Prostomial muscles. Scale bars = 50 μm, anterior is to the left. A. Adult specimen with ten chaetigers: ventral view, phalloidin staining. Inset shows phalloidin staining (red) and anti-acetylated tubulin staining (green). Frames mark the regions used for 3D-reconstruction in panels B-E and Figure 3. Pharynx (phar), ventral longitudinal muscle (VLM) and median ventral longitudinal muscle (mVLM). B. Close up of frame depicted in A, phalloidin staining, depth coded (in μm). The prominent straight ventral longitudinal muscles (sVLM) and the diagonal ventral longitudinal muscles (dVLM) connect anteriorly with the outer dorsal longitudinal muscles (oDLM). A small muscle strand splits from each dVLM muscle and extends ventroanteriorly (circle). The most central pair of longitudinal muscle strands cross each other and run toward the opposite lateral side of the prostomium (cDLM). A transverse muscle (PStm1) lies between the dorsal and ventral longitudinal muscles. One part of the PStm1 reaches into the antennae (square). PStm2 – prostomial transverse muscle 2. C – E. 3D-reconstruction of muscles from the area depicted in Figure B. Labels are as follows: sVLM – straight ventral longitudinal muscle, dVLM – diagonal ventral longitudinal muscle, with ventroanteriorly running muscle portion (circle), PStm1 – prostomial transverse muscle 1, contributing to the antennae (square), PStm2 – prostomial transverse muscle 2, lying dorsal to the longitudinal muscles, oDLM, mDLM, iDLM – outer, middle, and inner straight-running branches of dorsal longitudinal muscle, cDLM – central pair of dorsal longitudinal branches, crossing each other. C. Ventral view. D. Ventroanterior view. E. Dorsal view.