PURSCHKE, G.; M�LLER, M.C.M.; University of Osnabrueck, Germany; University of Osnabrueck, Germany: Evolution of Body Wall Musculature

A body wall musculature comprising an outer layer of circular fibers and an inner layer of longitudinal fibers is generally seen as the basic plan in Annelida. Additional muscles may be present such as parapodial, chaetal, oblique and dorsoventral muscles. The longitudinal muscle fibers do not form a continuous layer but they are arranged in distinctly separate bands in polychaetes. There are four to six bands, of which prominent ventral and dorsal bands are usually present. The ventral nerve cord lies between the two ventral bands. In certain polychaetes with reduced parapodia and in Clitellata a more or less continuous layer of longitudinal fibers is formed. Recent investigations employing phalloidin staining in combination with confocal laser scanning electron microscopy revealed that absence of circular muscles is much more widely distributed within the polychaetes than was previously known. This necessitates thorough reinvestigations of polychaete muscle systems and this feature has to be taken into account in further discussions of the phylogeny and evolution of Annelida.