Hydrostatic Skeletons

Hydrostatic Skeletons
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 and thicken, 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 patterns.