Florida Box Turtle Terrapene carolina|
| Chelonia mydas in Kona, Hawaii.|
A turtle with eyes closer to the end of the head. Keeping only the nostrils and the eyes above the water surface.| 1. | Snapping Turtle Tail. Blue Hills Reservation, Massachusetts| "Chelonia" (Testudines) from Ernst Haeckel's Kunstformen der Natur, 1904| The Western Hermann's Tortoise (Testudo hermanni hermanni) is a cryptodire.Cryptodires hide their head inwards.| The Western Hermann's Tortoise (Testudo hermanni hermanni) is a cryptodire.Cryptodires hide their head inwards.| Turtles are reptiles of the order Testudines (the crown group of the superorder Chelonia), 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 (living) 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. Of the many species alive today some are highly endangered.Like other reptiles, turtles are ectotherms—varying their internal temperature according to the ambient environment, commonly called cold-blooded. However, leatherback sea turtle have noticeably higher body temperature than surrounding water because of their high metabolic rate.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. Neck foldingTurtles are broken down into two groups, according to how they evolved a solution to the problem of withdrawing their neck into their shell (something the ancestral Proganochelys could not do): the Cryptodira, which can draw their neck in while contracting it under their spine; and the Pleurodira, which contract their neck to the side. HeadMost 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. ShellThe upper shell of the turtle is called the carapace. The lower shell that encases the belly is called the plastron. The carapace and plastron are joined together on the turtle's sides by bony structures called bridges. The inner layer of a turtle's shell is made up of about 60 bones that includes portions of the backbone and the ribs, meaning the turtle cannot crawl out of its shell. In most turtles, the outer layer of the shell is covered by horny scales called...