Skeleton



In biology, the skeleton or skeletal system is the biological system providing physical support in living organisms. (By extension, non-biological outline structures such as gantries or buildings may also acquire skeletons.)

Types and Classification
Skeletal systems are commonly divided into three types—external (an exoskeleton), internal (an endoskeleton), and fluid based (a hydrostatic skeleton), although hydrostatic skeletal systems may be classified separately from the other two, because they lack hardened support structures. An internal skeletal system consists of rigid or semi-rigid structures, within the body, moved by the muscular system. If the structures are mineralized or ossified, as they are in humans and other mammals, they are referred to as bones. Cartilage is another common component of skeletal systems, supporting and supplementing the skeleton. The human ear and nose are shaped by cartilage. Some organisms have a skeleton consisting entirely of cartilage and without any calcified bones at all, for example sharks. The bones or other rigid structures are connected by ligaments and connected to the muscular system via tendons.

Hydrostatic skeletons are similar to a water-filled balloon. Located internally in cnidarians (coral, jellyfish etc.) and annelids (leeches, earthworms etc.), among others, these animals can move by contracting the muscles surrounding the fluid-filled pouch, creating pressure within the pouch that causes movement. Animals such as earthworms use their hydrostatic skeletons to change their body shape, as they move forward, from long and thin to shorter and wider.