Turtle

Turtle, any reptile with a body encased in a bony shell, including tortoises. Although numerous animals, from invertebrates to mammals, have evolved shells, none has an architecture like that of turtles. The turtle shell has a top and a bottom. The carapace and plastron are bony structures that usually join one another along each side of the body, creating a rigid skeletal box. This box, composed of bone and cartilage, is retained throughout the turtle’s life. Because the shell is an integral part of the body, the turtle cannot exit it, nor is the shell shed like the skin of some other reptiles. The turtle’s shell is an adaptation that protects it from predators, which compensates for the reptile’s slow crawling speed. The carapace and plastron each arose from two types of bone: dermal bones that form in the skin and endochondral bone derived from the skeleton. Evolution has intricately linked these two types of bone to produce the shell of modern turtles. The carapace consists of 10 trunk vertebrae and their ribs, which are overlain by and fused to dermal plates. Another series of dermal plates forms the perimeter of the carapace. The plastron usually contains four pairs of large plates and a single one centred near the front these plates are large dermal bones, although the anterior ones may contain parts of the shoulder girdle. The shell is variously modified and shaped to meet the needs of defense, feeding, and movement.