They are one of the few species so ancient that they watched the dinosaurs evolve and become extinct. In addition to these reptilian traits, all species of turtles have evolved a bony outer shell which protects them from predators, as turtles are not known for their speed. The dorsal portion of the shell is known as the carapace and is covered with large scale-like structures called scutes . The ventral portion of the shell is known as the plastron . The carapace and plastron are connected at the sides by hard-shelled plates known as lateral bridges .
Openings exist between the carapace and plastron for the head, tail, and limbs.
While most species of land turtles and tortoises are able to retract their heads into their shells for added protection, sea turtles are not able to do so, and their heads remain out at all times. The sea turtle's body is wonderfully adapted to life in the ocean. Their shells are lighter and more streamlined than those of their terrestrial counterparts, and their front and rear limbs have evolved into flippers making them efficient and graceful swimmers, capable of swimming long distances in a relatively short period of time. Sea turtles have been known to move through the water as fast as 35 mph. When active, sea turtles swim to the surface every few minutes in order to breathe.