Biomechanics: Hydroskeletal

Many animals lack hard skeletons and create their skeletal systems using muscles and the incompressibility of fluid. Some have a fluid-filled cavity surrounded by muscles and connective tissue, whereas others, termed muscular hydrostats, lack such cavity and are mainly composed of a tightly packed array of muscles. The arrangement of muscle fibers in different orientations combined with sensory and control systems enables complex and diverse movements without any rigid element. The interaction of biomechanics and control in hydrostatic skeletons is discussed using three examples: octopus arm movements, tongue protrusion in frogs, and whole-body movements in the leech.