This paper deals with the body wall helical muscles of Tubificidae (Annelida, Oligochaeta). The majority of the longitudinal muscles are ribbonlike; their primary myofilaments are $6 \mu$ long and 300 Å thick; the secondary-primary filament ratio is about 7-8. Another kind of longitudinal muscle can be seen close to the basament membrane: they show a triangle-like section, and are easily recognizable because of the diameter of primary myofilaments (up to 600 Å), while the secondary-primary filament ratio is 15-16. The circular muscles are even more irregular, and they do not show a definite helical structure, though in transverse sections A and I zones are evident, with primary filaments 500-600 Å in diameter, and a very great number of the secondary ones.
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Con il contributo del Ministero per i Beni e le Attività Culturali