Functional morphology of undulatory burrowing
Undulating movements of Armandia brevis resemble those of nematodes both in qualitative behavior and in body shape, characterized by the ratio of amplitude to wavelength (Dorgan et al., 2013). Like nematodes, A. brevis has thick bands of longitudinal muscle that contract unilaterally for undulatory bending. Bending during undulatory burrowing requires unilateral contraction of longitudinal muscles simultaneously with a mechanism to resist radial expansion and axial shortening on the side of muscle contraction. As the wave of contraction passes posteriorly, longitudinal muscles on the non-bending side contract, extending the contracted longitudinal muscles and serving as a restoring force. The contracted oblique muscles presumably also extend when the body reaches the opposite curvature, although contraction of the ventral longitudinal muscle would likely extend relaxed oblique muscles as well (Law et al., 2014).
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