Figure 7.2: The animal depicted above is an Echinoderm - a starfish - which uses its tube feet for
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 ex-
pand, 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.
Exoskeleton DUMMY
An exoskeleton is an external skeleton that supports and protects an animal’s body. The
skeleton is non-living and consists of a cuticle strengthened by chitin, a substance secreted by
the epidermis (skin). Crustaceans such as crabs have their exoskeleton further strengthened
Chapter 7. Support systems in animals 193