Fig. 4. Transmission electron micrographs of transverse sections of the ventral mantle of a newly hatched Sepioteuthis lessoniana showing circular muscle fibres cut parallel to their long axes. (A) View of a superficial mitochondria-rich (SMR) and a central mitochondria-poor (CMP) circular muscle fibre. Note the core of mitochondria (Mt) in each. A radial muscle cell (RM) is also visible. (B) Magnified view of a CMP muscle cell showing a single sarcomere and its thick filaments (TF). Note that the length of the thick filaments is about 1 µm. The scale bar in each panel is 1 µm. DB, dense bodies.