Sea turtles
Sea turtles are ancient creatures. They have traveled our planet for more than 200 million years, tracing a highly successful evolutionary path, living in a variety of environments from dry land to the open sea. This makes them fascinating, albeit tough, subjects to study. Scientists have uncovered precious few secrets of the turtles' life in the sea. Nevertheless, what they have already learned provides some valuable clues to understanding these intriguing animals and their important part in both land and marine ecosystems.

The Turtle Family Tree
In order to understand better how different life forms are related, biologists classify them into a hierarchical system, from Kingdom to species.
Of the 16 Orders of reptiles that evolved during the last 310 million years, that of the Testudines, or turtles, is one of only four that remain today (the three other Orders are Crocodilia (alligators, caimans and crocodiles), Squamata (snakes and lizards) and Rhyncocephala (Tuataras).
Turtles, terrapins and tortoises all belong to the Order Testudinata (also called Chelonia), which comprises two Sub-Orders, namely Cryptodira and Pleurodira. The Pleurodires (2 Families) are also known as "side-necks" because they curl their necks into a horizontal S-shape when retracting their heads into their shells. The Sub-Order Cryptodira, or the straight-necked turtles, bend their necks in a vertical S when they pull their heads into the shell. The Cryptodira is the largest Sub-Order, comprising 11 Families, which include 2 Families of living sea turtles. Of the more than 220 species of living turtles, only 7 are marine.

Green Turtle
Chelonia mydas, commonly known as the green turtle is a large sea turtle belonging to the family Cheloniidae. It is the only species in the genus Chelonia. The range of the species extends throughout tropical and subtropical seas around the world, with two distinct populations in the Atlantic and Pacific Oceans.
The green turtle is the quintessential sea turtle, possessing a dorsoventrally-flattened body covered by a large, teardrop-shaped carapace and a pair of large, paddle-like flippers. Despite the turtle's common name, it is lightly-colored all around while its carapace's hues range from olive-brown to black in Eastern Pacific green turtles. The turtle is actually named for the greenish coloration of its fat and flesh. Unlike other members of its family such as the hawksbill and loggerhead turtles, Chelonia mydas is mostly herbivorous. The adults are commonly found in shallow lagoons, feeding mostly on various species of seagrass.
Like other sea turtles, green turtles are known to migrate long distances between their feeding grounds and the beaches they hatched from. Many islands worldwide have been called Turtle Islands primarily for the large amounts of green turtles that nest on their beaches each year. Female turtles dredge themselves onto beaches and lay eggs in nests that they dig during the night. After a period of time, hatchlings emerge from the nests and head for the water. Those that survive grow to maturity and live to a maximum of eighty years.