Fig. 2. (A) Dissection photograph of a yellowfin tuna (Thunnus albacares) showing the location and structure of the mid-lateral septum. The head is on the right and the tail on the left, and the dorsal half of the left fillet has been removed to reveal this view from above. As described by Westneat et al. (1993), the anterior oblique tendons run outwards and posteriorly and are composed of tendon precipitated around ribs. Four of them are highlighted in green. Posterior oblique tendons run inwards and posteriorly, and four of these are highlighted in yellow. Scale bar, 5 cm. (B) Dissection photograph of the posterior region of a yellowfin tuna (Thunnus albacares) with the skin removed to reveal the dwindling posterior muscle volume and the progressive replacement of muscle with tendon. These tendons form from myosepta, but progressing towards the peduncle (white arrow) they completely replace the muscle and form dense, twisting ropes of solid tendon. The yellow arrows indicate the long tendons from the subdermal sheath that originate in the myosepta but form mechanical connections to the skin, indicated by the tendons still attached to the skin after an attempt to peel them away. This also suggests a potential trajectory for forces generated in anterior myotomes (see text). The presence of a mechanical connection can be seen in the way the skin `tents up' when the tendon is pulled on with forceps in the lower, reflected flap of skin. Scale bar, 5 cm.

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