Organization of the muscular system in different larval stages of Phoronopsis harmeri . In all photographs, the apical pole of larva is to the top. A. Live 6-day-old larva, lateral view, ventral side is to the right. B – I. 3D-reconstructions of the muscular system of larvae stained with phalloidin. B. Whole 6-day-old larva viewed from the ventral side. C. Dorsal side of the base of the preoral lobe in 6-day-old larva. The chiasm of the radial muscles is indicated by arrowheads. D. A 9-day-old larva viewed from the ventral side. E. An 11-day-old larva viewed from the ventrolateral side. F. A 15-day-old larva, lateral view, ventral side is to the right. G. Anterior body part of a 15-day-old larva viewed from the dorsal side. The most dorsal optical sections have been excised. H. Anterior part of a 15-day-old larva viewed from the ventral side. The most ventral optical sections have been excised. Processes of the esophageal muscle cells, which contact the collar ring muscle, are shown by arrowheads. I. The trunk of a 24-day-old larva, ventral view. Abbreviations: am – annular muscle of the preoral lobe; cr – collar ring muscle; es – esophageal musculature; hd – hood depressors; lg – left group of radial muscles of the preoral lobe; mg – midgut; phd – precursor of the hood depressor; rg – right group of radial muscles of the preoral lobe; t – tentacle; tc – transverse collar muscles; te – tentacle elevator; tf – telotroch flexor; tg – tentacular ridge; tr – trunk retractor; vlm – ventrolateral muscles.