The structure and function of a muscle articulation-type jaw joint of a polychaete worm

The arrangement of the musculature and the fibers of the extracellular matrix (ECM) in the flexible jaw joint of the sandworm Alitta virens (Annelida, Polychaeta) was studied using dissection and histology. The jaws are capable of a wide range of motions principally related to defense and feeding. The left and right jaws are embedded in and moved by a compact pharyngeal bulb of muscle and ECM that also forms the mouth and esophagus. Eight pharyngeal bulbs were removed and dissected to document gross anatomical features or preserved and embedded in plastic for sectioning in multiple planes. The sections were stained with toluidine blue and basic fuchsin to differentiate muscle and ECM. The sections were then digitized and used to develop a three-dimensional computer illustration. We hypothesize that the muscle and fibers in the ECM are arranged as a muscular hydrostat to support the movement of the jaws. Four specimens were recorded using a digital video camera and a tank with an angled mirror to record lateral and ventral views of jaw movements during locomotion and biting associated with burrow guarding and feeding. Frame by frame kinematic analysis of this video showed that the jaws move symmetrically in a roughly horizontal plane. Although the angle between the jaws increases and then decreases after maximum gape has been reached, the jaws also translate relative to each other such that the axis of rotation is not fixed. Together, these functional morphological and behavioral data identify the jaw mechanism as a flexible joint known as a muscle articulation. As muscle articulations have been previously described only in the beaks of cephalopods and flatworms, this study implies that this type of joint is more common and important than previously recognized.