TOOTH ADVANCEMENT MUSCLES IN THE SAND DOLLAR ECHINARACHNIUS-PARMA

As the teeth of sand dollars are chipped and abraded during use, they are continuously renewed by growth in the plumule. During biting, the teeth are secured in place and held immobile by mutable collagenous de ntal ligaments, which react the axial forces upon the teeth. During ad vancement, they are propelled along the dental slides by a pair of den tal promoter muscles flanking each tooth, close to the plumule. At the abaxial (or plumular) end of the tooth slides are small, paired stylo id processes. The dental promoter muscles originate in the superficial stromal spaces of these processes and insert on the dental membrane. Upon contraction of the promoter muscles, force is transferred to the tooth by tension in its surrounding membrane, which is well endowed wi th collagen fibrils and bundles. Although some of the bundles wrap aro und trabecular pillars of the tooth surface, there does not appear to be any direct connection of the muscles to the teeth. However, the mus cle fibers do penetrate into the stromal spaces of the jaw and seem to be tethered to trabeculae by bundles of collagen. Smaller muscular re gions of the dental membrane, between the plumule and promoter muscles , are possibly antagonists responsible for elongation of the promoter muscles.