Fig. 1. Mantle morphology. (A) Photograph of a cross section of the formalin-fixed mantle of a juvenile Doryteuthis pealeii. The section was taken approximately from the midpoint along the length of the mantle. The diameter of the mantle is about 25 mm. Ctenidia (Ct), mantle (M), mantle cavity (MC), and viscera (V). (B) A schematic illustrating only the mantle muscles. The circular muscles compose the majority of the mantle musculature but regularly spaced bands of radial muscle fibres (RM) are also present. There are two types of circular muscles: central mitochondria poor (CMP) and superficial mitochondria rich (SMR). The schematic exaggerates the size of the muscle fibres and the proportion of SMR to CMP fibres. In an adult D. pealeii, the outer and inner layers of SMR fibres together compose only 4–6% of the thickness of the mantle wall. (C) A schematic of a muscle preparation illustrating the foil clips and the muscle. The gray ovals represent the cut ends of the radial muscle fibres.