A broad comparative survey has been made correlating ultrastructure of cross-striated fibers with contractile properties in both invertebrates and vertebrates. Most of the muscles were found to be heterogeneous in fiber-composition as indicated by: length of sarcomere, extent of SR, number of invaginating tubules, numbers of mitochondria, etc. Z discs and M bands have markedly different structures in different fibers. The general concept of the “fibrillar” nature of striated muscle is challenged. It is suggested that following excitation the responses of individual sarcomeres, or parts of sarcomeres, are relatively independent. The possibility that all striated muscles contain a very thin elastic filament in parallel with actin and myosin, which may also be contractile, is raised.