when landing after a jump.Depending on their means of locomotion, terrestrial animals needed to adapt their shapes
and skeletons to overcome the effects of gravity. Limbless animals, such as snakes, had to
overcome drag and friction. Flying animals such as birds and bats need light skeletons and
very strong sternums for wing muscle attachment. Animals that support their bodies clear
of the ground needed an energy efficient way of maintaining balance. For this reason, the
leg bones of most animals are held directly underneath the body. In this position they act as
props or struts and it is the bones rather than the muscles that take most of the strain of the
body’s weight.Hydrostatic skeleton DUMMY
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
coelomand in some animals this cavity is filled with a blood-like substance calledhaemo-
coel. 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) ossi-
cles (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).Figure 7.1: The animal above is a jellyfish. It uses its muscles to contract against the hydrostatic
skeleton to bring about movement.192 7.2. Skeletons