Octopuses, elephants, and humans all make use of hydrostatic pressure to move various body parts (tentacles, trunks, and tongues, respectively). If you think about it, there is no muscle that can stick out the tongue, says physiologist James O’Reilly of Northern Arizona University. So you do it by having fluid in a chamber in the tongue and having muscle wind around it. When you put the fluid under pressure, it changes the shape of the tongue.
This is the favored mode of locomotion of many invertebrates, which have no skeletons on which to anchor muscles for movement the way vertebrates do. Earthworms, for example, are basically segmented bags of fluid oozing their way through soil. Biologists have assumed that no vertebrate used this seemingly primitive form of propulsion, but O’Reilly has recently found one that does--the legless, wormlike, burrowing amphibian called a caecilian.
Unlike other vertebrates, caecilians have muscles that ring the body wall, running from the belly to the back (the muscles in most vertebrates tend to run lengthwise, from head to tail). By contracting those muscles, O’Reilly found, the caecilians pressurize the fluid in their body cavity, creating a force that goes in the direction of the head, driving the animal forward, just as a water balloon elongates when squeezed. In a water balloon, however, part of the balloon swells as it gets longer, says O’Reilly. But because of the way these animals also have connective tissue wound around their bodies, all of that volume change goes into their becoming longer and thinner.
This technique is remarkably efficient: a caecilian can generate about twice the force of a similar-size burrowing snake, which uses the muscles that run along the vertebral column to twist and arch itself through the earth. What the caecilians can do that snakes can’t, O’Reilly says, is apply nearly 100 percent of their muscular energy toward forward motion. They are using their entire body as a single-chambered hydrostatic organ.