Fig. 5. Transmission electron micrographs of transverse sections of the ventral mantle of a juvenile Sepioteuthis lessoniana showing circular muscle fibres cut parallel to their long axes. (A) Portion of a superficial mitochondria-rich (SMR) muscle fibre to illustrate the length of the thick filaments (TF). The approximate dimensions of the sarcomere and the long axis of the thick filaments are indicated by the arrow. Note that the lengths of the longest thick filaments are 4-5 µm. (B) View of a central mitochondria-poor (CMP) cell that shows a portion of one sarcomere. The arrow indicates the approximate length of one sarcomere and the long axis of the thick filaments. Note that the lengths of the longest thick filaments are 1.5 µm. Scale bars, 1 µm. MT, mitochondria.