Editing Hydrostatic skeleton (section)

== Structure ==

[[File:Bilaterian body plan.svg|thumb|upright=1.7|[[Nephrozoa|Many animals]] with a [[worm]]like cylindrical body have a hydrostatic skeleton with a flexible skin and a water-filled body cavity (coelom or pseudocoelom). They move by [[peristalsis]], using opposed circular and longitudinal [[muscle]]s, which act on the hydrostatic skeleton to change the body's shape.]]

Hydrostatic skeletons are typically arranged in a cylinder. Hydrostatic skeletons can be controlled by several different muscle types. Length can be adjusted by longitudinal muscle fibers parallel to the longitudinal axis. The muscle fibers may be found in continuous sheets or isolated bundles, and the diameter can be manipulated by three different muscle types: circular, radial, and transverse.<ref name=":1" /> Circular musculature wraps around the circumference of the cylinder, radial musculature extends from the center of the cylinder towards the surface, and transverse musculature arrange in parallel and perpendicular sheets crossing the diameter of the cylinder.<ref name=":1" />

Within the cylinder lies fluid, most often water. The fluid is resistant to changes in volume. Contraction of circular, radial or transverse muscles increases the pressure within the cylinder, and results in an increase in length. Contraction of longitudinal muscles can shorten the cylinder.<ref name=":1" />

Change in shape is limited by connective tissue fibers. Connective fibers, often collagenous, are arranged in a helical shape within the wall of the hydrostatic skeleton. The helical shape formed by these fibers allows for elongation and shortening of the skeleton, while still remaining rigid to prevent torsion. As the shape of the cylinder changes, the pitch of the helix will change. The angle relative to the long axis will decrease during elongation and increase during shortening.<ref name=":1" />