This order consists of over 260 species through out the world reported from tropical beaches & was found 200 million years ago. Diversity among them is based on the habitat, variation in shell form & food habits. Marine forms are often known as turtles, terrestrial forms as tortoises & fresh water forms as terrapins. There are five marine turtles, three fresh water turtles & one land tortoise recorded in Sri Lanka. Most in this order show an amphibious life.
The most characteristic feature of Chelonian organization is the shortening & broadening of the body, with developed bony plates. The bony plates form a box like structure. The head & limbs can be withdrawn in to it when needed. The dorsal part of the shell is known as the carapace which is supported by the backbone & ventral part as the plastron. Each of these is made of inner plates of bone, covered by separate outer plates of horny material, comparable to the scales of other reptiles.
The carapace is made up of five rows of bony plates such as, median neural, paired costals, & marginals. The thoracic vertebrate & ribs are usually fused with bony carapace. The plastron is developed from the expanded dermal bones of the sternum. Laterally, the carapace & the plastron are hinged together. This arrangement makes the pectoral girdle to lie within the ribs rather than outside it. This is a unique condition shown by the Chelonians. The character of the ribs is that it does not take part in respiratory movements.
High dome shaped shell is often found in slow moving, heavy land tortoise. Slightly flattened shell belongs to turtles living on both water & land. Flattened shell is present in pond turtles allowing it to dive under water reaching the bottom of the pond. Streamline shell belongs to sea turtles helping them to swim. It contains fin like ridges at the edge of the shell.
Chelonian jaws contain horny sheaths but not teeth. Their coupulatory organ is unpaired. They are oviparous with hard shelled eggs. Cloacal opening is longitudinal.
The forelimb is modified in to a flipper in turtles to help them to swim. All fingers in them are attached by fold skin. Hind limbs are clawed. Head & neck are not completely retractile. Skull & the shell are light & porous. Tail is very short. Young are known to be carnivorous while the adults are herbivorous. Female turtles reach the beaches only in nesting season. They migrate a long way but reaches a particular location for egg laying. Leatherback turtle is the larges of living turtles, which can also be found in Sri Lanka.
Terrapins have no horny scales on the shell. It has a soft skin cover. Carapace & plastron are not joined either. Each limb bears three claws.
Tortoises have feet adapted to walk on land. Number of the phalanges is reduced in to two in each digit. Each digit terminates in a claw. Carapace is very convex & plastron is concave. They are known to be diurnal & herbivorous. But some times they also feed on worms & insects. They have a very long life span usually up to 100 years or more. Their age cannot be detected by the growth rings found in some turtle shells.
Killing of turtles & selling their eggs are banned in Sri Lanka. There is a conservation centre in hikkaduwa where they collect eggs to hatch & release off spring to the sea.