The Diversity, Morphology and Biomechanics of Muscular-Hydrostats (#187)
The tongues of many mammals and lizards, the arms and tentacles of cephalopods and the trunk of the elephant lack the rigid skeletal elements that characterize the skeletal support systems of vertebrates and arthropods. They also lack the large fluid-filled cavities typical of the hydrostatic skeleton of many soft-bodied invertebrates including many worm-like animals and polyps such as sea anemones. These structures, termed ‘muscular hydrostats’, instead consist of a densely packed three-dimensional array of muscle that both produces the force for movement and also provides the skeletal support. Their function relies on the fact that the muscle resists volume change and any decrease in one dimension must therefore result in an increase in another. Since the musculature is typically arranged such that all three dimensions can be actively controlled, a wide diversity of movements and deformations can be produced. Typical movements include elongation, shortening, bending, torsion, and active control of stiffness. Elongation is created by contraction of circumferential, radial or transverse fibers. Shortening occurs by contraction of longitudinally arranged muscle fibers. Active bending movements require unilateral longitudinal muscle contraction simultaneous with circumferential, radial or transverse muscle activity. Torsional movements result from contraction of helically arranged muscle fibers; selective contraction of right- and left-handed helical muscle fibers allow torsion in both directions. The remarkable complexity and diversity of movements observed in muscular hydrostats may be facilitated by their mechanism of support and movement, although this may require more complex neural control. Bending and other deformations can occur at any point along the length, so movement is not restricted to joints as it is in animals with rigid skeletons. Unlike conventional hydrostatic skeletons, where localized muscle contraction increases hydrostatic pressure throughout, contraction of a small group of fibers has a localized effect, with the potential for greater precision and intricacy of deformation.