Editing Hydrostatic skeleton (section)

{{short description|Flexible skeleton supported by fluid pressure}}
A '''hydrostatic skeleton''' or '''hydroskeleton''' is a type of [[skeleton]] supported by [[hydrostatic]] [[fluid pressure]] or liquid,<ref name=":0">{{Cite book |title=Vertebrates {{!}} Comparative Anatomy, Function, Evolution |edition=7th |last=Kardong |first=Kenneth V. |publisher=Mc Graw Hill Education |year=2015 |isbn=978-0078023026 |pages=426, 496}}</ref> common among [[soft-bodied organism|soft-bodied]] [[invertebrate]] [[animal]]s colloquially referred to as "[[worm]]s". 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'', where the two may have the same capabilities but are not the same.<ref name=":0" />  As the [[prefix]] ''hydro-'' meaning "water", being hydrostatic means being fluid-filled.<ref name=":1">{{Cite journal |last=Kier |first=William M. |date=2012-04-15 |title=The diversity of hydrostatic skeletons |journal=Journal of Experimental Biology |volume=215|issue=8 |pages=1247–1257 |doi=10.1242/jeb.056549 |pmid=22442361|doi-access=free |url=https://cdr.lib.unc.edu/downloads/vm40z222s }}</ref>

As a skeletal structure, a hydroskeleton 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.<ref name=":1" /> These structural factors also persist in a hydrostatic organ.

A non-helical hydrostatic skeleton structure is the functional basis of the [[mammalian penis]],<ref>{{cite journal |last1=Kelly |first1=DA|title=The functional morphology of penile erection: tissue designs for increasing and maintaining stiffness |journal=Integrative and Comparative Biology |date=April 2002 |volume=42 |issue=2 |pages=216–221 |doi=10.1093/icb/42.2.216 |pmid=21708713 |url=http://www.bio.umass.edu/biology/kelly/pdf/Kelly-2002.pdf|doi-access=free }}</ref> which fills the [[corpus cavernosum penis|corpus cavernosa]] with [[blood]] to maintain physical rigidity during [[coitus]]. Helically reinforced hydrostatic skeleton structure is typical for flexible structures as in soft-bodied animals.<ref name=":1"/>