Hydrostatic skeleton

[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.

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.

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.

Organisms with complete hydrostatic skeletons need to be in an environment that allows them to re-fill themselves with their fluid that is necessary for survival.

Terrestrial organisms that have hydrostatic skeletons generally have a lack of strength because they are not in a fluid environment.

Also, hydrostatic nature is common in marine life such as jellyfish and sea anemones.

Earthworms have rings of muscles that are filled with fluid, making their entire body hydrostatic.

[5] An example of a simple Deuterostome containing a hydrostatic skeleton would be Enteropneusta, with the common name of acorn worm.

This organism is classified as a Hemichordate, and they are marine worms that use their hydrostatic skeleton to tunnel and anchor themselves into the ground.

This can be used for locomotion, but also can aid in the defense of the organism against outside forces as the worm can try to "hide" itself within the ocean floor.

Instead of connective fibers arranged in a helical shape, the penis contains a layer called the corpus cavernosum.

The corpus cavernosum contains connective fibers arranged both parallel and perpendicular to the longitudinal axis.

Helical muscles may be present, which can create torsion, an ability that is restricted in hydrostatic skeletons.