[Show abstract][Hide abstract] ABSTRACT: 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.
Preview · Article · Aug 2006 · Integrative and Comparative Biology
[Show abstract][Hide abstract] ABSTRACT: This paper is part of a series investigating the muscular architecture of various "Polychaeta", aiming to ascertain the presence of circular muscles in the body wall, which have recently been thought to be lacking more often than hitherto known. The F-actin muscular subset of Dorvillea kastjani was labelled with phalloidin and the architecture three-dimensionally reconstructed by means of confocal laser scanning microscopy. Three pairs of longitudinal, two transverse and numerous radial muscles ensure shape and flexibility of the prostomium. Mobility of the sub-biramous parapodia and their chaetae is achieved by seven different muscle types. The body wall contains transverse and bracing muscles and in total five longitudinal muscle strands: two dorsolateral, two ventral and one ventromedial. Numerous transverse fibres extend from the dorsal side peripherally and ventrally and become concentrated into six or seven pairs of bundles per segment on the ventral side. They terminate before reaching the midline, leaving a gap of only a few micrometres between them. Within the intersegmental furrows these fibres form complete rings in a supralongitudinal postion. Thus, true circular fibres exist in D. kastjani but are weakly developed.