THE CONCEPT OF SKELETON

THE CONCEPT OF SKELETON 
The skeletal system includes all of the bones and joints in the body. Each bone is a complex living organ that is made up of many cells, protein fibers, and minerals.  
The human skeleton is the internal framework of the body. It is composed of around 270 bones at birth – this total decreases to around 206 bones by adulthood after some bones get fused together. 
Skeletal system is the system of bones, associated cartilages and joints of human body. Together these structures form the human skeleton. Skeleton can be defined as the hard framework of human body around which the entire body is built. Almost all the hard parts of human body are components of human skeletal system. Joints are very important because they make the hard and rigid skeleton allow different types of movements at different locations. If the skeleton were without joints, no movement would have taken place and the significance of human body; no more than a stone.     
TYPES OF SKELETONS 
A skeletal system is necessary to support the body, protect internal organs, and allow for the movement of an organism. There are three different skeleton designs that provide organisms these functions: hydrostatic skeleton, exoskeleton, and endoskeleton. 
HYDROSTATIC SKELETON 
A hydrostatic skeleton is one formed by a fluid-filled compartment within the body: the coelom. The organs of the coelom are supported by the aqueous fluid, which also resists external compression. This compartment is under hydrostatic pressure because of the fluid and supports the other organs of the organism. This type of skeletal system is found in soft-bodied animals such as sea anemones, earthworms, Cnidaria, and other invertebrates.  
Hydrostatic skeleton: The skeleton of the red-knobbed sea star (Protoreaster linckii) is an example of a hydrostatic skeleton. 
Movement in a hydrostatic skeleton is provided by muscles that surround the coelom. The muscles in a hydrostatic skeleton contract to change the shape of the coelom; the pressure of the fluid in the coelom produces movement. For example, earthworms move by waves of muscular contractions (peristalsis) of the skeletal muscle of the body wall hydrostatic skeleton, which alternately shorten and lengthen the body. Lengthening the body extends the anterior end of the organism. Most organisms have a mechanism to fix themselves in the substrate. Shortening the 
muscles then draws the posterior portion of the body forward. Although a hydrostatic skeleton is well-suited to invertebrate organisms such as earthworms and some aquatic organisms, it is not an efficient skeleton for terrestrial animals.  
EXOSKELETON 
An exoskeleton is an external, hard, encasement on the surface of an organism. For example, the shells of crabs and insects are exoskeletons. This skeleton type provides defense against predators, supports the body, and allows for movement through the contraction of attached muscles. As with vertebrates, muscles must cross a joint inside the exoskeleton. Shortening of the muscle changes the relationship of the two segments of the exoskeleton. Arthropods, such as crabs and lobsters, have exoskeletons that consist of 30–50 percent chitin, a polysaccharide derivative of glucose that is a strong-but-flexible material. Chitin is secreted by the epidermal cells. The exoskeleton is further strengthened by the addition of calcium carbonate in organisms such as the lobster. Because the exoskeleton is acellular and does not grow as the organism grows, arthropods must periodically shed their exoskeletons.  
Exoskeletons: muscles attached to the exoskeleton of the halloween crab (gecarcinus quadratus) allow it to move. 
ENDOSKELETON 
An endoskeleton consists of hard, mineralized structures located within the soft tissue of organisms. An example of a primitive endoskeletal structure is the spicule of sponges. The bones of vertebrates are composed of tissues, whereas sponges have no true tissues. Endoskeletons provide support for the body, protect internal organs, and allow for movement through contraction of muscles attached to the skeleton.  
Endoskeletons: The skeletons of humans and horses are examples of endoskeletons. They provide bodies with support, protection of organs, and aid in movement. 
The human skeleton is an endoskeleton that consists of 206 bones in the adult. It has five main functions: providing support to the body, storing minerals and lipids, producing blood cells, protecting internal organs, and allowing for movement. The skeletal system in vertebrates is divided into the axial skeleton (which consists of the skull, vertebral column, and rib cage), and the appendicular skeleton (which consists of the shoulders, limb bones, the pectoral girdle, and the pelvic girdle). 
https://courses.lumenlearning.com/boundless-biology/chapter/types-of-skeletal-systems/ 
http://www.innerbody.com/image/skelfov.html 
http://www.mananatomy.com/body-systems/skeletal-system 
https://en.wikipedia.org/wiki/Human_skeleton