A form of exoskeleton is the shell of calcium or silica secreted by certain protozoans known as foraminiferans. Commercial sponges have an exoskeleton consisting of spongin, which is a tough, elastic substance. Cnidarians secrete a wide variety of exoskeletal substances, ranging from the elastic covering of the jellyfish to the stony material deposited by coral. The familiar shells of most mollusks are composed of calcium carbonate and an organic ground substance known as conchiolin. Among insects, each of the three principal divisions of the body—the head, the thorax, and the abdomen—is enclosed in a framework of horny plates. The plates of each primary division are separated from those of the next division by elastic tissue that permits flexibility of motion. The appendages are enclosed by sheaths projecting from the exoskeleton; elastic tissue similar to that between the plates joins the segments of the appendages and attaches them to the body.
The human skeleton is a strong, flexible framework of 206 bones that supports the body and protects internal organs. In addition, the bones of the skeleton store calcium, a mineral essential for the activity of nerve and muscle cells. The soft core of bone, the bone marrow, is where red blood cells, certain white blood cells, and blood platelets form.
Bones come in different shapes and sizes, each adapted to perform specific functions. The breastbone, for example, is a flat plate of bone that helps to protect the heart and lungs in the chest. The fused bones of the skull safely encase the brain. The short, delicate bones in the wrist and hand enhance dexterity, providing flexibility for small, precise motions. The long, heavy femur bone in the leg acts as a strong lever for powerful or speedy movement. Cartilage is flexible connective tissue that provides support to skeletal bones and allows joints to move without rubbing against each other.
Vertebrates have a more or less rigid group of structures composed of cartilage or bone or of a combination of these two connective tissues. The most primitive of these structures is the notochord, which is a backbone of cartilage occurring in fishes. Animals higher on the evolutionary scale have an axial skeleton, consisting of the skull, spinal column, and ribs, and an appendicular skeleton, made up of the pelvic and pectoral girdles and the appendages.
In higher animals, the skeleton formed in the embryo is initially cartilaginous; bone and calcium are deposited as the organism matures. In humans, the process of bone hardening, or ossification, is completed at about the age of 25. The last bone to ossify is the breastbone.
The total number of bones in any animal varies with its age; many bones fuse together during the ossification process. The average number of distinct skeletal structures in a young human is 200, exclusive of the 6 ossicles found in the ears. The human skeleton is subject to a number of pathological conditions, most important of which are fracture and a deficiency disease that is known as rickets.