The Skeletal System

Skeleton Types

Hydrostatic

In the animal kingdom there are three types of skeletons -- hydrostatic, exoskeleton, and endoskeleton. The prefix "hydro-" implies liquid, water, or moisture. Hydrostatic skeletons are found in soft-bodied invertebrates and consist of muscle layers that surround a fluid-filled body cavity. [1] Earthworms and jellyfish are examples of animals with a hydrostatic skeleton.

Exoskeleton

Exoskeletons are found in arthropods, such crabs and other crustaceans. Insects also have exoskeletons. Made of chitin, a strong, yet bendable complex carbohydrate, an exoskeleton provides the animal with protection and stability, and prevents if from drying out, enclosing the animal's tissues and organs in a hard case.

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Muscles attach to the exoskeleton, enabling movement. An exoskeleton does not grow with the animal. Therefore, the animal must molt periodically when the exoskeleton becomes too small. The shell of a mollusk is an example of a simple exoskeleton. [3]


Front view diagram of the human skeleton.

Endoskeleton

Vertebrates have endoskeletons. As the prefix "endo-" meaning "within, inside" indicates, endoskeletons are internal. Endoskeletons are composed of bone and cartilage. Bone is a dynamic living tissue with the capability to self-repair and remodel in response to physical factors. Bone is also capable of growth. Vertebrates' endoskeletons are divided into two main parts, an axial skeleton and an appendicular skeleton. [1]

Axial & Appendicular

As a medical term, appendicular describes parts of the anatomy that are associated with limbs, i.e., the arms and legs and includes the hands and feet. Thus, the appendicular skeleton consists of the bones of the limbs and the pectoral (shoulder) and a pelvic girdle. The axial skeleton consists of the bones of the head, neck and trunk. [1] Within the axis of the body are the vertebral column (spine or backbone) and the thoracic cage, i.e., the chest.

Bones of the Human Skeleton

The typical adult human has 206 bones that make up the skeleton. The components of the human skeleton -- bones, cartilage, and ligaments -- provide a framework for support and work in tandem with the muscular system to allow movement.

It is the construction of the skeleton in coordination with the muscles that permits us to stand upright. The bones of the human skeleton also provide protection for vital internal organs. For example, the skull protects the brain and the rib cage protects the heart and lungs. The skeleton serves as the site of attachment for skeletal musclesand a framework for muscles and skin, which in turn provides more protection for internal organs.

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See a list of the bones in the human body.
Also see The Muscular System.


References
1 Biology, Seventh Edition; Peter H. Raven, George B. Johnson, Jonathan B. Losos, Susan R. Singer; McGraw-Hill, New York, New York; 2005.
2 Essentials in Biology; Sylvia S. Mader; McGraw-Hill, New York, New York; 2007.
3 Life: The Science of Biology, Seventh Edition; William K. Purves, David Sadava, Gordon H. Orians, H. Craig Heller; Sinauer Associates, Sunderland, Massachusetts; 2004.
4 Science 101: Biology; George Ochoa; HarperCollins, New York, New York; 2007.

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