A hydrostatic skeleton or hydroskeleton is a type of skeleton supported by hydrostatic fluid pressure or liquid,[1] common among soft-bodied invertebrate animals colloquially referred to as "worms". 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.[1] As the prefix hydro- meaning "water", being hydrostatic means being fluid-filled.[2]
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.[2] These structural factors also persist in a hydrostatic organ.
A non-helical hydrostatic skeleton structure is the functional basis of the mammalian penis,[3] which fills the 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.[2]