Provide no protection and could not support a large land animal. Hard encasement deposited on the surface of an animal. Mollusks are enclosed in calcareous shells secreted by the mantle. Arthropods are enclosed in a cuticle secreted by the epidermis that contains chitin and also contains quinones that add extra strength in areas where protection is most important. Hard supporting elements buried within the soft tissues of an animal.
Sponges possess hard spicules of inorganic material or softer protein fibers.
Echinoderms have ossicles composed of magnesium carbonate and calcium carbonate forming hard plates beneath the skin. Bones of the vertebrate act in support and as levers when their attached muscles contract. Muscles move skeletal parts by contracting Animal movement is based on contraction of muscles working against some kind of skeleton. Ability to move a body part in opposite directions requires that muscles be attached to the skeleton in antagonistic pairs. Two kinds of myofilaments are found in each myofibril.