All reptiles grow throughout their lives and are ectothermic, or dependent on external heat for regulating body temperature. Most reptiles lay tough shelled eggs in pits dug in the soil, and do not nurture their young. Chelonia contains 230 species of turtles, tortoises and terrapins. Even though we commonly call all shelled reptiles turtles, turtles are actually aquatic reptiles with shells, tortoises are land reptiles with shells, and terrapins are shelled reptiles that live both on land and in water. The chelonian shell is made of modified scales and bones with the spine and most of the ribs connected to it.
Chelonians also have jaws equipped with horny beaks, instead of teeth.
Twenty-one species of crocodilians are found worldwide in the tropics and subtropics. This group consists of crocodiles, alligators, caiman, and gavials. These large, well-armored animals have sculptured heads, protruding eyes and nostrils, and heavily muscled tails. Crocodililans are able to close off the ear canals and throat with valves. These adaptations come in handy for attacking prey underwater.