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

Skeletal systems are commonly divided into three types - external (an exoskeleton), internal (an endoskeleton), and fluid based (a hydrostatic skeleton), though hydrostatic skeletal systems may be classified separately from the other two since they lack hardened support structures.

Large external skeletal systems support proportionally less weight than endoskeletons of the same size, and thus many larger animals, such as the vertebrates, have internal skeletal systems. Examples of exoskeletons are found in arthropods and shellfish: the skeleton forms a hard shell-like covering protecting the internal organs.

The phyla arthropoda and mollusca both have exoskeletons. Since exoskeletons necessarily limit growth, phyla with exoskeletons have come up with various solutions. Most molluscs have calcareous shells and as they grow, the diameter of the shell is enlarged without altering its shape. On the other hand, arthropods shed their exoskeletons to grow, a process known as molting (or ecdysis). During molting the arthrpod breaks down their old exoskeleton and then regenerates a new one which they then harden through various processes (such as calcification or sclerotization).

An internal skeletal system consists of rigid structures within the body, moved by the muscular systemThe muscular system is the biological system of animals that allows them to move internally and externally. The muscular system in vertebrates consists of three different types of muscles: cardiac, skeletal and smooth. Cardiac muscle is a striated muscle. If the structures are mineralized or ossified , as they are in humans and other mammals, they are referred to as bonesThe bones are a musical instrument which, at the simplest, consists of a pair of bones, human or animal, or pieces of wood or similar. They are played by holding them between one's fingers and moving one's wrist in such a way that they knock against each. CartilageCartilage is a translucent dense connective tissue. In the adult, cartilage contains no blood vessels or nerves, and consists of a relatively homogeneous extracellular matrix, in which cells called chondrocytes are sparsely distributed. These cells are re is another common component of skeletal systems, supporting and supplementing the skeleton. The human earFor an alternative meaning, see ear (botany). human ear. An ear is an organ used by an animal to detect sound. The term may refer to the entire system responsible for collection and early processing of sound (the beginning of the auditory system), or mere and noseAnatomically, a nose is a protuberance in vertebrates that houses the nostrils, or nares, which admit and expel air for respiration. In most mammals, it also houses the nosehairs, which catch airborne particles and prevent them from reaching the lungs. are shaped by cartilage. Some organisms have a skeleton consisting entirely of cartilage and without any calcified bones at all, for example sharksee text Sharks are a group (superorder Selachimorpha of fish, with a full cartilaginous skeleton, a streamlined body plan with between 5 and 7 gill slits along the sides (most often) or side of the head (the first modified slit is behind the eye and calls. The bones or other rigid structures are connected by ligamentA ligament is a short band of tough fibrous connective tissue composed mainly of long, stringy collagen molecules. Ligaments connect bones to other bones in joints. They do not connect muscles to bones; that is the function of tendons. Anatomy.s and connected to the muscular system via tendonA tendon or sinew is a tough band of fibrous connective tissue, attached on one end to a muscle and on the other to a bone. They are similar to ligaments except that ligaments join one bone to another. Due to their poor blood supply, tendons and ligamentss.