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 oblique, parapodial, chaetal, and dorsoventral muscles. The longitudinal muscle fibers do not form a continuous layer but are arranged in distinct bands in polychaetes. Mostly there are four to six bands, usually including prominent ventral and dorsal bands. However, other patterns of muscle band arrangement also exist. The ventral nerve cord lies between the two ventral bands in certain polychaetes, and is covered by an additional longitudinal muscle band of comparatively small size. In many polychaetes with reduced parapodia and in Clitellata a more or less continuous layer of longitudinal fibers is formed. Clitellata is the only group with a complete layer of longitudinal musculature. Circular fibers are usually less developed than the longitudinal muscles. However, recent investigations employing phalloidin staining in combination with confocal laser scanning 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.