Muscles provide the force to move a muscular hydrostat. Other vertebrates sometimes utilize a modified hydrostatic skeleton called a muscular hydrostat. The bending of a muscular hydrostat is particularly important in animal tongues. The most important biomechanical feature of a muscular hydrostat is its constant volume. Contraction of helical fibers causes elongation and shortening of the hydrostat. Contraction of orthogonal fibers causes torsion or twisting of the hydrostat. Elephant trunks and tapir proboscises also utilize a muscular hydrostat. A muscular hydrostat is a biological structure found in animals. The pendulum and hydrostat control was invented by Robert Whitehead. As a muscular hydrostat, the trunk moves by precisely coordinated muscle contractions. In many cases, the muscular hydrostat can be manipulated in all three dimensions. The hydrostat is a mechanism that senses pressure; the torpedo's depth is proportional to pressure.