Turtles are members of the Phylum Chordata, Class Reptilia, and Order Testudines. Their unique shell (Figure 1), lack of teeth, and bony jaws, which are covered with a hard, keratinized beak somewhat like that of birds, make them unusual. A turtle shell has as many as 60 bones. It has two sections: a carapace, covering the animals back, and a plastron, covering its belly. The carapace and plastron are connected on the turtles right and left sides by a bony bridge, which is formed by extensions of the plastron. The shell is fashioned from bones originating in the skin, which fuse with one another as well as with the ribs, vertebrae, and parts of the shoulder girdle (Figure 2). In most species, large scales, called scutes, overlay the bones. However, in softshell turtles, a tough, leathery skin replaces the scutes.
Most Illinois turtles are able to withdraw their head and neck into the shell by bending the neck into a vertical S-shaped curve. In species such as box turtles and mud turtles, the plastron is hinged, allowing it to close on the carapace. This feature provides the animal with more complete protection. Turtles usually have prominent tails that vary in size with sex (tails of males are longer and heavier than those of females) and with species (snapping turtles have the longest tails of Illinois species). Turtles use their limbs to propel themselves in water as well as over land. The toes of most species are extensively connected by webbing, an adaptation that aids them in aquatic locomotion.
Showing Bone Structure (plastron removed)
Cross section of turtle showing relationship between skeleton and shell.