The functional morphology of the suckers of several benthic octopus species was studied using histology and cinematography. The suckers consist of a tightly packed three-dimensional array of musculature. Three major muscle orientations are found in the wall of the sucker: (1) radial muscles that traverse the wall; (2) circular muscles that are oriented circumferentially around the sucker, including a major and minor sphincter muscle; and (3) meridional muscles that are oriented perpendicular to the circular and radial muscles. The connective tissue of the sucker includes inner and outer fibrous connective tissue layers and an array of crossed connective tissue fibers embedded in the musculature of the sucker. Attachment is achieved by reducing the pressure inside the sucker cavity. We propose the following mechanism to explain this pressure reduction. Contraction of the radial muscles thins the wall and thus increases the enclosed volume of the sucker. If the sucker is sealed to the substratum, however, the cohesiveness of water resists this expansion. Thus, contractile activity of the radial muscles reduces the pressure of the enclosed water. The radial muscles are antagonized by the circular and meridional muscles so that the three-dimensional array of muscle functions as a muscular-hydrostat. The crossed connective tissue fibers of the sucker may store elastic energy, providing a mechanism for maintaining attachment over extended periods.