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 ...
