UYENO, T.A.*; KIER, W.M.; Univ. of North Carolina, Chapel Hill: The muscle articulation in polychaetes and cephalopods: joints made of multifunction muscle
Joints allow relative movement between rigid structural elements (termed links). There are two categories found in animals: sliding and flexible joints. Sliding joints (e.g. vertebrate articulated joints) transmit compressional forces directly between contacting links. The shape of the contact surfaces and their connective tissue capsules limit the degrees of freedom and range of motion. Flexible joints include a pliable connection between two links that allows motion. These joints are normally loaded in tension because compressional forces buckle the connection. This study focuses on a form of flexible joint first identified in the Octopus buccal mass: the muscle articulation. Here, muscle and connective tissue form the connection between links. The soft tissues move and transmit compressional forces between the links and create pivot areas. Thus, with appropriate neural control, a greater diversity and complexity of joint movement may be possible. The pharyngeal bulb of the polychaete Nereis also appears to possess muscle articulation characteristics. The curved jaws, used in burrowing, feeding, and fighting, are embedded in a complex mass of connective tissue and muscle fibers. The longitudinal, radial, and circumferential muscle fiber bundles and the connective tissue fiber sheathes of the bulb probably function as a muscular hydrostat. A previous morphological study assigned single functions to individual muscle fiber bundles. Our biomechanical analysis indicates, however, that the function of a given muscle bundle is context-dependent: both the morphology and preliminary observations of the jaws in use suggest that some fiber bundles may function as pivots and jaw openers or closers depending on the activity of other fibers. Thus, Octopus and Nereis jaws share the characteristics of muscle articulations and suggest that such multifunctional joints may be more common than previously recognized.
