Turtle

Scientific classification
Kingdom:Animalia
Phylum:Chordata
Subphylum:Vertebrata
Class:Reptilia
Order:Testudines

Turtles are reptiles of the order Testudines characterised by a special bony or cartilaginous shell developed from their ribs that acts as a shield. "Turtle" may either refer to the Testudines as a whole, or to particular Testudines which make up a form taxon that is not monophyletic—see also sea turtle, terrapin, tortoise, and the discussion below.
The order Testudines includes both extant and extinct species. The earliest known turtles date from 215 million years ago, making turtles one of the oldest reptile groups and a more ancient group than lizards and snakes. About 300 species are alive today, and some are highly endangered.
Like other reptiles, turtles are ectotherms—varying their internal temperature according to the ambient environment, commonly called cold-blooded. Like other amniotes (reptiles, dinosaurs, birds, and mammals), they breathe air and do not lay eggs underwater, although many species live in or around water. The largest turtles are aquatic.

Anatomy and morphology
The largest chelonian is the great leatherback sea turtle, which reaches a shell length of 200 centimetres (6.6 ft) and can reach a weight of over 900 kilograms (2,000 lb). Freshwater turtles are generally smaller, but with the largest species, the Asian softshell turtle Pelochelys cantorii, a few individuals have been reported up to 200 centimetres (6.6 ft). This dwarfs even the better-known Alligator Snapping Turtle, the largest chelonian in North America, which attains a shell length of up to 80 centimetres (2.6 ft) and a weight of about 60 kilograms (130 lb). Giant tortoises of the genera Geochelone, Meiolania, and others were relatively widely distributed around the world into prehistoric times, and are known to have existed in North and South America, Australia, and Africa. They became extinct at the same time as the appearance of man, and it is assumed that humans hunted them for food. The only surviving giant tortoises are on the Seychelles and Galápagos Islands and can grow to over 130 centimetres (51 in) in length, and weigh about 300 kilograms (660 lb).
The largest ever chelonian was Archelon ischyros, a Late Cretaceous sea turtle known to have been up to 4.6 metres (15 ft) long.
The smallest turtle is the Speckled Padloper Tortoise of South Africa. It measures no more than 8 centimetres (3.1 in) in length and weighs about 140 grams (4.9 oz). Two other species of small turtles are the American mud turtles and musk turtles that live in an area that ranges from Canada to South America. The shell length of many species in this group is less than 13 centimetres (5.1 in) in length.

Neck folding
Turtles are broken down into two groups, according to how they evolved a solution to the problem of withdrawing their neck into their shell the Cryptodira, which can draw their neck in while contracting it under their spine; and the Pleurodira, which contract their neck to the side.

Head
Most turtles that spend most of their life on land have their eyes looking down at objects in front of them. Some aquatic turtles, such as snapping turtles and soft-shelled turtles, have eyes closer to the top of the head. These species of turtles can hide from predators in shallow water where they lie entirely submerged except for their eyes and nostrils. Sea turtles possess glands near their eyes that produce salty tears that rid their body of excess salt taken in from the water they drink.
Turtles are thought to have exceptional night vision due to the unusually large number of rod cells in their retinas. Turtles have color vision with a wealth of cone subtypes with sensitivities ranging from the near Ultraviolet (UV A) to Red. Some land turtles have very poor pursuit movement abilities, which are normally reserved for predators that hunt quick moving prey, but carnivorous turtles are able to move their heads quickly to snap.
Turtles have a rigid beak. Turtles use their jaws to cut and chew food. Instead of teeth, the upper and lower jaws of the turtle are covered by horny ridges. Carnivorous turtles usually have knife-sharp ridges for slicing through their prey. Herbivorous turtles have serrated-edged ridges that help them cut through tough plants. Turtles use their tongues to swallow food, but they cannot, unlike most reptiles, stick out their tongues to catch food.