the hard, protective and supportive parts of body forms skeleton (skeletal system).

In our body, bones & some cartilages form this system & provide a frame to our body which is protective & supportive.

1. Hydro skeleton àin which fluid forms the skeleton of body. In hydro, flatworms, annelids or roundworms.

2. Endoskeleton

1. It covers the body internally.
2. It is produced by mesoderm. (middle layer of body)
3. It is made up of living materials like bones, cartilage.
4. They does not stops the growth of body.
5. It has muscles attached to its outer surface.

 (c)   exoskeleton

1.  It covers the body eternally.
2. It is produced by ectoderms (outermost layer of body).
3. It is made up of non-living materials like chitin, keratin (hair & nails are made up of keratin).
4. They stops the growth of body.
5. It has muscles attached to its inner surface.

Difference between bone and cartilage :-

Bone	Cartilage
It is solid, rigid connective tissue & made up of tough inflexible ossein. They have small cavities called bone narrow at centre, where new cell are formed. It is highly vascular with blood vessels. Osteoblasts are bone forming cells.	It is connective tissue. Which is made up of flexible material chondrin. They don’t have such cavities. It is non-vascular. There is no specific cartilage forming cells.

Functions of cartilage :-

1. It provide shape, support and protection to body.
2. It helps in movement of body parts.
3. Bone narrow in bones help in the formation of new cells.
4. Cartilage of larynx (voice box) help in sound production.
5. Can ossicles (bone) of ear help in hearing.