Turtle Anatomy
Turtle Terminology
There are a variety of unique terms used to refer to turtles and their anatomy - here's what some of them mean:
Chelonian: a member of the order of reptiles called Chelonia, made up turtles.
Tortoise: common term for primarily terrestrial turtles.
Terrapin: common term for largely aquatic turtles.
Side-necked turtles: group of turtles which lack the ability to pull their heads back into their shells; they retract their heads under their shells by turning them sideways under the margin of their shells.
Anatomy
Scute: large scale like structures made up of keratin (like fingernails) that cover the shells of turtles. They do have nerve endings, and the turtle can detect something touching its shell. Turtles can regenerate damaged scutes, and on some turtles the have rings similar to growth rings on trees and can be used to estimate age.
Plastron: The ventral (bottom) portion of a turtle's shell (under the abdomen/chest). It is made up of bony plates and covered with scutes. In some turtles, the some of the bony plates making up the plastron are hinged.
Carapace: The dorsal (top) portion of the turtle shell (over the back). The carapace is also made up of bony plates, that are fused to the ribcage and backbone of the turtle. The seams of the bony plates do not necessarily coincide with the seams between scutes.
Cloaca: common opening of the digestive, urinary and reproductive tracts.
Box turtles share a physical feature with all other turtles and tortoises; they have a shell that protects their soft organs. Box turtles can also retract their head and limbs into their shells and close the shell for added protection. There are two parts to the shell. The top part of the shell, or the carapace, is a dome made from the actual back bones and ribs, which have expanded and fused together. The shoulders and pelvic bones are within the ribcage instead of outside it, as is the case with humans and other animals with arms and legs. The lower part of the shell is called the plastron and connects to the carapace on both sides by the bridge. The plastron has a hinge that divides the bottom shell into two halves which can be drawn up to the carapace and close the shell up like a box. Hatchling box turtles do not have a hinge, but develop one after a year or two.
The bony part of the shell is covered with a thin layer of keratin. The keratin layer is divided into segments called scutes. These scutes and the underlying bones grow incrementally, thus allowing the box turtle to expand and get larger. Turtles have had over 200,000,000 years to evolve this body type and its obviously served them well. Turtles have outlived the dinosaurs and many other prehistoric creatures to become one of the oldest living families in the animal kingdom.
There are several differences between turtle internal organs and what we are used to in mammals. For one thing, they have a three-chambered heart instead of four-chambered heart. Consequently, oxygenated blood from the lungs and veinous blood are in the heart chamber at the same time. However, a partial wall in the heart called an incomplete septum helps to keep the blood apart. Also, there is a pressure difference between the two blood types and little mixing occurs.
The location of internal organs are also different. The lungs, for example, are situated right under the bony shell and above all the other organs. Muscles in the abdomen push the other organs against the lungs and move them in and out like bellows.
The turtle brain has highly developed sight and smell centers, so we know turtles depend on them greatly. In fact, a turtle who has lost its sight or cannot smell due to pneumonia will not eat. These turtles may need force-feeding if their recuperation takes too long. The turtle brain can also withstand longer periods of oxygen depletion than mammal brains. For this reason never assume a box turtle that has been submerged in water to be dead. Try to resuscitate it by pumping its back legs in and out while it is lying on its plastron with its head pointed slightly downwards. The legs are used to push against the lungs and expel water.
Respiratory System
Oxygen is needed for metabolism, during which harmful carbon dioxide is produced. It is the job of the respiratory system to take in oxygen and dispose of carbon dioxide. Turtles have lungs and must breathe oxygen from the air. Even sea turtles, which are the most aquatic reptiles, must surface from time to time to get air. Marine turtles also inflate their lungs to keep themselves afloat when basking. Some turtles, such as softshell turtles, snapping turtles, and maybe marine turtles, can exchange gases through areas of their skin, cloaca, and throat. The amount of oxygen obtained is slight, but is enough to keep them alive during times of low oxygen need (such as hibernation).
Mouth and Pharynx
Cavities inside the skull where food, water, and air enter.
Glottis
Slit-like opening behind the tongue. It connects the pharynx and larynx. It closes when the turtle is underwater or eating.
Larynx
The upper end of the respiratory duct. It is connected to the glottis and leads into the trachea.
Trachea and Bronchii
The trachea is a long hollow tube between the lungs and the larynx. Halfway down, it divides into two tubes, called bronchii (singular: bronchus). One bronchus enters the left lung, the other the right lung.
Right and Left Lungs
Large, reddish, spongy organs lying underneath the carapace. Air passes from the bronchii into smaller tubes in the lungs called bronchioles. The bronchioles get progressively smaller until they end into a small cluster of air sacs. These air sacs, known as alveoli (singular: alveolus), located at the end of the bronchioles are where gases are exchanged. Oxygen from the air disolves into the blood, while carbon dioxide diffuses out of the blood into the air.
