Anatomy. A brief look at the anatomical structure of a typical chelonian perhaps will help us to understand the creatures a little better. Theoretically, the turtle's anatomy is little different from ours; it has all the same parts, but they are put together in a different way! The most obvious part of a turtle is its shell, which most probably arose from a mixture of the rib cage and the original reptilian scaly skin. There are in fact two layers: the dermal shell, which is the well known collection of horny plates, scutes, or laminae that takes the place of normal scales in other reptiles, and the bony plates beneath, which are a fusion of ribs and vertebrae. In chelonians the conventional reptilian scales occur only on the head and limbs and other areas of exposed skin. The upper part or dome of the shell is known as the carapace, while that below is called the plastron. The carapace and plastron are joined by bridges that allow for spaces through which the head and limbs can be extended or retracted as necessary. In some species the plastron may be hinged at the front or rear so that the animal can, if it wishes, shut itself away completely from the outside world, as in the typical box turtles. One genus of tortoise, Kinixys, even has a hinged carapace that allows added protection for the hind limbs.
The laminae are arranged in a symmetrical pattern on the shell and seldom correspond to the bony plates beneath, thus giving added strength to the shell. Epidermal cells that lie between the two hard layers are the growing part of the shell. As the animal increases in size, new rings of horny material are added to the periphery of each lamina. Tortoises are unique among the vertebrates in that both the pectoral and pelvic girdles are contained within the rib cage, while the main vertebrae are fused to the inner bony plates of the carapace. There are eight vertebrae in the neck of most species, making the head extremely mobile. In the suborder Cryptodira the neck forms a vertical "S" as the head is withdrawn, while in the suborder Pleurodira the "S' is horizontal. The turtle skull is relatively simple. The bottom jaws are fused at the chin. There are no teeth, but the jaw edges are usually covered with sharp-edged horny plates that perform in the manner of shears.
Male or Female? If you're looking for a suitable mate for your turtle, you will have to choose from nearly fullgrown animals. The younger the turtle, the more difficult it is to distinguish between the sexes. In the males of many species the abdominal shell, or plastron, is curved inward more deeply than that of the females. Males, as a rule, also possess a somewhat longer tail, narrower at its base, and the cloaca is located closer to the tip of the tail. This can be determined, however, only in a direct comparison of several animals of approximately the same size. Halfgrown male painted turtles can be recognized quite clearly by the front claws, which are plainly longer than those of the females.
How Old Is the Turtle? If you don't know the turtle's year of birth, you will have to rely on estimates. After three years the animal attains about onethird of its final size, after three more years it reaches twothirds of its final size. This is a fairly rough indication, because the growth rate depends very heavily on the living conditions. The rate or growth slows down increasingly with age. It is not true that age can be read by the "annual rings" on the plates, or scutes, of the dorsal shell, or carapace.