Statement A is incorrect because muscle bundles are themselves called fascicles; they are held together by a collagenous connective tissue layer called fascia. Statement B is correct as the sarcoplasmic reticulum is the storehouse of calcium ions . Statement C is correct because the striated appearance is due to the distribution pattern of actin and myosin proteins . Statement D is incorrect because the functional unit of contraction is the sarcomere, defined as the portion of the myofibril between two successive 'Z' lines , whereas the 'M' line is a thin fibrous membrane holding thick filaments together.