Metamorphosis of Phoronopsis harmeri . In all images, the apical is to the top. A. First stage of the metamorphosis (3 minutes after metamorphosis has begun) – animal with completely everted metasomal sack, which forms the juvenile body, is divided into the anterior trunk part (atp), the posterior trunk part (ptp), and the ampulla (a) (SEM). B. Head region of animal at the first step of metamorphosis, stained with phalloidin. 3D-reconstruction of muscular system; lateral view. C. Second step of the metamorphosis (10 minutes after metamorphosis has begun) – animal with engulfed preoral lobe (pl) (SEM). D. The same stage, 3D-reconstruction of musculature of animal head region stained with phalloidin; ventro-lateral view. E. Tentacles with degenerated (arrows) and complete (arrowheads) muscles. F. Oral view of newly formed juvenile 20 minutes after metamorphosis has begun. 3D-reconstruction of musculature of head region. Degenerated muscles are shown by arrows. G. The same stage; higher magnification of tentacles and lophophore with newly formed tentacle longitudinal muscles (tlm) and circular muscle of the tentacular coelom (mc2). H. Oral view of head region of 4-day-old juvenile. Degenerated muscles are shown by arrows. Abbreviations: am – annular muscle of the preoral lobe; ao – apical organ; cr – collar ring muscle; dt – definitive tentacles; es – esophageal musculature; fo – frontal organ; hd – hood depressor; lm – longitudinal muscles of the trunk; mt – metamorphic tentacles; tc – telotroch constructor; te – tentacle elevator; tf – telotroch flexor; tr – trunk retractor; tt – telotroch.