Testudines, or Testudinata is the order of reptiles that includes turtle and tortoises.

Sometimes, testudines are called chelonians.

Turtles and tortoises can easily be recognized by their bony, protective shells, which have evolved from parts of their spinal columns and rib cages.

The largest of the testudines is the leatherback sea turtle (Dermochelys coriacea), which can be over 6 feet long and weigh over a ton.

The smallest of the testudines is the speckled padloper tortoise (Homopus signatus), which lives in South Africa. It is about 3 inches long and weighs about 5 ounces.

Most testudines have long lifespans. Some tortoises are recorded to have lived for over 150 years.

Turtles and tortoises do not have teeth. They use their upper and lower jaws, which are covered by horny ridges, to cut and chew their food.

Testudines have three-chambered hearts.

Some testudines hibernate.

Turtles and tortoises can be found all over the world, although most species live in the tropics.

Some testudines live entirely on land and some spend most of their lives underwater.

The earliest testudine fossils that we have found are about 215 million years old.

Fossils records show that human beings have been eating testudines for at least 2 million years.

Many species of testudine have become extinct. Now there are only about 260 living species of testudine.

There are two suborders of living testudines - the hidden-necks (Cryptodira) and the side-necks (Pleurodira).

As their name suggests, hidden-necks can retract their heads completely into their shells. They do this by bending their necks vertically and then drawing their heads directly under their spines.

A side-neck must bend its neck horizontally before it withdraws its heads into its shell.  After it retracts its head, some of its head and neck remain exposed.

Difference Between Turtles and Tortoises

The word "tortoise" is usually used to describe testudines that live on land and are adapted to dry environments.

Some people use the word "turtle" to mean only those testudines that usually live in or near water.

To other people, any testudine is a turtle. Therefore, a tortoise is a kind of turtle.

People sometimes called turtles that live on land "land turtles" and turtles that live in the ocean "sea turtles".

Testudines that spend time in both land and water, for example, next to ponds and rivers, are sometimes known as terrapins.

Shell

The shell of a turtle or tortoise is made up of about 60 bones and is divided into an upper section and a lower section.

The upper section is known as the carapace and the lower section is known as the plastron.

Bony structures known as bridges join the carapace and the plastron together at the testudine's sides.

The shells of most testudines are covered with horny scales that are known as scutes.

Turtles and tortoises use their shells to store calcium.

During hibernation, calcium ions that are stored in the shell are used to prevent metabolic acids from building up in the blood stream.

Some of the calcium that females store in their shells is used to make eggshells.

The leatherback sea turtle has a shell that differs from the shells of other testudines. It has a flexible shell that is made of tiny bones that are covered with leathery skin. It does not have scutes.

This soft shell allows the leatherback sea turtle to dive deep beneath the surface of the ocean without being crushed by the pressure of the water. If it had a heavy, inflexible shell like other turtles, it would be crushed to death.

Respiration

All testudines have lungs and breathe air.

Even turtles that spend most of their time underwater must come to the surface regularly in order to breathe.

Because a testudine's ribcage is rigid and is used to form its shell, a testudine cannot breathe by expanding and contracting its ribs, as most vertebrates do.

Turtles and tortoises breathe by relaxing and contracting respiratory muscles in their abdomens.

A testudine has one set of muscles that pulls its internal organs toward the front and rear of its shell. This increases the size of its body cavity, which causes air to be drawn into the lungs.

Another set of muscles causes the internal organs to be pushed against the lungs. This causes air to be expelled from the lungs.

Testudines also breathe using a technique known as buccal pumping. They draw air into their mouths and then push it into their lungs by repeatedly moving the floor of the throat up and down.

Some sea turtles have pairs of sacks in their cloacas (openings that are used for excretion and reproduction) that are known as cloacal bursae. These cloacal bursae are lined with many blood vessels.

Oxygen that is dissolved in the water is absorbed through these blood vessels, and carbon dioxide is released through them, so that the cloacal bursae behave like gills.

Other sea turtles have tongues that are covered with buds known as papillae, which are also full of blood vessels.  Gases in the water can be exchanged through these blood vessels, so that the papillae also function like gills.

Turtles that can filter dissolved oxygen from the water can spend more time underwater than turtles that breathe only through their lungs. However, turtles that can breathe oxygen that is dissolved in the water still must come to the surface to breathe air through their lungs in order to obtain enough oxygen to survive.

Reproduction

Testudines are oviparous - they lay eggs.

Their eggs have a leathery covering.

Turtles and tortoises usually bury their eggs in soil, sand, mud or rotting vegetation.

Some testudines lay their eggs in the open.

Testudines do not incubate their eggs or care for their young.

Testudines and other Reptiles

Testudines differ from all other living reptiles in that testudines do not have temporal fenestrae.

Temporal fenestrae are holes in the sides of the skull, either above or below the eyes.

Every other reptile either has two temporal fenestrae on each side of its skull, or has an ancestor with two temporal fenestrae on each side of its skull.

Amniotes that do not have temporal fenestrae, and whose ancestors did not have temporal fenestrae, are known as anapsids.

If an amniote has two temporal fenestrae on each side of its skull, or has ancestors with two temporal fenestrae on each side of its skull, it is known as a diapsid.

The only anapsids that are alive today are the testudines. However, there have been many species of anapsids on Earth that are now extinct.

Living diapsids include all reptiles besides turtles, as well as all birds.

Dinosaurs were diapsids.

Reptiles other than turtles are more closely related to birds and dinosaurs than they are to turtles.