Analysis and reconstruction of unusual obliquely striated fibres in Lumbriculids (Annelida, Oligochaeta).

The longitudinal muscle fibres of Lumbriculids (Annelida, Oligochaeta) have been studied by means of electron microscopy. These fibres are obliquely striated (flattened circomyarian) and possess Z-elements and sarcotubules which frequently cross the centre of the fibre connecting the cell surface on the opposite sides. The myosin filaments have, in cross-section, a rectangular profile with the long side parallel to the fibre surface; they look wavy (S-shaped) on the XZ plane, thus crossing the whole thickness of the sarcomere. The three-dimensional interpretation of the contractile apparatus was facilitated by the comparison of the micrographs from ad hoc sections of the three spatial planes with corresponding calculated "sections', obtained with a dynamic computer model. The real periodicity of the different contractile structures has been determined and the wavy shape of the thick filaments on the XZ plane has been demonstrated by indirect proofs: computed images in which rectangular filaments are parallel to the fibre axis cannot be made to yield a situation comparable with real images. Furthermore, the distances (centre to centre) between thick filaments in fibres at different states of contraction are so distributed in each "sarcomere' as to be compatible only with the wavy filaments.