The similarity in swimming style and external body shape between dolphins and scombrid fishes, especially tunas, is a textbook example of evolutionary convergence. I identify additional morphological features of the musculoskeletal system shared by dolphins and tunas. Specifically, these swimmers share a pattern of force transmission through a complex, three-dimensional system of collagenous fabrics, which are stiffened by muscular hydrostatic pressure. This force transmission system increases both the displacement advantage and moment arm of contracting axial muscle. These features represent a functionally significant design for steady swimming vertebrates.