[Morphological and functional variations of the muscles of invertebrates].

The problem of muscle evolution among Invertebrates has been considered, mainly in obliquely strated fibers of Annelida and in trasversally striated fibers of Insecta. In both groups the morphological variations run parallel to the functional requirements; similar modulations, in fact, have been observed in fast and slow acting muscle fibers independently from the basic pattern of Insecta and Annelida fibers. Fast acting muscles are characterized by a lesser diameter of myosin filaments and a low thin-to-thick filament ratio. On the contrary, the slow acting muscles show a greater diameter of the myosin filaments and a higher thin-to-thick filament ratio. These observations agree with the hypothesis of a common origin of almost all muscular systems.