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The diversity of hydrostatic skeletons
William M. Kier


A remarkably diverse group of organisms rely on a hydrostatic skeleton for support, movement, muscular antagonism and the amplification of the force and displacement of muscle contraction. In hydrostatic skeletons, force is transmitted not through rigid skeletal elements but instead by internal pressure. Functioning of these systems depends on the fact that they are essentially constant in volume as they consist of relatively incompressible fluids and tissue. Contraction of muscle and the resulting decrease in one of the dimensions thus results in an increase in another dimension. By actively (with muscle) or passively (with connective tissue) controlling the various dimensions, a wide array of deformations, movements and changes in stiffness can be created. An amazing range of animals and animal structures rely on this form of skeletal support, including anemones and other polyps, the extremely diverse wormlike invertebrates, the tube feet of echinoderms, mammalian and turtle penises, the feet of burrowing bivalves and snails, and the legs of spiders. In addition, there are structures such as the arms and tentacles of cephalopods, the tongue of mammals and the trunk of the elephant that also rely on hydrostatic skeletal support but lack the fluid-filled cavities that characterize this skeletal type. Although we normally consider arthropods to rely on a rigid exoskeleton, a hydrostatic skeleton provides skeletal support immediately following molting and also during the larval stage for many insects. Thus, the majority of animals on earth rely on hydrostatic skeletons.


  • Funding

    This work was supported in part by the National Science Foundation [grant IOS-0951067].

  • Glossary

    Each of the five radially arranged bands in echinoderms, together with their underlying structures, through which the double rows of tube feet protrude.
    Antagonistic muscle
    A muscle that opposes the action of another muscle, thereby allowing a shortened muscle to be re-elongated.
    Bulk modulus
    The ratio of the change in pressure acting on a volume to the fractional change in the volume.
    The central sac-like digestive cavity of animals in the Phylum Cnidaria.
    Crossed-fiber helical array
    An arrangement of connective tissue fibers, typically collagen, commonly found reinforcing the walls of hydrostatic skeletons and wrapping the body or organ in sheets of parallel fibers, in both right- and in left-handed helixes.
    A jellylike layer in cnidarians and ctenophores between the ectoderm and the endoderm.
    Neutral plane
    A surface in a bent beam along which the material is neither compressed nor extended.
    Obliquely striated muscle
    A striated muscle fiber type, common in invertebrates such as annelids, nematodes and molluscs, in which the thick and thin myofilaments and the dense bodies to which the thin filaments are anchored are arranged in a staggered pattern, forming oblique bands across the fiber.
    A ciliated groove found in sea anemones and some corals that extends from the mouth into the pharynx and generates a current of water directed into the coelenteron.
    One of the minute pores in the epidermis of a plant leaf or stem through which gases and water vapor pass.
    A dimensionless number that characterizes the change in dimensions of an object during a deformation.
    The force per unit area that tends to deform the body on which it acts.
    The natural contraction of a cross-striated muscle with a maximum length change greater than threefold effected by penetration of the myofilaments through perforated Z-discs.
    Water vascular system
    A system of water-filled canals derived from the coelom that connects the tube feet of echinoderms.
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