Skeletons can be armor outside or scaffolding within
A skeleton is the structural frame supporting most animals—exoskeleton shells, endoskeleton bones, or hydrostatic fluid bags. Vertebrates build endoskeletons around a vertebral column of bone and cartilage; invertebrates lack that column and invent other supports. The word traces to Greek for dried up.
Animals hold their shape in three basic ways. An exoskeleton is a rigid outer case, an endoskeleton is an internal frame that organs and soft tissue hang from, and a hydrostatic skeleton is a muscle-walled cavity stiffened by fluid pressure, as in jellyfish, flatworms, nematodes and earthworms. Skeletons can also be pliant, bending and springing back like the hinge of a clam shell or a jellyfish's mesoglea, or rigid, which is the norm on land. Even single cells have a cytoskeleton that keeps their shape, moves cargo and powers cilia and flagella.
Armour has a weight problem. Arthropods must moult their shells to grow, and because an exoskeleton gets heavy relative to body mass, land animals that wear one tend to stay small, while water's buoyancy lets bigger animals such as the southern giant clam carry massive shells. Invertebrates improvise widely, from cuttlebones in some cephalopods to the ossicle rods of starfish and sea urchins.
Vertebrates build around a backbone of bone and cartilage. Bone is mostly mineral, chiefly hydroxyapatite, plus an organic matrix and water; its hollow tubes resist compression, its marrow makes blood cells, and it stores calcium and phosphate. Sharks, rays and chimeras use cartilage alone, a lighter material that saves energy when swimming. Turtles turned their ribcage into a shell, snakes can have more than 300 vertebrae against about 65 in lizards, birds fly on thin hollow bones and fused elements, and whales have lost their hind legs and usually fuse their neck vertebrae for stability.
An adult human has 206 bones, though the total shifts depending on whether the hip bones and tailbone are counted as single or multiple pieces. The femur is the largest and the stapes of the middle ear the smallest. The three ear ossicles connect only to one another, and the hyoid in the neck, which anchors the tongue, touches no other bone. Full development takes about 20 years. The word itself comes from the ancient Greek for dried up.
Source: Skeleton
