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.
Thin filaments consist of two strands of actin and one strand of regulatory protein coiled together.
Thick filaments are staggered arrays of myosin molecules. Unit of organization of skeletal muscle. I bands are areas near the edge of the sarcomere containing only thin filaments.