Editing Hydrostatic skeleton (section)

=== Vertebrates ===
The [[Penis|mammalian penis]] is a hydrostatic organ. The hydrostatic fluid, in this case blood, fills the penis during an [[erection]]. Unlike the hydrostatic skeletons of many invertebrates, which use the bending of the animal for locomotion, the penis must resist bending and shape changes during [[sexual intercourse]]. Instead of connective fibers arranged in a helical shape, the penis contains a layer called the [[corpus cavernosum penis|corpus cavernosum]]. The corpus cavernosum contains connective fibers arranged both parallel and perpendicular to the longitudinal axis. These fibers remain folded when the penis is flaccid, but unfold as the penis fills with blood during an erection, which allows the penis to resist bending. The penises of [[turtle]]s are structured similarly, although they evolved separately.<ref name=":3" />

Other vertebrates sometimes utilize a modified hydrostatic skeleton called a [[muscular hydrostat]].<ref name=":1" /> Muscular hydrostats do not contain a fluid-filled cavity. These structures are constructed of muscle and connective fibers, densely packed into a 3-D structure. In many cases, the muscular hydrostat can be manipulated in all three dimensions. This allows for more precise movement compared to a typical hydrostatic skeleton. While in typical hydrostatic skeletons, movement is generated by applying force to a fluid-filled cavity, muscular hydrostats generate movement by muscle contractions. When one muscle contracts and decreases in area, other muscles within the structure must expand in response. Helical muscles may be present, which can create torsion, an ability that is restricted in hydrostatic skeletons. Muscular hydrostats are found in mammalian, reptilian, and amphibian tongues. Mammalian tongues have the structure of a central core of muscle fibers surrounded by bundles of longitudinal muscles and alternating parallel sheets of transverse muscle fibers. Elephant trunks and tapir proboscises also utilize a muscular hydrostat. These structures are composed of longitudinal fibers surrounded by radial and helical fibers.<ref name=":3" />