The fine structure of the proboscis of Helobdella stagnalis has been investigated by means of the electron microscope. The longitudinal external and internal, radial and circular muscle fibres are analyzed in detail for the following characters: overall submicroscopic organization, diameter of myosin filaments, thin to thick filament ratio, thickness of the Z elements and fractional mitochondrial volume. These data are compared with the corresponding ones for the different body wall muscles. Ultrastructural findings on the proboscis fibres suggest that these fibres all work as slow tonic muscles; the smaller mitochondrial volume of the longitudinal fibres in respect to the radial and circular ones, however, would indicate that the former are less active metabolically than the latter.
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Con il contributo del Ministero per i Beni e le Attività Culturali