Turtle Anatomy

At first glance the anatomy of a turtle may appear to be simple, however, underneath of its shell the turtle can be fairly complex.

 

 

 

 

 

 

 

 

 

 

• Head: foremost part of a tortoise which is attached to the trunk
• Neck: part of a tortoise between the head and the body 
• Nuchal shell: hard plate that forms the shell of the tortoise close to the nape of the neck
  
• Vertebral shell: scale above the tortoises spinal column
  
• Costal shell: side scale
  
• Pygal shield: scale situated above the tail
  
• Tail: extension of the spinal column of a tortoise
  
• Nail: nail of a tortoise
  
• Hind leg: rearmost limb of the tortoise
  
• Marginal shell: each of the scales that form the edge of the tortoise’s shell
  
• Front leg: foremost limb of the tortoise
  
• Mandible: lower jaw
  
• Mouth: entrance to the digestive tract
  
• Nostril: entrance to the respiratory system
  
• Eye: sight organ of the tortoise

 

 

Turtle Shell

The turtle has a very unique appearance that can be easily identified trough the presence of its shell. The turtle shell plays a vital role in protection  and very little about its form or function has changed over the 200 million years of the turtle’s evolution.

There are two main parts of a turtle’s shell. The upper shell of the turtle is called the carapace and the lower shell that encase 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 include portions of the backbone and the ribs. That means the turtle cannot crawl out of its shell.

In most turtles, the outer layer of the shell is covered by horny scales called scutes that are part of its outer skin – epidermis. Scutes are made of keratin (the primary substance in hair, nails and hooves of other animals). The pigment melanin, present in the scutes, can form different designs and brightly colored patterns in some species.

Some turtles do not have horny scutes. Like the leatherback sea turtle and the soft-shelled turtles have shells covered with leathery skin instead of horny scutes.

Although the scutes form the familiar outer layer of the shell, it is the bony layer underneath which actually provides the shape, support and protection of the turtle shell.

Baby turtles shells are much like human babies skulls. When born the plates are not fused or connected in any way. Nonetheless, over time the bone like plates will grow together and once the turtle has reached its maximum size the plates will begin to fuse together. Keratin continues to grow and layer over top of the plates throughout the turtles life. The continuous growth of keratin aids the turtle in its amazing regenerative abilities. If a section or part of a shell were harmed or damaged in any way the other areas of the shell would continue to grow and the hurt portion would possibly heal over time.

Tortoises, being land-based, have rather heavy shells. In contrast, aquatic and soft-shelled turtles have lighter shells that help them avoid sinking in water and swimming faster with more agility. These lighter shells have large spaces called fontanelles between the shell bones. The shells of leatherback sea turtles are extremely light because they lack scutes and contain many fontanelles.

The color of a turtle’s shell may vary. Turtle shells are commonly colored brown, black, or olive-green. In some species, shells may have red, orange, yellow, or grey markings and these markings are often spots, lines, or irregular blotches. One of the most colorful turtles is the eastern painted turtle which includes a yellow plastron and a black or olive shell with red markings around the rim.

The shape of the shell gives helpful clues to how a turtle lives. Most tortoises have a large, dome-shaped shell that makes it difficult for predators to crush the shell between their jaws. One of the few exceptions is the African pancake tortoise, which has a flat, flexible shell that allows it to hide in rock crevices. Most aquatic turtles have flat, streamlined shells which aid in swimming and diving. American snapping turtles and musk turtles have small, cross-shaped plastrons that give them more efficient leg movement for walking along the bottom of ponds and streams.

Although the shell provides excellent protection from many predators, it can also make the turtle vulnerable to health problems if it is not given good care in captivity.

 

 

Head

Most turtles that spend most of their lives 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.

 

 

Limbs

Terrestrial tortoises have short, sturdy feet. Tortoises are famous for moving slowly, in part because of their heavy, cumbersome shell, which restricts stride length.

The amphibious turtles normally have limbs similar to those of tortoises, except the feet are webbed and often have long claws. These turtles swim using all four feet in a way similar to the dog paddle, with the feet on the left and right side of the body alternately providing thrust. Large turtles tend to swim less than smaller ones, and the very big species, such as alligator snapping turtles, hardly swim at all, preferring to simply walk along the bottom of the river or lake. As well as webbed feet, turtles have very long claws, used to help them clamber onto riverbanks and floating logs, upon which they like to bask. Male turtles tend to have particularly long claws, and these appear to be used to stimulate the female while mating. While most turtles have webbed feet, some, such as the pig-nosed turtle, have true flippers, with the digits being fused into paddles and the claws being relatively small. These species swim in the same way as sea turtles.

Sea turtles are almost entirely aquatic and have flippers instead of feet. Sea turtles fly through the water, using the up-and-down motion of the front flippers to generate thrust; the back feet are not used for propulsion, but may be used as rudders for steering. Compared with freshwater turtles, sea turtles have very limited mobility on land, and apart from the dash from the nest to the sea as hatchlings, male sea turtles normally never leave the sea. Females must come back onto land to lay eggs. They move very slowly and laboriously, dragging themselves forwards with their flippers.

 

 

Skin and molting

As mentioned before, the outer layer of the shell is part of the skin; each scute (or plate) on the shell corresponds to a single modified scale. The remainder of the skin is composed of skin with much smaller scales, similar to the skin of other reptiles. Turtles do not molt their skins all at once, as snakes do, but continuously, in small pieces. When kept in aquaria, small sheets of dead skin can be seen in the water (often appearing to be a thin piece of plastic) having been sloughed off when the animal deliberately rubs itself against a piece of wood or stone. Tortoises also shed skin, but a lot of dead skin is allowed to accumulate into thick knobs and plates that provide protection to parts of the body outside the shell.

By counting the rings formed by the stack of smaller, older scutes on top of the larger, newer ones, it is possible to estimate the age of a turtle, if one knows how many scutes are produced in a year. This method is not very accurate, partly because growth rate is not constant, but also because some of the scutes eventually fall away from the shell.

 

 

• Trachea: the windpipe, which connects the larynx and bronchi of the tortoise
• Lung: respiratory organ of the tortoise
• Stomach: part of the digestive tract of the tortoise between the esophagus and the intestine
• Pancreas: gland in the tortoise that produces digestive enzymes
• Rectum: final part of the tortoise’s digestive tract
• Anus: outlet of the tortoise’s digestive tract
• Bladder: pocket in which urine collects before it is evacuated
• Ovary: egg-producing reproductive gland
• Intestine: part of the digestive tract of the tortoise after the stomach
• Liver: bile-producing digestive gland
• Heart: blood-pumping organ of the tortoise
• Esophagus: part of the digestive tract of a tortoise between the mouth and the stomach

 

 

 

Respiratory System

The rigid shell means turtles cannot breathe as other reptiles do, by changing the volume of their chest cavity via expansion and contraction of the ribs. Instead, turtles breathe in two ways. First, they employ buccal pumping, pulling air into their mouth, then pushing it into the lungs via oscillations of the floor of the throat. Secondly, by contracting the abdominal muscles that cover the posterior opening of the shell, the internal volume of the shell increases, drawing air into the lungs, allowing these muscles to function in much the same way as the mammalian diaphragm.

Mouth, Pharynx and Cloaca
Food, water and other nutrients are ingested into two main cavities in most turtles, the mouth and the pharynx. These two cavities are located inside of a turtles skull. However, sea turtles also have the ability to take in oxygen through their cloaca. This special feature is used in extreme situation typically to keep the turtle alive when oxygen levels are low (i.e. deep sea diving, hibernation).

Glottis and Larynx
The glottis of the turtle is a small opening positioned behind the tongue that acts as a barrier between the pharynx and the larynx when swimming underwater, diving or eating. The larynx is connected to the glottis and leads to the trachea. It is considered to be the upper most portion of the respiratory duct.

If you have ever heard a turtle hiss and wonder if it means that they are mad…rest assured, they are not mad. They are just frightened. So when they are frightened, and they need to pull their head and legs into their shell, they won’t quite fit in because of full inflated lungs. What they do is expel the air within their lungs out as fast as they can so that they will have room for their appendages to fit into the security of their hard shells. With this rush of air leaving their bodies, it makes a hissing noise and leads one to believe the turtle is ticked off and warning them to stay away.

 

 

Digestive system

Most vertebrates have similar digestive systems and the turtle is no exception. Turtles are not strictly herbivores. All species are found to eat at least some meat which causes them to have powerful digestive enzymes. In addition turtles swallow their food with very little chewing. Food particles are often whole or in fairly large chunks. The salivary glands of the turtle help to soften and break down the food to make swallowing possible.

Mouth and Tongue
The mouth of a turtle is a cavity found inside its skull. The tongue is flat and wide and fastened securely to the bottom of its mouth to prevent it from moving.  Air, water, food and other essential nutrients enter the turtle’s body most often by way of the mouth.

Esophagus and Stomach
The esophagus is tubular, digestive structure that is connected to the stomach. Powerful digestive enzymes and acids within the turtle’s stomach decompose the food. The walls of the organs in the digestive system are composed of smooth muscle that helps to push the food through the system and aids in decomposition of the food by churning during the digestive process.

Liver, Gall Bladder and Pancreas
The liver is the largest organ in the body of a turtle. The liver has numerous functions and capabilities among which one is bile production. The gall bladder, on the other hand,  is a small organ hidden behind the liver. It transfers the bile produced in the liver to the small intestine in the digestive process. Finally, the pancreas sliver like gland located next to the small intestine. It aids in the digestive process by introducing digestive enzymes into the small intestine as well.

Small and Large Intestine
The small intestine is connected from the stomach to the large intestine. Villi (or villus singular) located on the small intestine wall absorb food into the body. The large intestine, which is also known as the colon, reabsorbs excess waste and water produced by the digestive system.

Anus
The last part of the digestive system, the anus is where the feces (or wastes) exit the turtle’s body.

 

 

 

 

 

• Skull: bony case of the brain of the tortoise
• Phalanges: small bones forming the fingers
• Humerus: arm bone
• Proscapular process: bone of pectoral girdle of a tortoise, situated in front of the coracoid
• Back bone: vertebral column of a tortoise
• Femur: thigh bone
• Tibia: one of the two leg bones
• Phalanges: small bones forming the toes
• Fibula: one of the two leg bones
• Pelvic girdle: set of bones to which the limbs of a tortoise are attached
• Coracoid: bone of the pectoral girdle of a tortoise
• Scapula: shoulder bone
• Radius: one of the two bones of the forearm
• Ulna: one of the two bones of the forearm
• Vertebra: each of the bones forming the spine of a tortoise
• Mandible: lower jaw of the tortoise

 

 

 

Skeletal system

Bones make up the majority of the skeletal system in turtles as opposed to amphibians who have a large amount of cartilage in their system. Connective tissue in the turtles is mineralized and becomes bone and the interior of their bones consists of sponge like marrow.

The turtle skeleton is divided into two main sections, the endoskeleton and the ectoskeleton. The endoskeleton consists of all the internal bones and the ectoskeleton of a turtle is its shell. The endoskeleton is further divided into  two subsections called the axial skeleton and the appendicular skeleton. The axial skeleton is made up of the skull and both the cervical and thoracic vertebrae. The appendicular skeleton on the other hand consists the remaining bones in the skeleton.

 

 

Nervous system

The nervous system of the sea turtle is composed of the brain, nerves and spinal cord. In addition, specialized cells called neurons are the signal transmitters throughout the system. The brain is the center of turtle’s nervous system and it is there that the impulses carried by the nerves from the sensory organs are processed.  Although the brain of turtles is more advanced than an amphibian’s it is primitive in regards to birds and mammals. The spinal cord of the turtle extends down it’s back and is protected by the carapace vertebrae.  It is through the spinal cord that the information is carried to and from the brain. Turtles exhibit all the same senses as most organisms but the sense of smell is most advanced.