Kingsnake, Eastern Racer, Eastern Rat Snake, Ringneck Snake, Coachwhip, Common Garter Snake, Western Whiptail Lizard, Fence Lizard, Diamondback Terrapin, Eastern Box Turtle, and Painted Turtle. One subspecies of Milk Snake, called the Eastern Milk Snake, is tan with brownish-red blotches, while another subspecies, called the Scarlet Kingsnake, has black, red, and yellow rings. These two subspecies of the Milk Snake look very different, but they can breed with each other and produce young.
Multicellular animals first appeared about 600 million years ago in the early Paleozoic and there was a rapid rise in number of families during the Cambrian and Ordovician. Diversity remained relatively constant (perhaps even declining) up until about 200 million years ago and then it rose again to its current all-time high of close to 800 families.Reptiles such as the large sail-backed carnivore Dimetrodon, were common at this time. Some of these early reptiles had elaborate sails on the back. The purpose of these structures is not known although they are thought to have been involved in temperature regulation; that is, to radiate heat when the animal was too hot and to absorb it when the animal was too