Morphology of the muscle articulation joint between the hooks of a flatworm (Kalyptorhynchia, Cheliplana sp.).

Schizorhynch kalyptorhynchs are meiofaunal turbellarian predators that possess an eversible proboscis that can be armed with two stout hooks. The hooks grasp and manipulate prey using a wide range of rotations and translations. These diverse motions are possible because the hook supports may function as a muscle articulation type joint-that is, a joint formed of muscle and connective tissue that connects, separates, and moves the microscopic hooks. We analyze the morphology of the flexible joint in a species of Cheliplana by using three types of microscopy: light, laser scanning confocal, and transmission electron. Radial myofilament bundles are present in the core of the hook supports, and lateral divaricator muscle fibers are located on their lateral surfaces. We develop a novel model for movements of the proboscis and describe the tensile function of the basement membrane that surrounds each hook support's medial glandular region. Contraction of divaricator muscle fibers antagonized by contraction of radial myofilaments causes the lateral bending of the hook supports and opening of the hook apparatus. Relaxation of the divaricator fibers and maximal contraction of the radial myofilaments, which put the medial basement membranes in tension, may cause medial bending in the hook supports and closing of the hook apparatus. During proboscis retraction, closure may also be aided by the compression of the hook apparatus as the proboscis is drawn through the rostral pore. The study provides new insights into the principles of support and movement in muscle articulations.