Define hydrostatic skeleton.

Hint:-Skeleton is the internal framework of the body which consists of individual or fused bones supplemented and supported by muscles, ligaments, cartilage and tendons. It serves as a scaffold and protection of the organs such as the brain, spinal cord,etc. Skeleton is formed by fluid- filled compartments and supported by the aqueous fluid is called hydrostatic skeleton. Let us know more about hydrostatic skeleton.

Complete Answer:-
As said earlier, hydrostatic skeletons are the skeletons supported by fluid-pressure and common among simple invertebrate organisms.
- Because it is a skeletal muscle, it can affect the shape and movement.
- The body wall and muscle layer are its two mechanical units.
- Typical arrangement of hydrostatic skeleton is cylindrical.
- Several muscle types can control the hydrostatic skeleton.
- Length can be adjusted by longitudinal muscle fibers parallel to the longitudinal axis.
- Continuous sheets or isolated bundles are the two alternatives in which these muscle fibers can be found.
- Circular, radial, and transverse are the muscle types which can manipulate the diameter of these fibers.
- Fluid, mostly water, filled within the cylinder is resistant to volume change.
- Length can be increased by the contraction of circular, transverse or radial muscles as the pressure within the cylinder increases.
- On the other hand, contraction of the longitudinal muscles shortens the cylinder.
- Connective tissue fibers, often collagen, limit the change in length. Within the wall of the hydrostatic skeleton, these fibers are arranged in a helical shape allowing for shortening and elongation of the skeleton in a way that they remain rigid to prevent torsion the pitch of the helix changes in response to change in shape of the cylinder. During elongation, the angle relative to the long axis decreases and increases during shortening.

Note:- Hydrostatic skeletons are more common in primitive organisms, whereas hydrostatic organs are in general a feature of advanced organisms.
Non-helical hydrostatic skeleton structure forms the function basis of the mammalian penis.