Relationship between diameter and length of a constant volume cylinder. The positions on the graph of shapes A through D (drawn to scale) are indicated. A small decrease in diameter from shape B to D causes a large increase in length. From Kier and Smith (Kier and Smith, 1985).

Diagram illustrating the differences between a pressurized cylinder reinforced with fibers in an orthogonal array (A–D) and a cylinder reinforced with fibers in a crossed-fiber helical array (E–H). Orthogonal fiber reinforcement prevents length change (B), provides stiffness in bending until failure occurs by kinking (C), and allows torsion or twisting around the long axis (D). Cylinders reinforced with a crossed-fiber helical array can change length (F), bend in smooth curves (G) and resist torsion (H). After Wainwright (Wainwright, 1988).

Relationship between the volume contained by the crossed-fiber helical system and the fiber angle. The actual volumes of several nemertean and turbellarian worms are indicated with fine horizontal lines; the heavy lines show the measured range of elongation and contraction. Note that a worm with a smaller relative volume (e.g. Lineus longissimus) is typically capable of a greater range of elongation and shortening than a worm with a greater relative volume (e.g. Geonemertes). Some worms (e.g. Polycelis) do not reach the limits set by the crossed-fiber helical system because of other morphological constraints. From Clark and Cowey (Clark and Cowey, 1958).

Schematic cutaway view of a sea anemone showing the circular muscle fibers (C), parietal longitudinal muscle fibers (L), mouth (M), pharynx (P), retractor muscles (R), sphincter muscle (S), and tentacles (T). After Ruppert et al. (Ruppert et al., 2004).

Diagram showing the stages in locomotory cycle of an earthworm. Regions undergoing longitudinal muscle contraction are stationary (indicated by large dots) and are drawn twice as wide as those regions undergoing circular muscle contraction. The tracks of individual points on the body through time are indicated by the lines connecting each drawing, extending from left to right on the page. From Gray and Lissman (Gray and Lissman, 1938).

Schematic cutaway diagram of the arrangement of the tube feet (T), ampullae (A), and the lateral (LC) and radial canals (R) of the starfish Luidia clathrata, showing the trajectories of the connective tissue fibers (CT), longitudinal muscle (L) of the tube feet, and circular muscle (C) of the ampullae. From McCurley and Kier (McCurley and Kier, 1995).

Transverse sections showing the muscular arrangement of three examples of muscular hydrostats. (A) Transverse muscle fibers (T) occupy the core of the squid tentacle and extend to interdigitate with bundles of longitudinal muscle fibers (L) arranged near the surface. Scale bar, 1 mm. (B) Radial muscle fibers (R) extend from the center of the trunk of the elephant between bundles of longitudinal muscle (L). (C) Circular muscle fibers (C) surround two large bundles of longitudinal fibers (L) in the tongue of a monitor lizard. Scale bar, 1 mm. Panels A and C are histological sections stained with Masson’s Trichrome. Panel B is after Boas and Paulli (Boas and Paulli, 1908).