|Figure 31-5 How an earthworm moves forward.
circular muscles contract, longitudinal muscles
are stretched by internal fluid pressure and the
worm elongates. Then, by alternate contraction
of longitudinal and circular muscles, a wave of
contraction passes from anterior to posterior.
Bristlelike setae are extended to anchor
animal and prevent slippage.
Not all skeletons are rigid; many invertebrate
groups use their body fluids as
an internal hydrostatic skeleton. Muscles
in the body wall of the earthworm,
for example, have no firm base for
attachment but develop muscular force
by contracting against the coelomic
fluids, which are enclosed within a
limited space and are incompressible,
much like the hydraulic brake system
of an automobile.
Alternate contractions of the circular
and longitudinal muscles of the
body wall enable the worm to thin
setting up backwardmoving
waves of motion that propel
the animal forward (Figure 31-5).
Earthworms and other annelids are
helped by septa that separate the
body into more or less independent
compartments (Figure 17-1,).
An obvious advantage is that if a
worm is punctured or even cut into
pieces, each part can still develop
pressure and move. Worms that lack
internal compartments, for example,
the lugworm Arenicola
(Figure 17-5,), are rendered helpless if body
fluid is lost through a wound.
There are many examples in the
animal kingdom of muscles that not
only produce movement but also provide
a unique form of skeletal support.
The elephant’s trunk is an excellent
example of a structure that lacks any
obvious form of skeletal support, yet is
capable of bending, twisting, elongating,
and lifting heavy weights (Figure
31-6). The elephant’s trunk,
tongues of mammals and reptiles, and
tentacles of cephalopod molluscs are
examples of muscular hydrostats
Like the hydrostatic skeletons of
worms, muscular hydrostats work
because they are composed of incompressible
tissues that remain at constant
volume. The remarkably diverse
movements of muscular hydrostats
depend on muscles arranged in complex