Organization of the muscular system in early and young larva of Phoronopsis harmeri . In all photographs, animals are viewed from the lateral side; the ventral side is to the right, and apical side is to the top. A, C. Photographs of live larvae. B, D. 3D-reconstructions of muscular system (blue) of larvae stained with phalloidin (yellow). A, B. Early actinotrocha. C, D. Young larva. E – H. 3D-reconstructions of muscular system of a young larva stained with phalloidin (E – H) and with Hoechst (F). The position of the mouth is indicated by asterisk. E. Whole larva. F. Internal surface of the preoral lobe with radial muscles (rm) and their basal nuclei (arrowheads). G. Musculature of the edge of the preoral lobe includes annular muscle (am) and the bases of radial muscles. H. Esophageal musculature. Abbreviations: ap – apical plate; ar – archenteron; bc – blastocoels; es – esophageal musculature; lfc – laterofrontal cells are marked by long thick microvilli; t – lateral tiers of muscle cells; mc – muscle cells; pl – preoral lobe; plp – precursor of the preoral lobe; of – oral field; st – stomach; tc – transverse collar muscles; tg – tentacular ridge; tm – transversal muscles of the preoral lobe; vlm – ventro-lateral muscle.