Horizontal CLSM projections (plane of optical sections perpendicular to DV axis of specimens) of specimens after regenerating for two days stained with fluorophore-tagged phalloidin. Anterior facing upwards in all panels. Black arrowheads point to bursal nozzle, white arrows to male primordium, white double arrowheads to swallow’s nest receptors. A. Whole specimen. B. Magnification of posterior end of specimen in A. Note the concentration of body-wall muscles and parenchymal muscles. C. Further developed male primordium with circular orientation on ventral side and numerous parenchymal muscles forming a distinct sphere. D. Dorsal projection. Note the density of circular muscles in the mid-body area and their absence at the posterior end. White arrowheads point to pairs of fibres, black double arrowheads to forked or furcated ends of muscle fibres. E. Ventral projection of the same specimen as in D. Note the density of circular muscles at the level of the mouth and their absence at the posterior end. White arrowheads point to pairs of fibres. Inset: Magnification showing the circular orientation of muscle fibres in the ventral part of the primordium. Abbreviation: m, mouth. Scale bars: A 100 μm; B, C 20 μm; D, E 50 μm.