By definition invertebrates should have no skeleton at all, but insects, crayfish and a number of others cheat and achieve support with chitonous exoskeletons. You already know this but guppies, bullfrogs, Cheshire cats and other vertebrates are supported by internal skeletons made of bone and cartilage. They are considered to have true skeletons, but to distinguish them from those of cheating invertebrates the supportive structures are termed endoskeletons. The soft bodies of snails and clams are protected and supported by yet another structure, simply known as a shell. These different forms of support and protection, for the most part, are governed by strict monopolies with the different groups within the animal kingdom having exclusive rights to particular designs.

   And then you have the turtle, a vertebrate with a shell that functions as an exoskeleton that contains an endoskeleton within. It is by all standards a bizarre design. Close examination, however, shows the body plan is only a modest modification of that of a the typical vertebrate. The rib cage, which protects the heart, lungs and other vital organs in other reptiles has become modified for maximum slow lane life insurance. Like medieval armor, having a heavy skeleton on the outside results in some major advantages and some minor inconveniences.

   Lets examine the armor itself. A turtle's shell is a bony box composed of a carapace, the upper shell, and the plastron, the lower shell. The two sections are joined together by a bridge. The shell itself is formed from three living, growing layers. The carapace is built over reptile ribs which have become broadened and flattened and wrap under the top shell. The turtle's back and trunk vertebrae are fused to the underside of the ribs and have lost all of their flexibility and most of their processes for muscle attachments. The back muscles, out of work, and with no real place to attach have all but disappeared. The ribs are covered with many effectively fused bony plates (membrane bones), fed by many small blood vessels. Their small ragged joints actuary lock together into a solid shell. All of this is fused to the ribs. The exterior is covered with individual shields termed scutes, which are similar in composition to the scales of most other reptiles. The scutes are hardened layers of skin which form over the bone and serve as an external protective membrane. The sutures of the external scutes and those of the underlying bone and ribs do not align. The offset positioning provides additional structural strength to the shell. This arrangement prevents the shell from splitting open when turtles are chewed on by large predators. The plastron and bridge are structured in a similar way except they lack the underlying ribs.

   The transformation of the skeleton from an internal one to that of a bony box takes place during embryonic development. Turtle embryos are at first similar in structure to those of other reptiles. A forerunner of the carapace, which first appears as a raised area in the center of the embryo's back, grows outward in all directions, expanding downward like the flow of very thick molasses. The ribs at first form independently, beneath the expanding "molasses", but soon are influenced and positioned by its expansion. The embryonic growth of what will become the top shell is so rapid, and the influence on the ribs so pronounced, that the forming limb girdles are soon enclosed, without having a chance to assume their exterior position. Thus, the embryonic rib cage and limbs of turtles do not reposition themselves, they are simply taken over by a faster forming shell. Eventually this shell will become the dominant feature of the embryo and will represent about one-third of its total weight.

   The advantage of the shell is of course protection, but increased protection comes with the cost of the loss of speed and agility and a number of subtle problems which arise from living inside a box. In order for the turtle to achieve complete protection it must also be able to withdraw its head, legs and tail into the shell. Turtles have eight neck vertebrae (most mammals have 7). The increased number of vertebrae and some odd muscle arrangements allow for the "s" like bend necessary for the withdrawal of the head, and on most turtles the scaly front legs fold in like shutters to provide protection for the head. The withdrawn hind legs, and the folded over tail fill up the opening in the rear of the shell. Its all a tight fit, but together the four legs prevent predators from gaining access to the turtle's vital parts. The legs them selves are built for sturdy lifting power, but of course lack maneuverability because of the confines of the shell's leg openings. The connection of the legs to the rest of the skeleton necessitated some redesign. Not only are the joints inside the shell, but the limb girdles are used for breathing. In that the rib cage cannot expand and contract to work the lungs, air is pumped in and out by movement of the leg girdles. To fill the lungs the bones of the shoulder rotate inward and forward and those of the pelvis are moved backwards and downward. To exhale the turtle reverses the movements. Watching a turtle breath it would appear that head and leg movements are responsible for filling the lungs, but it the real action is deeper within the shell. The head and legs do pump in and out, but only because of space constraints within the shell. Since the rib cage and shell are so rigid the expanding lungs are another problem. Turtles use special muscles to push aside the intestines and other abdominal organs to make room for the inflated lungs. The displaced organs move down into loose pockets of skin around the base of the legs. Most turtles can remain tightly tucked away in their shells for long periods as they have large lungs and a low rate of oxygen consumption. They must relax and partly release their sealed in appendages however in order to breathe.

   Its a good design allowing individual turtles to live well past the century mark and leaving basic turtle body plans unchanged for some 150 million years.