- a hard encasement deposited on the surfave of an animal

- as the animal grows, the exoskeleton changes size by the enlargement of the

       diameter of the shell when new layers are added to the outer edge

- Cuticle (a polysaccharide similar to cellulose) makes up almost half of the cuticle. 

       Fibrils of the chitin are embedded in  a matrix made of protein and form composit

       material which increases the flexibility and strength of the exoskeleton.

- hardened via the addition of organic compounds and non-organic salts

- as the organism grows the exoskeleton must be molted and relaced by a larger

       one.

- Typically seen in arthropods and mullusks

- consists of hard supporting elemnts buried within the soft tissues of an animal

-Characteristic of Sponges, echinoderms and chordates

        -Sponges have hard spicules of inorganic material or softer protein fibers to

           reinforce them.

       -Echinoderms have hard plates beneath their skin called ossicles which are

           either tightly bound together by protein fibers (ex: sea urchins) or loosely

           bound for changing shape (ex: sea stars).

       -Chordates have endoskeletons of bone and/or cartilage.  The mammalian

           skeleton consists of over 200 bones and is divided into the Axial and

           Appendicular skeletons.