Hydrostatic skeleton

A hydrostatic skeleton, or hydroskeleton, is a skeleton supported by fluid pressure. Hydrostatic skeletons are common among simple invertebrate organisms. While more advanced organisms can be considered hydrostatic, they are sometimes referred to as hydrostatic for their possession of a hydrostatic organ instead of a hydrostatic skeleton. A hydrostatic organ and a hydrostatic skeleton may have the same capabilities, but they are not the same. Hydrostatic organs are more common in advanced organisms, while hydrostatic skeletons are more common in primitive organisms. As its name suggests, containing hydro meaning "water", being hydrostatic means that the skeleton or organ is fluid-filled.

As a skeletal structure, it possesses the ability to affect shape and movement, and involves two mechanical units: the muscle layers and the body wall. The muscular layers are longitudinal and circular, and part of the fluid- filled coelom within. Contractions of the circular muscles lengthen the organism’s body, while contractions of the longitudinal muscles shorten the organism’s body. Fluid within the organism is evenly concentrated so the forces of the muscle are spread throughout the whole organism and shape changes can persist. These structural factors also persist in a hydrostatic organ.

University of Massachusetts researcher Diane Kelly documents a non-helical hydrostatic skeleton structure as the functional basis of the mammalian penis which must function similarly to a rigid element in use. Helically reinforced hydrostatic skeleton structure is typical for flexible structures as in soft-bodied animals.

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. 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.

Within the cylinder lies fluid, most often water. The fluid resists 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.

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.

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