A hydrostatic skeleton is a structure found in many cold-blooded and soft-bodied organisms. It consists of a fluid-filled cavity, which is surrounded by muscles. The cavity is called a coelom and in some animals this cavity is filled with a blood-like substance called haemocoel. The fluid presses against the muscles, which in turn contract against the pressure of the fluid. The fluid is incompressible and thus maintains a constant volume against which the muscles can contract. The hydrostatic skeleton prevents the collapse of the body. The muscles in the body act against the fluid and in doing so bring about movement. If the body is segmented, the pressure of the fluid is localised in a few segments at a time. Hydrostatic skeletons occur in flatworms, round worms, earthworms, starfish and slugs.
Note that starfish and other Echinoderms have an outer skeleton of calcareous (chalky) ossicles (little bones) or spicules which are like little spines for protection. This outer skeleton encloses a water vascular system with tube feet that are moved by fluid pressure changes (it serves as a hydrostatic skeleton which controls movement).
Advantages of a Hydrostatic Skeleton
- Fluid shape: This allows organisms with hydrostatic skeletons to fit through oddly shaped passages, which is useful for burrowing or swimming.
- Strength: Creatures with hydrostatic skeletons can squeeze between spaces and expand, making a ‘prying open’ movement which allows them to force their way into various regions of rock and soil surfaces.
- Healing: Healing takes place faster in organisms with hydrostatic skeletons than in organisms with bone structures. This is because the haemocoel contained within the hydrostatic skeleton is made up mostly of water, and thus, can be refilled quickly. This allows many organisms with hydrostatic skeletons such as earthworms to grow back their body mass after damage.
- Lightweight: The hydrostatic skeleton allows the animal to move in a more flexible manner as it requires very little muscle mass for movement.
- Circulation: The fluid cavity allows circulation of nutrients and waste.
- Protection: The hydrostatic skeletons cushions the internal organs of the animal from shock.
- Suited to environment: Hydrostatic skeletons are suited for life in moist or aquatic environments, depending on the animal’s adaptations.
Disadvantages of a Hydrostatic Skeleton
- Structure and surface for attachment: The hydrostatic skeleton lacks a structure and does not have surfaces for the attachment of muscles or limbs.
- Lack of protection: There is very little protection for the internal organs.
- Dessication: A moist or water habitat is essential for survival of these animals in order to prevent dessication (drying out).
- Limited strength: Terrestrial animals with hydrostatic skeletons cannot increase their body size as they would collapse under their own body weight.