Exoskeletons are hard segments that cover the muscles and visceral organs. Muscles for movement attach to the inner surface of the exoskeleton. The bulk and weight of the exoskeleton and associated mechanical problems limits the size animals can attain. Spiders use a combination of an exoskeleton for protection and fluid pressure for movement. Image from W.H. Freeman and Sinauer Associates, used by permission.
Muscles are on the outside of the endoskeleton.
Cartilage and bone are types of connective tissue . Image from W.H. Freeman and Sinauer Associates, used by permission. The skeleton and muscles function together as the musculoskeletal system. Certain cells in the bones produce immune cells as well as important cellular components of the blood. Bone also helps regulate blood calcium levels, serving as a calcium sink.